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Biochemistry Services

Introduction

The Clinical Biochemistry department provides a routine and emergency service (analytical and consultative) to assist clinicians with the diagnosis and management of their patients through the biochemical examination of body fluids and products. In addition to the main Clinical Biochemistry service, the Department provides:-

Maintenance and training for all Roche Blood Gas Analysers located on Neonatal Unit, Intensive Therapy Unit, High Dependency Unit, Acute Medical Unit, Emergency department Resusitation, West Wing Theatres and the Delivery Suite.

Quality control support for extra-laboratory blood glucose testing.

To ensure the optimum use of the service, clinicians are encouraged to discuss the selection, performance and interpretation of tests with the Department, and use of the advisory service provided by the Biochemistry Consultants, supported by senior scientific staff, is emphasised.

Repertoire and Sample Requirements

BLOOD:

The following lists are not comprehensive: more unusual tests may be available after discussion with senior laboratory staff.

Most "blood" assays are performed on serum from clotted blood in a SST tube (yellow capped Vacutainer). Where a different specimen type is required this is indicated in the test list below. Failure to supply the correct tube may result in the test being reported as unsuitable.

The [] refer to the notes which follow this list.

ACTH [D, MS]

AFP (Tumour marker) [R]

AFP (Maternal) [R]

Alanine amino transferase (ALT) [Y]

Albumin [Y]

Alcohol [1, GY]

Alkaline phosphatase [Y]

Alkaline Phosphatase. isoenzymes [2, M]

Amino acids [G]

Amylase [Y]

Androstenedione [R]

Angiotensin converting enzyme (ACE) [R]

Bicarbonate [Y]

Bile acids (serum) [R]

Bilirubin [Y]

Bilirubin (Paediatric)

Blood gases [3]

CA125, CA 15-3, CA 19-9 [R]

Caffeine [R]

Calcium [Y]

Calculi analysis

Carbemazepine [Y]

CEA [R]

Caeruloplasmin [R]

Chloride [Y]

Cholesterol [Y]

Copper [R]

Cortisol [R]

Creatine Kinase (CK) total [Y]

Digoxin [R]

Ethosuximide [R]

FSH [Y]

Gamma glutamyl transferase [Y]

Gastrin [D, GS]

Glucagon [GS]

Glucose [GY]

Glycated haemoglobin [M]

Growth hormone [R]

HCG β (Tumour marker) [R]

HCG β (? ectopic pregnancy) [4,R]

HDL Cholesterol [Y]

Homocystine [G]

17-Hydroxyprogesterone [R]

IGF I [R]

Insulin antibodies [D, R]

Insulin [D, G]

Iron [5,Y]

LDH [R]

LH [Y]

Lithium [R]

Magnesium [Y]

Oestradiol [Y]

Osmolality [R]

Pancreatic polypeptide [D, GS]

Paracetamol [Y]

Phenobarbitone [R]

Phenytoin [Y]

Porphyrin [D]

Potassium [Y]

Progesterone [Y]

Prolactin [Y]

PSA [Y]

PTH [R]

Salicylate [Y]

Sodium [Y]

Testosterone [R]

Theophylline [R]

Free thyroxine [Y]

Total protein [Y]

Triglycerides [6,Y]

Troponin I [Y]

TSH [Y]

Urea [Y]

Uric Acid [Y]

Valproate [Y]

VIP [D, GS]

Vitamin A [R]

Vitamin D [R]

Zinc [R]

Notes:

The alcohol method is NOT suitable for medico legal purposes.

Alkaline phosphatase isoenzymes are usually analysed on the specimen submitted for LFT or Calcium profile.

Blood gas analysis, Blood gas analysers are situated at various locations outside the Biochemistry Department.

βHCG analysis should only be used for the diagnosis of ectopic pregnancy. It should not be used merely as a pregnancy test.

Iron analyses are offered in cases of iron poisoning and in cases of suspected haemochromatosis. Ferritin assay is offered for the routine monitoring of patients' iron status.

Elevated triglycerides cannot be interpreted on a non-fasted sample.

D         Discuss with laboratory in advance.

R         Use red capped (plain) Vacutainer

Y          Use yellow capped (SST) Vacutainer

M         Use mauve capped (EDTA) Vacutainer

G         Use green capped (Lithium Heparin) Vacutainer

GY       Use grey capped (Fluoride Oxalate) Vacutainer

GS      Contact laboratory for special heparinised tubes

MS      Contact laboratory for special EDTA tubes

 

URINE:

Analyte

Type

Preservative

Albumin/Creatinine Ratio

Random or EMU

None

Amino acids

Fresh, random

None

Amylase

Random

None

Calcium

24 hour

Hydrochloric Acid

Catecholamines

24 hour

Hydrochloric Acid

Cortisol

24 hour

None

Creatinine

24 hour

None

Cysteine

Random or 24 hour

None

Drug Screen

Random

None

Glucose

24 hour

None

5-Hydroxy indole acetic acid (5HIAA)

EMU

Hydrochloric Acid

Organic acids

Fresh, random

None

Osmolality

Fresh, random

None

Oxalate

24 hour

None

Phosphate

24 hour

Hydrochloric Acid

Porphobilinogen

Fresh, random

None

Porphyrins

Fresh, random

None

Potassium

Random or 24 hour

None

Reducing substances

Fresh, random

None

Sodium

Random or 24 hour

None

Steroid profile

24 hour

None

Urate

24 hour

None

Urea

Random or 24 hour

None

Urobilinogen

Fresh, random

None

Notes:

  • Samples marked with ‘Fresh’ MUST be delivered to the Department as soon as possible.
  • Where a preservative is indicated, bottles containing the appropriate preservative are obtainable from the Biochemistry Department.
  • Catacholamines. Many drugs can interfere and ideally patients should be off medication when urine is collected. The following should, if possible, be discontinued: monoamine oxidase inhibitors, phenothiazines, guanethidine, levodopa, imiprimine, methyldopa, labetalol. Catecholamines occur in significant amounts in vanillin containing foods such as bananas, coffee, chocolate, tea, broadbeans, nuts, custard, blancmange, sponges, cakes, biscuits and sweets. Intake of these should be restricted over the two days before and during the course of the urine collection.
  • 5HIAA. No walnuts, bananas, pineapple, tomatoes, avocados, plums or large amounts of other fruits for 48 hours before or during the course of the urine collection. The following drugs can interfere and should, if possible, be discontinued: chlorpromazine and other phenothiazine derivatives, indomethacin.
  • Androgen, oestrogen or glucocorticoid steroid profiles are available after discussion with the laboratory. 

 

MISCELLANEOUS:

  • Faecal Elastase
  • Faecal Calprotectin
  • Faecal porphyrins
  • CSF glucose
  • CSF total protein
  • CSF Xanthochromia

 

Composition of Test Profiles

The Biochemistry Department offers two admission test profiles for Medical Admission and Surgical Admission, and a number of common organ test panels.

Liver Function Tests (LFTs)

Albumin
Total protein
Alkaline phosphatase (ALP)
Alanine amino transferase (ALT)
Total Bilirubin

Direct Bilirubin and Gamma glutamyl transferase (GGT) are also available separately.

Cardiac Profile

Troponin T
Creatine Kinase (CK)

Electrolytes

Sodium
Potassium
Urea
Creatinine

Bicarbonate and chloride are not measured routinely but are available on request.

Thyroid Function Tests (TFTs)

Thyroid-Stimulating Hormone, TSH (as first-line screen)

Free T4 will be analysed on those samples with an abnormal TSH, on all patients on thyroid active drugs, suspected or known pituitary/hypothalamic disease or as otherwise indicated after discussion with senior laboratory staff. Free T3 is also available when required.

Calcium Profile

Calcium
Alkaline Phosphatase (ALP)
Albumin
Total protein
Adjusted calcium is also report

For a combination of these panels, one full SST tube is required. A second SST is required if TSH and any other tests are requested. If tests for departments other than Biochemistry are required, then extra samples must also be sent.

 

CLINICAL PROTOCOLS AVAILABLE

It is recommended that the selection, performance and interpretation of these procedures is discussed with clinical laboratory staff in each individual case in order to make the most effective use of these tests or services.

Test

Indication

Aldosterone and Renin

Suspected hyperaldosteronism

Ammonium chloride loading test

Suspected renal tubular acidosis (RTA)

Combined pituitary function test

Suspected pituitary dysfunction

Long Dexamethasone suppression test

Distinguishing causes (low and high doses) of Cushing's Syndrome

Short Dexamethasone suppression test

Suspected Cushing's Syndrome (overnight test)

Fasting hypoglycaemia test

Suspected Insulinoma

Fluid Deprivation test

Suspected Diabetes Insipidus

Gastrin and related dynamic tests

Suspected Zollinger Ellison Syndrome

Glucose tolerance test (OGTT)

Suspected Diabetes mellitus

Gonadotrophin releasing hormone

To test the pituitary-gonadal axis

Growth hormone during OGTT

Suspected Acromegaly

Growth hormone

Growth hormone deficiency (before and after exercise) in children

Insulin and C-peptide

Suspected insulinoma

Lactose tolerance test

Suspected Lactose Intolerance

Neonatal hypoglycaemia investigations

Tests to determine underlying cause of hypoglycaemia

Porphyria investigations

Tests to determine suspected porphyria

Sweat test (check with Biochemistry)

Suspected Cystic fibrosis

Synacthen stimulation test (short)

Suspected adrenocortical hypofunction

Synacthen stimulation test (long)

Distinguishing causes of adrenocortical hypofunction

Thyrotrophin-releasing hormone test

Investigating secondary and tertiary thyroid disorders

 

SPECIAL TOPICS

BLOOD GAS ANALYSIS

The blood gas analysers are situated on Neonatal Unit, Intensive Therapy Unit, High Dependency Unit, Acute Medical Unit, Emergency department Resuscitation, West Wing Theatres and the Delivery Suite, for the following tests on each machine: pH, pCO2, pO2, calcium, glucose, lactate, sodium, potassium, co-oximetry. Bilirubin is available on NNU only. These are operated under the supervision of the Blood Sciences Department. Non-laboratory staff wishing to use these instruments must be trained in their correct operation by Blood Sciences Department staff and issued with a user identification number. In the event of a fault occurring with any of these gas analysers out of hours, staff should contact the Blood Sciences out of hours BMS.

The Roche blood gas analysers can only be operated by staff with a valid user ID number. This number enables an audit trail to be established for the analysis of patient samples. The patient's unit number, name and date of birth is required before results are issued by the analyser.

Incorrect sample handling before analysis leads to significant artefacts. For accurate estimation of pO2 and pCO2 it is necessary to avoid frothing, to expel all air bubbles within two minutes and to inject the sample into the machine within ten minutes if the syringe is kept at room temperature. Samples kept at 0°C by immersion in a crushed ice/water mixture remain stable for far longer. Syringes should be capped to prevent contact with the atmosphere. If heparin makes up 10% or more of the total volume of a sample for gas analysis important errors, especially in apparent pCO2, may occur.

The blood gas analysers are located on Neonatal Unit, ITU, HDU, AMU, A&E, West Wing Theatres and the Delivery Suite, and are maintained on a daily basis by staff in the department with support from Blood Sciences. The instruments are primarily for use by these departments and may only be used by other wards after consultation with them.

MARKERS OF MYOCARDIAL DAMAGE

Troponin T

Cardiac Troponin T does not start to rise until 4 - 6 hours post MI but by 12 hours after the onset of pain diagnostic sensitivity is close to 100%; tissue specificity of Troponin T for cardiac muscle is also high, although false positives can occur. Troponin levels remain elevated for over a week following MI.

The main indication for requesting Troponin T in this Trust is risk stratification in patients with Acute Coronary Syndrome (ACS); see Trust protocol on ACS (on the Trust intranet).

Resolving diagnostic ambiguity with conventional tests, e.g. if non-cardiac CK release is a possibility (NB, if the ECG and CK are both repeatedly normal the probability of an abnormal Troponin is very low);

Troponin T is available 24 hours a day, 7 days a week.

Diagnosis of late-presenting MI.

For Troponin T:

Analyte

Result

Interpretation

Troponin T

< 14 pg/ml

no evidence of myocardial damage

Troponin T

> 14 pg/ml

evidence of myocardial damage

Cardiac enzymes

The enzyme offered by the Biochemistry Department, to provide supportive information, is creatine kinase (CK). Under ordinary circumstances, this test should be requested on two consecutive days, although in many cases a typical clinical presentation combined with diagnostic ECG changes may render enzyme measurements redundant. Unless a patient develops new symptoms or the clinical condition changes significantly there is no advantage in requesting enzyme analysis on the third day.

The following table shows the typical time courses of enzyme changes following myocardial infarction:

Analyte

Level starts to be abnormal (hours)

Peak level of abnormality(hours)

Duration of abnormal level (days)

Total CK

5 - 12

18 - 30

2 - 5

The shorter duration of CK rises in comparison with Troponin T makes the former more useful should there be suspicion of extension of MI.

POINTS TO NOTE

Because of the delay between the onset of pain and the appearance of enzyme abnormalities, samples taken during the first few hours following the development of symptoms cannot be used to exclude myocardial infarction.

However, assuming that samples have been collected during the appropriate time 'window', CK is an extremely sensitive test, a normal result making myocardial infarction highly improbable.

Because total CK is not cardiac-specific, interpretation must take into account the clinical circumstances and ECG changes. Atypical patterns, including isolated elevations in the enzyme in the absence of other evidence of myocardial infarction should give rise to the suspicion of a non-cardiac explanation. Total CK increases may be due to skeletal muscle damage (e.g. surgery, resuscitation, intra-muscular injection).

Others

The CRP assay offered by this laboratory is not suitable for detecting early myocardial damage.

LIPIDS

When making requests for lipid analysis, please state explicitly on the request form whether the patient was in a fasted state when the sample was obtained (use the tick box provided).

Routine lipid profile - (all samples accepted, but need to state when ordering if fasting/non-fasting):

Total cholesterol
HDL cholesterol
Non-HDL cholesterol (calculated)
TC/HDL ratio (calculated)

Extended lipid profile - (all samples accepted, but need to state when ordering if fasting/non-fasting):

Total cholesterol
HDL cholesterol
Non-HDL cholesterol (calculated)
TC/HDL ratio (calculated)
LDL-cholesterol (on fasting samples only & if triglycerides <4.5 mmol/L)
Triglycerides

Triglycerides only - (as a separate test, no need to be fasting)

If no information on the patient's fasting state is given the sample will be assumed to be random/routine.

The UK Heart Protection Study indicated that most patients with established CHD or other occlusive arterial disease may benefit from statin treatment. Where Simvastatin is the chosen agent the optimum dose is probably 40 mg; in an adequately treated patient population (e.g. those on a practice CHD register) the target median total cholesterol is <4 mmol/l. With regard to primary prevention of CHD, initiation of drug therapy for lipids should not be undertaken without a formal CHD risk assessment; clinicians can contact senior laboratory staff for assistance in this matter. Fasting plasma glucose should be checked as a part of CHD risk assessment.

As a first line screen, it is reasonable to measure a random (i.e., not necessarily fasted) total cholesterol, avoiding excessive stasis during venepuncture.

If raised lipid levels are found on a non-fasted sample then request lipids on a sample obtained after an overnight fast (no breakfast, water only to drink).

Cholesterol levels fall within 24 hours as part of the metabolic response to injury and do not return to base line for several weeks or months. It is therefore recommended that lipid levels should be reassessed 8-12 weeks after a MI or other vascular event, unless it has been decided not to give lipid therapy for some reason. Annual lipid checks are recommended on patients established on long-term lipid therapy.

Remember secondary hyperlipidaemias. The commonest secondary cause of hypercholesterolaemia is hypothyroidism, while the most frequent secondary causes of hypertriglyceridaemia are diabetes mellitus and alcohol abuse. Beta-blockers and thiazide diuretics (and may other drugs) affect lipid profiles adversely.

Some genetic dyslipidaemias can be investigated by DNA methods. Such cases should be discussed with the Consultant Chemical Pathologist.

DIABETES MELLITUS

Oral glucose tolerance test

A protocol for this procedure is available from the Department. There is no need to carry out a glucose tolerance test if a patient has classical features of diabetes mellitus (e.g., polyuria and polydipsia, ketonuria, rapid weight loss) and unequivocal marked hyperglycaemia (random plasma glucose greater than 11 mmol/l) or a fasting plasma glucose of 7.0 mmol/l or above.

Normal fasting plasma glucose < 6.1 mmol/l

Full international expert committee recommendations for the diagnosis of diabetes mellitus are given below:

A patient is classified having diabetes mellitus if they have:

Diabetic symptoms (e.g. polyuria, polydipsia and unexplained weight loss)

Plus either

a random venous plasma glucose concentration ≥ 11.1 mmol/l;

OR

a fasting plasma glucose concentration ≥ 7.0 mmol/l (whole blood ≥ 6.1mmol/l);

OR

2 h plasma glucose concentration ≥ 11.1 mmol/l 2 h after the administration of 75 g anhydrous glucose in an oral glucose tolerance test (GTT).

Where there are no symptoms, diagnosis should not be based on a single glucose determination but requires confirmatory plasma venous determination. At least one additional glucose test result on another day with a value in the diabetic range is essential, either fasting, from a random sample, or from the two hour post glucose load. If the fasting or random values are not diagnostic the 2-hour value should be used.

It should be noted that children usually present with severe symptoms and diagnosis should then be based on a single raised blood glucose result, as above. Immediate referral to the Paediatric Diabetes Team should be made.

Impaired Glucose Tolerance (IGT)* is a stage of impaired glucose regulation (fasting plasma glucose < 7.0 mmol/ and GTT 2-hour value ≥ 7.8 mmol/l but < 11.1 mmol/l).

Impaired Fasting Glycaemia (IFG)* has been introduced to classify individuals who have fasting glucose values above the normal range but below those diagnostic of diabetes. (fasting plasma glucose ≥ 6.1 mmol/l but < 7.0 mmol/l).

*IGT and IFG are not clinical entities in their own right but rather risk categories for cardiovascular disease (IGT) and/or future diabetes (IFG).

A diagnosis of diabetes has important legal and medical implications for the patient and it is therefore essential to be secure in the diagnosis. A diagnosis should never be made on the basis of glycosuria or a stick reading of finger prick blood glucose alone, although such tests may be useful for screening purposes.

Glycated Haemoglobin

During the life span of a red cell following its release from the marrow into circulation, a small percentage of the haemoglobin (Hb) becomes glycated. The average glucose level to which the red cells are exposed during their life span determines the proportion of total Hb modified in this way. For this reason, glycated Hb tends to be increased in diabetes mellitus and, particularly in insulin-dependent diabetics, glycated Hb gives a better measure of overall glycaemic control over the previous 2-3 months or so than do random glucose measurements. Conditions which reduce the life expectancy of red cells in circulation (e.g., bleeding, in vivo haemolysis) give falsely low results and so invalidate the test. Some haemoglobinopathies can cause analytical interference, as can gross uraemia.

Since 2011, the World Health Organisation (WHO, 2011) has recommended that glycated haemaglobin A1c (HbA1c) can be used as a diagnostic test for diabetes in most situations. The main exceptions are rapid onset diabetes, anaemia, haemoglobinopathies and other diseases associated with changes in red cell turnover (e.g. malaria, drug-induced haemolysis) or glycation rates (e.g. chronic renal disease). In these situations, HbA1c is not recommended as the sole test to diagnose diabetes, and it is also inappropriate to use HbA1c to identify gestational diabetes mellitus.

An HbA1c of ≥48 mmol/mol is now the recommended cut off for diagnosing diabetes, but in an asymptomatic patient a repeat measurement is required to confirm the diagnosis (ideally after 1 month). An HbA1c ≥48 mmol/mol can also be used to confirm a diagnosis of diabetes in an asymptomatic individual with a fasting glucose ≥7.0 mmol/L or random glucose ≥11.1 mmol/L, precluding the need for a repeat glucose measurement or glucose tolerance test (except in the circumstances mentioned above, e.g. in pregnancy).

However, an HbA1c value <48 mmol/mol does not exclude diabetes diagnosed using glucose tests.

HbA1c values in the range 42–47 mmol/mol indicate a high risk of developing diabetes in the future, i.e. are pre-diabetic. Such patients should receive intensive lifestyle advice and warned to report any symptoms of diabetes (annual monitoring of HbA1c is recommended). The reference range provided for HbA1c is for identifying new diabetes or pre-diabetes only and is NOT a target range for optimal glycaemic control. We still recommend that all HbA1c results are reviewed.

Criteria for Glycaemic Control using HbA1c in Patients with Type 1 and Type 2 Diabetes:

 

                                         Good                      Borderline                   Sub-optimal

Type 1                               < 48 mmol/mol      48-58 mmol/mol         > 58 mmol/mol

                                               

                                         Low risk                  Arterial risk                 Microvascular risk

Type 2                               < 48 mmol/mol      >= 48 mmol/mol         > 58 mmol/mol

 

HbA1c levels only change slowly due to the red cell lifetime of approximately 120 days, so measurements more frequently than every 3 months are of limited value when monitoring those with known diabetes. Thus, repeat requests in known diabetic patients received within 3 months will generally be rejected (unless the patient is pregnant). 

Microalbumin

Microalbuminuria is the condition of pathologically increased urinary albumin excretion below the detection limit of common side-room tests for proteinuria (e.g. reagent sticks). As a first line screen, an albumin:creatinine ratio can be determined on a random daytime urine sample obtained from an ambulant patient. If the results in this test are abnormal, the urinary albumin:creatinine ratio should be repeated twice within 1 month; if two of the three tests are positive the patient is deemed to be at high risk of renal and other complications of diabetes.  For the two confirmatory tests, first morning urine samples should be obtained, where practicable.  This is because there is a postural effect on urinary albumin loss.  Timed urine collections are not recommended for the assessment of microalbuminuria. See also section 4.7.2 on proteinuria.

THYROID FUNCTION

The first line test performed in this Department is TSH, assayed by a highly sensitive 'third generation' technique. A normal TSH level makes primary thyroid disease extremely unlikely in patients not taking thyroid active medication.

Free T4, assayed by a labelled antibody technique, is used as a follow-up test when the TSH is abnormal, in patients on thyroid active drugs and when pituitary/hypothalamic disease is suspected.

Free T3 is occasionally useful in the investigation of suspected thyrotoxicosis or in patients on certain drugs, including Amiodarone. The need for T3 measurements will be assessed in the light of the clinical context and the results of first and second line tests. Serum T3 is generally of no diagnostic value in suspected hypothyroidism.

In general, tests of thyroid functions are most effective in patients in whom the objective is to confirm/exclude untreated primary thyroid disease. Thyroid function tests may be of value in monitoring patients either on thyroxine or anti-thyroid medication, although opinion is divided on interpretation in some circumstances. The fact that a patient is receiving thyroid-active drugs significantly affects the interpretations of the tests and relevant therapeutic details should always be supplied with TFT (TSH) requests, as well as an indication of the thyroid status suspected clinically.

Abnormalities in both free T4 and, to a lesser extent, TSH are common in patients with non-thyroid disease, particularly in the elderly and in patients with more severe intercurrent illnesses. Interpretation can be very difficult and, indeed, genuine primary thyroidal dysfunction may be masked by these non-thyroidal factors, so that both false positives and false negatives can occur. For these reasons, requesting TFTs as part of a metabolic screen in a patient admitted with non-thyroidal illness, and with no genuine suspicion of thyroid disease, is usually futile and potentially misleading. Therefore, TFTs should not be requested on an acutely ill patient or for six weeks after a major illness.

The interpretation of TFTs in pregnancy, the new-born and the elderly is complex and discussion in doubtful cases is encouraged.

INTERPRETATION OF HIGHLY-SENSITIVE TSH AND FREE T4 IN PATIENTS NOT RECEIVING THYROID-ACTIVE MEDICATION

Thyroid Function Interpretation

TSH

Low

Normal

High

Free T4

High

Primary hyperthyroid

Abnormal binding proteins or TSH-secreting tumour or abnormal T4 metabolism

Primary hypothyroid with abnormal binding proteins or TSH-secreting tumour

Normal

Non-thyroid illness or borderline thyrotoxicosis or T3-toxicosis

Euthyroid

Borderline primary hypothyroid

Low

Non-thyroidal illness or hypopituitarism

Non-thyroidal illness or hypopituitarism

Primary hypothyroid

Based on:

Beckett, Smith, Walker and Rae; Lecture Notes on Clinical Chemistry; Blackwell 1998

THE ADRENAL CORTEX

'Spot' serum cortisol is generally unhelpful in the investigation of either hypo- or hyper-corticoadrenalism, and assessment of the diurnal rhythm of cortisol secretion is also of limited value.

Suspected primary adrenocortical failure

The most useful screen for potential adrenocortical failure is the short tetracosactrin (Synacthen) test. A normal result excludes adrenal failure but abnormal outcomes should be followed up by more extended testing with a depot preparation of tetracosactrin. Such 'long' Synacthen tests are useful in confirming abnormalities found in the short screen. They may help discriminate between primary adrenocortical hypofunction (in which the response remains flat) and adrenal failure secondary to hypothalamic-pituitary disease (in which the adrenal response to repeated stimulation with tetracosactrin may increase over a few days). It may be appropriate to investigate pituitary function more fully in the latter situation. It may be useful to store a sample for possible ACTH assay (with due attention to sample handling requirements) before steroids are administered.

Strategy for investigation:

Screening test -

Short Synacthen Test

Store sample for possible future ACTH assay before giving steroids.

Follow up tests -

Long Synacthen Test

ACTH (Sample stored as a(ii) above)

Adrenal autoantibodies

Suspected adrenoglucocorticoid hypersecretion

Suspected hyperfunction of the adrenal cortex should be investigated in the first instance by either an overnight dexamethasone suppression test and/or measurement of free cortisol excretion in a 24 hour urine collection. Various test protocols are available for the follow-up of abnormal results. To help ensure that the most appropriate investigations are carried out and that procedures are correctly performed, close liaison between the laboratory and the requesting clinician is essential.

Strategy for investigation:

Outpatient screening tests -

Overnight dexamethasone suppression test

24 hour urinary free cortisol

Tests to confirm hypersecretion -

Low dose dexamethasone suppression test

Insulin hypoglycaemia

Tests to establish cause of hypersecretion -

ACTH assay

High dose dexamethasone suppression test

CRH test, selective venous catheterisation

Imaging techniques

Tumour markers

Reproductive Hormones

For menopausal status, request FSH and LH. Progesterone should not be requested for menopausal status.

For ovulation, request progesterone on a sample taken on or about day 21.

For other common conditions, specimens for hormone requests are best taken early in the follicular phase.

ALWAYS indicate hormones required when sending requests; DO NOT request "hormone profile".

RENAL FUNCTION

Glomerular function

Glomerular function can be assessed by measuring the serum concentration of urea or creatinine, or by estimating glomerular filtration rate (GFR)

As a simple screen serum urea (as part of the 'U&Es') is adequate, a normal urea level excluding a major glomerular problem in most circumstances. However, serum urea concentrations may be elevated for a variety of non-renal reasons, including upper gastro-intestinal haemorrhage ('blood meal'), catabolic states and hypovolaemia.

Serum creatinine is a more specific test of glomerular function than is serum urea (although ketoacidosis can cause false elevations in creatinine estimations as a result of analytical interference) but some patients with significant impairment of glomerular function may have serum creatinine levels within the population reference range.

Despite the imperfect sensitivity of serum creatinine for the detection of glomerular failure, measurement of creatinine clearance is rarely helpful, mainly because of the poor reproducibility of clearance determinations and because creatinine clearance overestimates the true GFR at all levels of renal function. For this reason, in chronic kidney disease, the estimation of GFR using a calculation based on the NICE EPI equation is now recommended.

Click here  to view the eGFR Flowchart 

Proteinuria

There is no need to perform 24-hour urine collections for the quantitation of proteinuria. A positive dipstick test for protein should result in a urine sample (preferably early morning, so as to eliminate the orthostatic effect on proteinuria) being sent to the laboratory for confirmation by measurement of the albumin:creatinine ratio, after exclusion of urinary tract infection. Patients with two or more positive tests for proteinuria, preferably spaced 1 to 2 weeks, should be classified as having persistent proteinuria.

Interpretation

Dipstick

Urinary albumin:creatinine ratio (mg/mmol)

Normal

Negative

<2.5 (males), <3.5 (females)

Microalbuminuria

Negative

≥2.5 (males), ≥3.5 (females)

"Trace" proteinuria

Trace

15-29

Clinical Proteinuria

1+ or more

≥30

 

Renal concentrating function:

A protocol is available for fluid-deprivation tests (with or without follow-up DDAVP test). This procedure tests the ability of the posterior pituitary to secrete AVP (ADH) appropriately and the responsiveness of the renal concentrating apparatus to the hormone. This test is sometimes helpful in the investigation of suspected cranial or nephrogenic diabetes insipidus.

Other causes of polyuria and polydipsia include-

diabetes mellitus

hypercalcaemia

hypokalaemia

chronic renal failure

primary ('hysterical') polydipsia

Urinary acidification:

A protocol is available for the ammonium chloride loading test, a test of distal tubular urinary acidification. Suspected renal tubular acidosis is one of the rare indications for assay of serum chloride, hyperchloraemia often being found in this condition.

Renal Calculi:

24-hour urine collection (in a plain container) should be collected on patients with a history of calculi or a high risk of developing calculi.

The following analysis will be carried out:

Calcium and Urate at the Manor Hospital

Oxalate and Citrate at the University College Hospital, London

Reference Ranges

Analyte

Gender

Range (mmol/24 h)

Calcium

Male

<7.5

Female

<6.2

Urate

Male

<4.8

Female

<4.5

Oxalate

Male & Female

0.10 - 0.46

Citrate

Male

0.6 - 4.8

Female

1.3 - 6.0

 

GASTROINTESTINAL FUNCTION TESTS

Fat absorption:

Faecal Fat estimation has now been replaced by Faecal Elastase estimation as an indicator of exocrine pancreatic function.

Tests of (proximal) small intestinal function may be arranged after discussion with the gastroenterologists.

BIOCHEMICAL MONITORING OF PATIENTS RECEIVING TOTAL PARENTERAL NUTRITION

The following suggestions are not intended to be didactic but rather to serve as broad guidelines for the monitoring of patients established on total parenteral nutrition (TPN). Over the first few days of TPN, patients' requirements may undergo rapid changes so that during the initiation of TPN, and at other times when the metabolic state is labile, monitoring should be intensive.

Daily on blood:

- urea,  sodium, potassium, glucose

In stable patients, measurements of these analytes on alternate days may be sufficient.

It is sometimes helpful to measure the electrolyte content of daily collections of other fluids, when there are excessive and rapidly changing losses.

Twice weekly:-

Blood:

- bilirubin, alanine aminotransferase (ALT), alkaline phosphatase, gamma-glutamyl transferase, albumin, total protein, calcium, phosphate, magnesium

Urine (24 hour collection):

- urea, sodium, potassium

If the serum urea is stable and the urine is not infected with urea-splitting organisms, the urinary excretion of urea can be used to estimate urinary nitrogen losses.

Other measurements:

In prolonged TPN, assays of zinc and other trace elements may be justified. Direct assay of vitamins is hardly ever necessary.

THERAPEUTIC DRUG MONITORING AND TOXICOLOGY

Therapeutic Drug Monitoring (TDM):

The table gives guidelines that are useful in deciding whether to assay a particular drug. There are very few situations where a request for Phenobarbitone or Valproate is justified, since these drugs exhibit a very poor relationship between serum levels, dose and clinical effect.

The Department sends some requests for therapeutic drug monitoring (Phenobarbitone, Ethosuximide, Gabapentin, Caffeine, Theophylline) to other laboratories, although Lithium, Phenytoin, Carbamazepine and Valproate are assayed at the Manor. Use of specialist facilities like this has been encouraged by the Audit Commission. We know that in the past many drug assays requested through this Department have been of doubtful clinical value and because of this we audit requests forwarded elsewhere. Our intention in doing this is not merely to avoid waste but also to help ensure that clinicians obtain the clearest possible answer to the clinical question that they are asking in making requests. Discussion about requests and the interpretation of results is welcomed.

We require a minimum amount of information before a sample will be sent for assay:

a. full patient details (including post code);

b. full details of all drugs the patient is on including dose and dose frequency;

c. time of last dose;

d. time sample taken;

e. clinical condition of patient and the reason for request (control, compliance, toxicity etc.)

TDM Guidelines

Drug

Recommended time of sampling

Therapeutic Range

Time (days) to reach steady state

Value of monitoring plasma

Frequency of monitoring

Detection limit

Carbamazepine

Before next dose (not critical) standardise timing

4-12 mg/l

2-3

Proven

monthly

2 mg/l

Digoxin 4

At least 6 hours after an oral dose and upto immediately before the next dose

0.5-2.0 µg/l

7

Proven

monthly

0.1-0.2 µg/l

Ethosuximide

Before next dose (not critical) standardise timing

40-120 mg/l 1

7-14

-

-

-

Gabapentin

Before next dose (not critical) standardise timing

2.0-20.0 mg/l 1

1-2

-

-

-

Lamotrigine

Before next dose (not critical) standardise timing

1.0-4.0 mg/l 1

4-6

-

-

-

Lithium

12 hr ± 30 minutes after last dose

0.6-1.0 mmol/l 3

5

Well proven

monthly

0.1 mmol/l

Phenobarbitone

Before next dose(not critical)standardise timing

15-40 mg/l 1

10-20

Unproven

not recommended

5 mg/l

Phenytoin

Before next dose (not critical) standardise timing

10-20 mg/l

7-10

Well proven

2 monthly

2.5 mg/l

Theophylline 2

Before or 2-4 hrs after an oral dose

10-20mg/l 1

1-2

Proven

monthly

2.5 mg/l

Valproate

Standardise timing (6-12 hrs) after last dose

up to 100 mg/l

1-2

Unproven

not recommended

10 mg/l

Vigabatrin

Standardise timing, before next dose

5-35 µg/l 1

5-10

-

-

-

Notes:

  1. Reference ranges as quoted by the measuring laboratory, Biochemistry, New Cross Hospital, Wolverhampton.
  2. For Theophylline monitoring in acutely ill patients baseline serum samples should be obtained. When on continuous intravenous therapy obtain serum samples every 12 hours for dose adjustment. The therapeutic range given is for childhood and adult asthma. For neonatal apnoea aim for 5-10 mg/l and for children 1 year of age aim for 5-15 mg/l.
  3. In view of the possible long term toxic effects of lithium, the lowest possible dose giving effective control of symptoms should be used. Patients on long term lithium therapy should have their renal and thyroid function monitored.
  4. Digoxin effect and toxicity is increased by hypokalaemia, hypercalcaemia, renal failure, hypothyroidism, advanced heart disease.
  5. Carbamazepine, Valproate, Phenytoin, Lithium and Digoxin are analysed on site Monday to Friday routinely. These tests can be performed urgently following discussion with the laboratory.

There are very few situations where a request for Phenobarbitone or Valproate is justified, since these drugs exhibit a very poor relationship between serum levels, dose and clinical effect.

Therapeutic ranges are target ranges associated with optimal therapeutic effect in the majority of cases. However, 'sub-therapeutic' or 'toxic' concentrations may be the effective concentrations in some patients. Ingestion of other drugs may also modify the concentration of the drug being measured or its pharmacological activity. The drug concentration value should always be used in conjunction with clinical observations.

Toxicology:

Paracetamol:

In cases of suspected paracetamol overdose blood should be collected at least 4 hours after ingestion of the drug. Serum levels measured less than 4 hours post-ingestion cannot be interpreted. Therapeutic levels of paracetamol are usually less than 10 mg/l. If serum levels are more than 200 mg/l at 4 hours after ingestion or more than 50 mg/l at 12 hours or detectable at all at 24 hours, liver damage is likely unless active antidote treatment is given. The prognostic accuracy of serum paracetamol levels after 15 hours is uncertain. The levels requiring antidote treatment in relation to time are given in the figure and table below:

Time after overdose (hours) above which liver damage is likely, unless antidote treatment given.

Serum paracetamol concentration (mg/l)

4

200

5

170

6

140

7

120

8

100

9

85

10

70

12

50

15

30

If the patient is to be treated with antidote, INR, serum creatinine and ALT should be monitored.

Certain patients, particularly chronic alcoholics or those who take certain drugs (including Phenytoin, Carbamazepine, phenobarbitone, Primidone, Rifampicin and St John's Wort) are at higher risk of paracetamol-induced hepatic necrosis, and should be treated at serum concentrations half as great as those indicated above. In addition, in patients likely to be glutathione deplete (e.g. eating disorders, cystic fibrosis), the serum paracetamol concentration at which antidote is indicated is halved.

In the case of patients presenting 8 - 15 hours after ingestion antidote should be given immediately if it is thought that more than 150 mg/Kg body weight or 12 g in an adult (whichever is smaller) has been ingested. DO NOT WAIT for the result of the serum paracetamol concentration; the efficacy of antidote declines rapidly during this period and it must therefore be given urgently.

Severe cases with liver and renal failure may be complicated by metabolic acidosis and hypoglycaemia.

Salicylate:

The therapeutic levels of serum salicylate for analgesia are of the order of 50-150 mg/l, for anti-inflammatory purposes 100-350 mg/l. Levels > 350 mg/l in children and >500 mg/l in adults are indications for active treatment, when the specimen is taken more than 6 hours after overdose. If the initial concentration exceeds 900 mg/l and if there is renal impairment or if other therapeutic measures fail, haemoperfusion or haemodialysis will usually be necessary. Serum salicylate levels should be measured during treatment as an indication of efficacy of treatment.

Other drugs:

In cases of acute poisoning with other agents that need to be identified for immediate patient management, please contact the Regional Laboratory for Toxicology at Birmingham City Hospital to discuss and justify the request. Specimens should, where possible, go through the Biochemistry Department to the City Hospital.

SERUM TUMOUR MARKERS

The Department does not perform any of these assays but offers a service for them by sending the samples to other laboratories.

It is important to remember that:

Measurements of serum tumour markers are useful for:

Monitoring of the progression of specific tumours diagnosed by other means;

Monitoring the response of specific tumours to therapy including recurrence & tachyphylaxis.

Most serum tumour markers are neither specific for:

Individual types of tumours;

nor

Even tumours in general.

They should not be used for attempting to diagnose tumours or identify primaries when metastases have been found.

An elevated serum tumour marker does not indicate the presence of a tumour.

A serum tumour marker within the reference range does not indicate the absence of a tumour.

There are very few exceptions to the guidance above but these include serum calcitonin for medullary carcinoma of the thyroid and AFP for hepatomas and germ cell tumours.

Each form requesting serum tumour markers is vetted by a senior member of the Biochemistry staff and specimens will not be processed if appropriate clinical details are not given. Clinical staff are encouraged to contact the Department to discuss individual cases.

Marker

Clinical Indication - remembering points above

AFP

Hepatoma, Germ cell tumours

CA125

Ovarian tumours

CA15-3

Breast

CA19-9

Pancreas

CEA

Colorectal tumours

HCG

Germ cell tumours, hydatidiform mole, choriocarcinoma

SPECIMEN REQUIREMENT AND TRANSPORT FOR ANALYSIS OF CSF FLUID

Whenever possible collect four sequential specimens.

First specimen should be 0.5 ml (minimum) of CSF placed in a Fluoride Oxalate tube for glucose estimation. This specimen should be labelled "first" and sent to Clinical Biochemistry.

Microbiology requires at least 5 ml of CSF in two separate 28 ml sterile universal containers labelled "second" and "third". These specimens must be delivered to the Microbiology Department as soon as possible.

A minimum of 1 ml of CSF should be placed in a 28 ml universal container for protein estimation and spectro-photometric scan, labelled "fourth" (N.B. 1 ml is about 20 drops from the Luer connector on a needle). Protect this sample from light by placing it in a thick brown envelope outside the usual plastic specimen bag. Please mention clinical indication for request, result of CT if possible, time of onset of symptoms and event and time of specimen taken and if the differential diagnoses include meningitis. This specimen should be taken to the Clinical Biochemistry Department as soon as possible.

A simultaneous blood specimen should be taken for serum bilirubin, total protein and glucose measurement that are needed to aid interpretation.

Record the timing of sample relative to that of possible haemorrhage. This should not be less than 12 hours

For further details please contact the Duty Biochemist in the department of Clinical Chemistry.

 

Birmingham Children's Hospital Pathology IMD Handbook

This document details the tests available and the samples required to investigate inborn metabolic disorders.

BCH IMD Handbook 2017

SUDI Protocol

The current Sudden Unexplained Death of an Infant protocol is available by clicking the following link - SUDI Protocol (updated July 2016).