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Whole blood is used for number and condition of cells but not used for biochemical testing whereas serum or plasma is preferred for testing. Biochemical tests are used as a screening tool for a large number of chemical constituents for diagnosis in patients where symptoms cannot be elucidated.

Proper collection, preservation, and preparation of sample is the key to accurate chemical analyses of biological fluids. Quality assurance of the results is assured by proper specimen collection, handling, preservation, and initial preparation or processing. Majority of chemical tests are done on whole blood obtained by venipuncture from which plasma or serum is separated.

Collection of blood is done by touching the tip to a large drop of blood while holding the capillary tube or microcontainer in a slanting position. Samples can be collected from a single puncture by using multiples tubes and then processed by centrifugation and sent to the testing laboratory.

All laboratory measurements should be carried out within 1hr of collection or the specimens are processed so that they can be stored and constituents to be measured are not altered. Processing the specimens includes separation of cells from serum or plasma, observation of specimen color, refrigeration, and freezing if necessary.

Biochemical specimens require processing venous blood specimens for separation of serum or plasma using SST or PST tubes. Blood gas determination is done using arterial blood. The liquid portion of blood with fibrinogen is called plasma while serum is obtained after clotting of blood and does not contain fibrinogen. A plain or SST tube is used for collection of blood for serum chemical analysis, where blood is allowed to clot at room temperature for 30 minutes. Centrifugation is done for serum separation for further testing. Silicon coating is done in the vacuum tubes to prevent clots from being adhered to the walls of tube and to minimize hemolysis during collection.

Serum is separated from cells with the use of Serum separator tubes containing inert silicone gel. The gel forms a barrier between cells and the serum and is shifted up inside the tube during centrifugation. Serum is transferred to different tube or aspiration of serum is done for analysis. The possibility of biohazardous transmission is reduced and time is saved by the use of SST tubes.

Invalid or inaccurate results are obtained by blood drawn from patients on certain medications since these drugs interfere with chemical analysis. Metabolites of some drugs or drug itself can affect the concentration of the analyte being measured or some drugs can have direct action on chemical reactions.

All laboratory assays should be done as quickly as possible but when the specimens need to be preserved until further tests can be done, use of refrigeration at 4 degree centigrade is generally used. This is a simple and reliable means of retarding bacterial action, and inhibiting enzymes. Bringing back the samples back to room temperature from refrigeration state before chemical analyses is very important for accurate results.

Removing cells from plasma and serum is another means of preventing analyte changes. Specimens needed for bilirubin assay must be protected from light or tested immediately since bilirubin is light sensitive substance.

Written by Phlebotomy Training specialist Dr Shahbaz A. Cheema, Course Director for Maxis Healthcare who run NHS Accredited Phlebotomy Training courses for medical and non medical practitioners. Learn the 3 Steps To Become a Phlebotomist