Biochemistry

The Clinical Biochemistry test repertoire includes general chemistry and endocrine tests, therapeutic drug monitoring, HbA1c and a range of more complex and specialised investigations.

The laboratory is equipped with an Abbott system for core biochemistry, routine endocrinology and measurement of troponin, haematinics, tumour markers and therapeutic drugs and with three Sebia analysers for HbA1c measurement.

The department also performs more complex Clinical Biochemistry investigations, which include specialised endocrine tests, analysis of trace metals, faecal inflammatory markers and measurement of drugs of abuse and immunosuppressants. Some of these assays are provided for other Trusts, both in the South West and other sites in the UK.

Results for the majority of routine tests are available the same day, often within two hours of specimen receipt in the laboratory. The more specialist test carried out in Clinical Biochemistry have a longer turn-around-times. Some highly specialised tests will require referral to other UK laboratories due to the complex nature of these investigations. Because of this, results may sometimes not be available for several weeks.

Duty Biochemist

A duty biochemist is available during standard working hours (Monday to Friday, 9am to 5.30pm) for clinical advice including enquiries about interpretation of results and appropriate follow-up tests. Please telephone 430900 (Ext 30900) or e-mail plh-tr.DutyBiochemist@nhs.net.

A Consultant Clinical Biochemist is available at all times and can be contacted via switchboard outside standard working hours.

Test information

It is essential that full clinical details are provided on request forms by the person requesting the test to allow meaningful interpretation of results and the immediate addition of any further appropriate tests.

Urine tests

The Clinical Biochemistry Department offers a number of urine tests, those that are most commonly requested are shown in the table below:


Urine tests
Test	Specimen	Note
ACR (albumin: creatinine ratio)	Plain universal (white top) either early am or random sample	This is inappropriate in patients with proteinuria on dipstick testing
Metadrenalines (Metanephrine + Normetanephrine) (Catecholamine are no longer measured at DCL).	Plain 24hr urine collection	For suspected phaeochromocytoma. Please note that patients should avoid caffeine and chocolate in the 3-4 days before sample collection
Calcium	Plain 24hr urine collection
Calcium/Creatinine Ratio	Plain random sample (white top universal) or 24hr collection
Copper	Plain 24hr urine collection
Cortisol	Plain 24hr urine collection	Please note that this is for suspected Cushing’s syndrome, NOT adrenal insufficiency.
Creatinine clearance	Plain 24hr urine collection	Not generally available, eGFR reported with serum creatinine
Electrolytes	Plain random sample (white top universal) or 24hr collection
5HIAA	Plain 24hr collection	For suspected carcinoid NOT for phaeochromocytoma requires 3 days of special diet**
Microalbumin	Plain (white top universal) early am only	See ACR
MSU		See Microbiology section
Myoglobin		No longer available; suggest measure serum CK & test urine for haem with dipstick
Organic acids	Plain (white top universal) random
Protein/ creatinine ratio	Plain (white top universal) early am sample
VMA		Please note that this test is not suitable for adult patients; please instead send 24-hour urine sample for analysis of metadrenalines.

Useful Telephone Numbers



	External	Internal
Dr Tony Avades, Consultant Chemical Pathologist	(01752) 430037	30037
Ms Roanna George, Consultant Clinical Scientist	(01752) 430199	30199
Dr Jinny Jeffery, Clinical Scientist	(01752) 430867	30867
Duty Clinical Biochemist	(01752) 430900	30900
Results	(01752) 433217	33217
Main Laboratory	(01752) 430868	30868
Special Investigations	(01752) 245254	55254

**For 24 hr urine collection for 5HIAA patients should avoid the following for 3 days before, and during the period of, the urine collection: avocados, bananas, chocolate, eggplants, figs, grapes, kiwifruit, nuts, pineapple, plums, tomato (or juices made from these products). List all medications (especially antihypertensives & antidepressants) on request form.

24 hr urine test patient information sheet

Your Doctor has asked you to perform a 24 hour urine: 24-Hour Urine Collection

Therapeutic Drug Monitoring

Measurement of therapeutic drug concentrations can be useful in patient management to tailor drug therapy to individual needs in order to optimise the beneficial effects and minimise toxicity and to assess compliance. However, individual drug pharmacokinetics may be influenced by many factors. For advice concerning interpretation or for information about drug assays not included in the table below, please contact the laboratory.


Drug
Drug	Sampling time	Therapeutic range
Carbamazepine	pre-dose	4-12 mg/L
Ciclosporin	pre-dose	differs according to indication
Digoxin	6-10hrs post-dose or trough, pre-dose	0.5-1.0 ug/L
Gentamicin	Trough, pre-dose
Lithium	12-24hrs Post-dose or trough, pre-dose	0.4-1.0 mmol/L
Phenobarbitone	pre-dose	10-40 mg/L
Phenytoin	predose peak useful if toxicity suspected 3-9 hours after oral dose, 2-4 hours after iv dose	10-20 mg/L
Sirolimus	pre-dose
Tacrolimus	pre-dose
Theophylline	oral: predose or 4-6 hours post slow-release preparation iv: pre-treatment if already on oral treatment, then at 6 & 18 hrs	10-20mg/L
Valproate	Pre-dose	No therapeutic range.
Vancomycin	Pre-dose

Clinical Biochemistry Investigations

Testing for diabetes mellitus

For the diagnosis of diabetes, either glucose or HbA1c should be measured. The diagnosis of diabetes in a symptomatic patient is made on the basis of one of the following:

A fasting plasma glucose concentration ≥ 7mmol/L
A random glucose concentration > 11.1 mmol/L
A HbA1C result of ≥48 mmol/mol

The glucose sample must be collected in a fluoride oxalate tube as glucose measurements on serum samples are unstable.

If asymptomatic, two elevated concentrations are required to make the diagnosis (for HbA1c the repeat test should be collected two weeks after the first raised measurement).

Please note that HbA1c is not appropriate for the diagnosis of diabetes in the following populations (information taken from Diabetes UK):

All children and young people
Patients of any age suspected of having Type 1 diabetes
Patients with symptoms of diabetes for less than 2 months
Patients at high risk who are acutely ill (e.g. those requiring hospital admission)
Patients taking medication that may cause rapid glucose rise e.g. steroids, antipsychotics
Patients with acute pancreatic damage, including pancreatic surgery
In pregnancy
Presence of genetic, haematologic and illness-related factors that influence HbA1c and its measurement (see reference: Use of Glycated Haemoglobin (HbA1c) in the diagnosis of Diabetes Mellitus WHO 2011)

Please refer to the NICE Guidelines for the management of type 1 and type 2 diabetes if you require information on the frequency of monitoring.

Short synacthen test for suspected adrenal failure

Indication

This is performed for the investigation of adrenal insufficiency.

Principle

Cortisol secretion is stimulated by ACTH from the anterior pituitary. This test evaluates the ability of the adrenal glands to produce cortisol in response to stimulation by a synthetic ACTH preparation (Synacthen).

Side effects

Hypersensitivity to Synacthen has been reported

Preparation

The test should ideally be performed in the morning.

Procedure

Take a basal serum blood sample for measurement of cortisol (gold-topped tube). Inject Synacthen 250 µg iv or im (children 36 µg/kg body weight). Take blood for cortisol after 30 and 60 minutes

Interpretation

There are different protocols and cut-offs in place around the UK due to variation in assay platforms used. A cut-off of >440nmol/L is used in the Derriford protocol, and a rise in cortisol above this concentration will usually exclude adrenal insufficiency.

Aldosterone sampling

Indication

The diagnosis of primary hyperaldosteronism (frequently due to adrenal aldosterone-producing adenomas) should be considered in hypertensive patients with spontaneous or diuretic-induced hypokalaemia. However, a number of patients with aldosterone secreting adenomas are normokalaemic.

Contraindications

None

Preparation

The renin-aldosterone axis is primarily regulated by renal blood flow. Patients under investigation should therefore be normally hydrated, have an adequate sodium intake and be normokalaemic. Any potassium replacement should be stopped on the day of the test.

There are a number of medications that can falsely alter the aldosterone:renin ratio. Therefore, if safe to do so, all drugs should be discontinued for at least two weeks prior to sample collection. See the table below for minimum length of withdrawal for specific medications. The β-blockers prazosin or doxazosin have minimal effect on the renin-aldosterone system and therefore can be used at the time of sampling. If there are medications that cannot be withdrawn then a list of all such medications must be provided with the request to aid interpretation of results.


Drug
Drug	Minimum length of withdrawal
• spironolactone	6 weeks
• oestrogen preparations	6 weeks
• ACE inhibitors	2 weeks
• angiotensin II receptor blockers	2 weeks
• direct renin inhibitors	2 weeks
• calcium channel blockers	2 weeks
• β-blockers (for exclusions see above)	2 weeks
• diuretics	2 weeks
• NSAIDs	2 weeks

Sample Requirements

A plasma EDTA sample (lavender top) for renin and a serum SST (gold top) for aldosterone and potassium are required for analysis. Samples must be taken to the laboratory immediately to allow the sample to be separated promptly, hence samples cannot be collected in Primary Care. If the test is required by a GP, then the samples can be collected in the venepuncture suite in Derriford (please contact the Duty Biochemist for more information).

Procedure

The patient should remain seated for five minutes prior to venepuncture.

Interpretation

High aldosterone and suppressed renin activity indicate primary hyperaldosteronism. Similar results may sometimes be seen in renal disease. In the case of equivocal results, repeat after withdrawal of all interfering medications (as listed above) may be indicated.

Clinical Biochemistry Guidelines

Thyroid function testing

Thyroid disease is associated with significant morbidity. It can present with non-specific symptoms hence thyroid function tests (TFTs) are frequently requested. As the results of TFTs can themselves by affected by intercurrent illness it is important that they are interpreted correctly and thyroid pathology is not assumed and treatment commenced inappropriately.

What should I do about a marginally raised TSH?

If a marginally raised TSH (4.9 –10 mIU/L) is found on a screening blood test and the fT4 is normal, measurement should be repeated in 3-6 months after excluding non-thyroidal illness and drug interference.

If TSH remains marginally elevated the patient’s thyroid autoantibody status should be determined. If thyroid autoantibodies are negative TFTs should be repeated every three years, unless symptoms develop (or the patient is pregnant/contemplating pregnancy). If thyroid auto-antibodies are positive TFTs should be repeated annually.

There is no evidence to support the benefit of routine early treatment with thyroxine in non-pregnant patients with a serum TSH between 4.9-10 mIU/L and a normal FT4, but physicians may wish to consider the suitability of a therapeutic trial of thyroxine on an individual patient basis and in patients who are pregnant/seeking fertility, children and those with a goitre.

How often should I check TFTs in a patient on thyroxine replacement for primary hypothyroidism?

In stable patients annual measurement is sufficient. After a change in dose two months should generally elapse before repeat measurement of TFTs to allow TSH to stabilise.

How do I know from TFT results that the dose of thyroxine is appropriate?

In uncomplicated primary hypothyroidism the biochemical target is a TSH within the reference range, however clinical and biochemical findings need to be taken into consideration.

Should I measure TFTs routinely in patients with diabetes mellitus?

Patients with Type 1 should have a check included in their annual review. Patients with Type 2 should have TFTs checked at diagnosis but routine annual testing is not necessary.

What do I do about a marginally low TSH?

Causes include non-thyroidal illness, medication and subclinical hyperthyroidism. Suggest repeat in 1-2 months, after excluding non-thyroidal illness and drug effects. With subclinical hyperthyroidism, AF is a strong indicator to treat the condition. In the presence of sinus rhythm, individuals with persistent TSH <0.1 mIU/L with symptoms of hyperthyroidism, goitre or osteoporosis should also be considered for referral to one of the endocrinologists and annual follow-up is certainly appropriate to ensure overt hyperthyroidism does not develop.

Is it necessary to check TFTs in patients presenting with hypertension?

This is no longer considered necessary.

How do I monitor TFTs in patients on amiodarone?

TFTs before commencing amiodarone and every 6 months during and for a year after treatment.

How do I monitor TFTs in patient on lithium?

TFTs before commencing and every 6 months whilst on treatment.

Familial Hypercholesterolemia

Patients who have cholesterol and LDL-C concentrations as per Simon Broome criteria (see table below) need to have further assessment and be referred to specialist lipid clinic if they have:

*Premature CHD younger than 60 years.

*Family history of raised total cholesterol: as in the table below.

*Lipid stigmata mainly Tendon Xanthomata or juvenile corneal arcus.