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Clinical Biochemistry

Introduction

 
The Clinical Biochemistry Laboratories provide services for the analysis of biochemical constituents in blood, urine, and other body fluids (e.g. cerebrospinal fluid, pleural fluid, ascitic fluid, etc.). With a repertoire of biochemical constituents analysed ranging from trace inorganic elements to large organic molecules, these measurements help clinicians in the diagnosis and management of a wide variety of diseases.

It is advisable to make prior arrangement with the laboratory concerning the nature of specimen, time of collection, type of container and necessary precautions before sending specimens for the more specialised tests. As a matter of policy, tests’ cancellations are only allowed if the laboratory has not performed the test at the time of notification.

Collection of Specimens for Biochemical Tests

Timing and Posture

In general, samples should be collected with the patient in the fasting state (e.g. before breakfast between 7am to 9am) and in a sitting position. This would avoid interference from variations arising from food intake, body position and stress. Turbidity caused by lipemia affects the accuracy of many laboratory analyses and should be avoided as much as possible. Fasting is important if estimations of triglycerides and phosphate are to be accurately interpreted.

The timing of specimen collection is important for biochemical constituents that undergo marked diurnal variation and for those used to monitor drug therapy.

Blood Collection

Venous blood collection is used for most tests while arterial blood is required for blood pH and blood gas measurements.

When collecting a venous blood specimen, avoid prolonged tourniquet application (it is recommended that the tourniquet be released soon after the needle enters the vein) as this may raise the level of protein and substances bound to it, e.g. cholesterol, calcium and drug. Trauma to surrounding tissues should be avoided as it may produce misleading increase in serum potassium level. Lysis should also be avoided as it would cause spuriously high levels of potassium, LDH, CK, AST, phosphate and spuriously low levels of insulin.

Do not use the limb into which an intravenous infusion is being administered for collecting blood specimens, as this would cause erroneously high results for the administered substances, and low results for other analytes due to dilution. Use the other limb.

Use the recommended anticoagulant for tests utilising whole blood or plasma. Fill the container up to the mark indicated so as to prevent excessive dilution of the biochemical constituents.

For analytes affected by strenuous physical activity or exercise such as creatine kinase (CK) and myoglobin, allow the patient to have sufficient rest before drawing blood.

For the assessment of acid-base status of patients, collect 3mL of arterial blood in a syringe containing heparin (preferably in a dried form). Care must be taken to ensure that there is no air space in the syringe. Any air bubbles present in the syringe must be immediately expelled, the needle removed and the syringe sealed or tightly capped to avoid exposing the blood to air. The syringe should be gently rotated between the palms of the hands to ensure heparin and blood are mixed. The sample should be clearly labelled and immersed in crushed ice and despatched immediately to the laboratory.

When multiple blood samples are required, to minimise the potential for pre-analytical error due to cross-contamination of tube additives, the recommended order of draw (CLSI document H3-A6 “Procedure for the collection of diagnostic blood specimens by venipuncture”, 6th Edition 2007) is as follows:

  1. sterile samples (eg blood cultures)
  2. citrate tubes (tri-sodium citrate) / ACD tubes (tri-sodium citrate, citric acid, dextrose)
  3. serum tubes (clot activator, gel polymer, plain)
  4. plasma tubes (lithium heparin)
  5. EDTA tubes (potassium EDTA)
  6. Fluoride oxalate tube (sodium fluoride/potassium oxalate) 

Delivery and Storage of Samples

Samples should be kept cold during transport and delivered to the laboratories as soon as possible (within two hours). When this is not possible, blood samples for tests utilising serum or plasma should be centrifuged and separated before storage either in the refrigerator or freezer. If a centrifuge is not available, serum may be collected once the clot has retracted, by means of a Pasteur pipette into a plain container. Samples for tests utilising whole blood should be kept in a refrigerator outside the freezing compartment. Some tests require additional precautions such as protection from light. Details of specific storage and precautions are given with the relevant tests.

Collection of Urine

For most biochemical estimations, urine needs to be collected over a 24-hour period. Timed collections are required for certain tests and details are given with the relevant tests.

It is very important that timed urine collections (24 hour and shorter periods) should be collected with great care. Clear instructions to the patient are essential regarding emptying of the bladder at the start of the collection period (discarding this urine) and at the end of the collection period (keeping this urine). Patients should be instructed to save the urine samples at the time of defecation and while taking a bath. These instructions are particularly important for short time collection, where large errors may arise if a sample of urine is discarded or retained.

Urine samples should be sent to the laboratory as soon as the collection has ended. The whole bottle of urine or an aliquot may be sent. If an aliquot is to be sent, the contents of the bottle should be mixed thoroughly, its volume measured and written on the request form. An aliquot of approximately 20 - 50 mL may be sent in a smaller container. If it is necessary to keep urine collections in the ward for more than 24 hours, the urine should be kept refrigerated.

It is important to add the correct preservatives to 24-hour urine collections. Preservatives added to urine collection containers are toxic/corrosive as indicated by the labels. Patients should be instructed not to discard the preservatives but to handle the urine collection containers and their contents with care, and to keep them out of reach of children.

Collection of Cerebrospinal Fluid (CSF) and Other Fluids

CSF should be collected into a plain sterile container and sent to the laboratory immediately. Similarly, other fluids, e.g. ascitic, peritoneal etc. should also be sent to the laboratory as soon as they are taken. If they cannot be sent immediately, they should be stored in a refrigerator.

Anticoagulants and Perservatives

For most of the tests, serum is required and blood need to be collected in a plain tube or preferably a tube containing a clot activator and a gel. For other tests, whole blood or plasma is required and blood should be collected in specific anticoagulants. Please refer to the test lists for the particular anticoagulant needed as collecting specimens in the wrong anticoagulant will invalidate test results.

Various urine tests require different preservatives to prevent change in analyte concentration. To obtain accurate results, please adhere to the recommended preservatives given in the test list.

Important Precautions to Ensure Quality of Test and Prevent Sample Rejection

Some of the common mistakes that result in sample rejection are listed as follows.

Inappropriate sites for venepuncture:

  • Arm on side of mastectomy
  • Edematous areas
  • Haematomas
  • Arm in which blood is being transfused or IV fluid being administered
  • Scarred areas
  • Arms with fistulas or vascular grafts
  • Sites above an IV canula

Ensure complete drying of alcohol swabbed skin before inserting needle for blood collection. Otherwise, lysis of red cells by alcohol would cause misleading test results. At the extreme, badly haemolysed samples are not suitable for analysis.

Order of draw for multiple blood samples is important. Contamination with anticoagulant can interfere with many analyses.

After blood collection, gently invert tubes 5 times to ensure proper mixing of blood with clot activator or anticoagulant.

Ensure correct blood collection tubes for the particular test – e.g. clotted samples will be rejected for HbA1c, G6PDH and Cyclosporine requests.

Finally, proper labelling is expected – unlabelled samples, missing requesting doctor’s signature, mismatch of patient’s identity on sample tube and request form, or nature of sample (e.g urine, fluid) will result in sample rejection.

Reference Intervals

Ninety-five percent of healthy individuals have results within the Reference Interval. System Internationale (SI) units and the 2.5th to 97.5th percentile reference intervals are used whenever possible. If on any occasion there is a need for conversion to conventional units, please consult the laboratory. For reference intervals to be included in the patient’s test report, age and sex of the patient must be provided in the request form. The reference interval is age and sex dependent and is appropriate for the registered patient only.

Whenever possible the values given are for the local population, otherwise published values for the method used (after validation) are given. When there are no differences for test values obtained between the sexes, only one set of reference interval is given. Otherwise, age and/or sex dependent reference intervals are given. For some tests, only adult reference intervals are available.

For non-serum/plasma specimens eg. pleural, ascetic, dialysis, knee or cell culture fluids, reference intervals may not be available and it will be on the onus of the requestor to interpret the findings. Similarly, reference intervals are not available for specimens from animals.

Call-Back Values of Critical Analytes

Critical biochemical analytes and their call-back values are listed below. Results are informed by telephone to the requestor and recorded in the test report. Click here to view critical test results.

Click here to view all Clinical Biochemistry tests