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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.

Pre analytical procedures starting from proper collection, preparation, and preservation of the samples are critical for accurate chemical analyses of biological fluids, including blood. Results are of good quality when specimen collection, handling, preservation, and initial preparation or processing collection of specimens is done correctly. Whole blood is processed to obtain plasma or serum, which is used in maximum chemical determinations.

For chemistry analyses blood can be collected in a capillary tube or microcontainer by touching the tip to a large drop of blood while the tube is held in slanting position. Several tubes can be filled from a single puncture, centrifuged, and transported to the 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.

Serum or plasma obtained from venous blood collection is generally used for most chemical determinations. Blood gas determinations use arterial blood. Plasma is derived from blood, and it contains fibrinogen and serum is obtained after clotting so it is devoid of fibrinogen. Blood is collected in a plain tube or a serum-separator gel tube, without anticoagulant and allowed to clot for 30 minutes at room temperature to derive serum for chemistry tests. Serum is separated by centrifugation of the sample. Blood clots are prevented from adhering to the walls of test tube by a silicone coating which also minimizes hemolysis.

An inert silicone gel is used for separating serum from cells in serum separator tubes. A barrier is formed between cells and serum and the displacement of gel up inside the tube during centrifugation. Analysis is carried out on aspirated serum or serum is transferred to a different tube for analysis. Aerosol formation is minimized by SST tubes which minimizes the possibility of biohazardous transmission and saves time.

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.

A simple and reliable method of retarding bacterial action and inhibiting enzymes is the preservation of the specimens until further tests can be done refrigeration at 4 degree centigrade. Chemical analyses require that refrigerated specimens should be brought to room temperature.

Changes in anlayte or the substance being measured are also brought about by removing cells from plasma or serum as soon as possible. Bilirubin assay is light sensitive and so the specimens for bilirubin assay must be tested immediately or protected from light till tested.

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