Biochemistry Assays

Specialised Testing Procedures

The service is comprehensive and there are many protocols available for specialized testing procedures. These include:-

Collection of 24 hour urine.
Dexamethasone suppression test.
Glucose absorbance test (GAT)
Growth hormone suppression test.
Gut hormone collection.
Oral Glucose tolerance test (OGTT).
Short synacthen stimulation test,
and many others.

Please contact the laboratory on 01271 322345 (or internal ext. 2345) before starting any of these procedures and we will be pleased to help.

Our commitment is that this department will provide accurate and timely results, to be of help in the diagnosis and care of the patient. Grossly abnormal results will be telephoned back to the ward or doctor. Tests may be added to your request or those which contribute little to your management may be deleted. However discussion is encouraged, particularly in the fields of endocrinology or toxicology and visits to the laboratory are welcomed.

All assays, are rigorously quality controlled, and are subjected to regular external quality assessment.  Our performance in these schemes is open for inspection.

The Head of Department is available to discuss the future directions of the department.

Analysis turnaround time
Time Limits for Requesting Additional Examinations
Specimen collection
Results enquiry

Key Factors Affecting Result Interpretation
Requesting groups
 

Analysis Turnaround Time

Times stated are from receipt in the laboratory to production of report visible to users on the Pathology computer. Time from patient to laboratory is beyond our control but will have significant effect on the overall turnaround time.

In the event of unforeseen circumstances, e.g. analyser or I.T. failure, reporting may be delayed.

Most requests are processed on arrival at the laboratory and reported within 4 hours (daily assays) if received before 16:00h. A few assays are batched and run at least weekly or more frequently depending on batch size and staff availability.

Assays that are referred to regional or supra-regional laboratories take considerably longer. Please contact the laboratory in case of difficulty.

Daily assays:Urea / Electrolytes, Liver Function tests, CRP, Cardiac enzymes, CEA, CA125, Glucose, Lipids, Calcium, Phosphate, Magnesium, Amylase, Osmolality, Pregnancy test, Cortisol, FSH / LH, Prolactin, Testosterone, TSH, Free T4, Free T3, PSA, Oestradiol, Progesterone, PTH, Vitamin D, Immunoglobulins, Urate, Microalbumin, Lactate, Ammonia, Lithium, B12, Ferritin, Folate, NTProBNP, & High Sensitivity Troponin T.
Weekly (at least):Protein electrophoresis, immunofixation, SHBG & ATP.

The list is not exhaustive and does not preclude the running of URGENT assays following discussion with the Laboratory (the usual urgent samples are generally processed and reported within 60 minutes of receipt in the laboratory).

We regularly audit the turnaround times of requests which includes the time from receipt in the laboratory to the time results are made available. Average turn-around times can be generated for specific tests upon request to the laboratory.

Time Limits for Requesting Additional Examinations
Samples are stored in the laboratory for up to five days, during which time certain additional tests may be requested, if appropriate.

Primary care providers may request additional tests via a dedicated email account ndht.addontests@nhs.net  Requests are actioned Mon –Fri 9am – 5.30pm.

Click here to see specific details for biochemistry add-on tests.

Specimen collection
Test results are meaningless or – worse totally misleading unless obtained from valid specimens.  A valid specimen must be from the correct patient, in the correct container collected at the correct time and receive the correct handling.

Generally, we  perform up to 4 assays on one full tube.  Exceptions are glucose, alcohol or lactate which require a fluoride preservative, (grey top), ciclosporin, TPMT and lead which use EDTA (purple top). Certain rarer tests may require a lithium heparin specimen tube, available from the laboratory on request.  For a full list of assays and containers see:-

Minimum specimen identification
Every specimen or container must clearly identify the patient to whom it relates.  As a general policy, based upon patient safety, incorrectly or inadequately labeled specimens are rejected, see the SPECIMEN ACCEPTANCE POLICY. Exceptions to this policy for unrepeatable specimens may only be made by the Principal Biochemist or Senior Biomedical Scientist staff.

Special requirements
Many tests rely on the specimen being in a specific type of container, being handled in a specific way, being collected at a specific time or being delivered in a timely manner. Information for CSF sample requirements can be found here.

Key Factors Affecting Result Interpretation

EDTA contamination
Contamination of blood specimens with potassium EDTA can cause problems in Biochemistry. EDTA is the anticoagulant used in Haematology and chelates divalent cations Ca++ and Mg++ and inhibits divalent cation- dependent proteolytic enzymes.

What are the effects of EDTA contamination? Increased potassium – leading to an invalid potassium result;  decreased calcium, magnesium and alkaline phosphatase.

How to avoid contamination: Follow the order on the Vacuette Selection Chart – serum samples must be taken before citrate and EDTA samples. If there are doubts about a result, then a fresh sample should be obtained and checked urgently before any treatment is commenced.

I.V. Contamination
For hospitalised patients, a common source of misleading results is contamination of the specimen with i.v.  fluid.

What are the effects of i.v. contamination? High results (e.g. infusion of a concentrate); low results (dilution of the specimen); Unpredictable effects (e.g. drug interference with an analytical reaction).

Haemolysis and Old specimens
Haemolysis and delay in arrival at the laboratory releases cellular contents, notably potassium, lactate dehydrogenase and phosphate, into plasma.  Haemolysis may also cause interference with some analytical reactions.  In vitro haemolysis can be caused by excessive manual drawing on a syringe, use of a small bore needle, squirting through a needle from a syringe into a vacuum collection tube etc.

N.B. when using ward-based analysers even specimens taken from catheters may be haemolysed.  Haemolysis cannot be seen in whole blood. This is a particular caveat when using blood gas analysers to estimate electrolytes.

Storage of Samples
Every effort should be made for samples to be delivered to Biochemistry. If this is not possible then advice should be sought from the department. (Tel: 01271 322345)What are the effects?Some results will be adversely affected e.g. potassium. For full guidance on storing specimens when delivery to the laboratory has been delayed, click here.

 

Requesting groups
N.B. The contents of these groups are subject to change from time to time.

Urea + Electrolytes (U&E) (NDDH)Urea, Creatinine, Sodium, Potassium & Chloride.
Urea + Electrolytes (U&E) (GPs)Creatinine, Sodium & Potassium.
Bicarbonate IS NOT contained in this group and must be requested separately.
Liver Function Tests (LFT)Total Bilirubin, Alkaline Phosphatase, ALT, Total Protein, Albumin
Cardiac Enzymes (CE)AST, LDH, CK
N.B. in general cardiac enzymes are not raised until some hours (about 9) after the event.
Thyroid Function (TFT)TSH
N.B. depending upon the result of the TSH assay and the given Patient clinical summary we will automatically cascade to the assaying of Free T4, Free T3 and Anti-thyroid peroxidase. If no relevant clinical summary is given the cascade cannot operate efficiently.
BoneCalcium, Phosphate, Alkaline Phosphatase, Albumin
CalciumTotal Calcium, Albumin
Total calcium levels are affected by changes in plasma protein concentrations. Most of the protein bound fraction of calcium is bound to albumin. North Devon Pathology use the following formula:

Corrected Calcuim = {[40-serum albumin] x 0.02} + measured calcium

Lipid ScreenCholesterol, Triglyceride, HDL & non-HDL cholesterol
HormonesIf the requesting practitioner does not specify the hormones required, the Laboratory will assay those deemed appropriate to the clinical summary.

Results enquiry
All results, whether urgent or not, are available on the hospital computer as soon as they are authorised and printed.  This is a more reliable way to access results since it negates the possibility of a transcription error and is more rapid than by telephone since it does not require a call to a frequently engaged extension.  Please see the section of this manual dealing with the pathology computer to obtain access to results enquiry.

Reference ranges for common analytes are made available with the reported results on the laboratory computer and printed reports but for those assays which require interpretation, specific comments will be added to the results also.   Other comments may be coded as:

HAEM=Haemolysed sample. Potassium, phosphate, bilirubin and enzymes invalid.
OLD=Sample from preceding day. Potassium will be artifactually high especially if stored in ‘fridge.
LIPID=Grossly lipaemic serum seen on centrifugation.
Most assays show water displacement effects and spectrophotometric interference.
SR=Specific report carrying appropriate comments issued.
NOTE: If printed, information on this page is only valid on day of printing.

Last updated: June 4, 2018