Blood: Most Common Body Fluid Specimen Analyze in Clinical Laboratory

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The body fluid most commonly analyzed in the clinical laboratory is blood. Whole blood can be allowed to clot in a clean plastic collection tube and then be centrifuged and the serum removed for testing in clinical chemistry. Conversely, clotting can be prevented by allowing the whole blood to mix with an anticoagulant such as lithium heparin so that it can be measured as whole blood or centrifuged to obtain the plasma for analysis. Other types of testing performed in the clinical laboratory, such as the complete blood count, require anticoagulated, well-mixed whole blood.

Serum specimens obtained from venous circulation are most commonly used in clinical chemistry. Therefore, proper collection technique or venipunture is needed in order to obtain an acceptable specimen, which minimizes preanalytical errors. Standards of practice in blood collection are found in other resources as those published by the Clinical Laboratory Standards Institute (CLSI; formerly the National Committee for Clinical Laboratory Standards [NCCLS]). A tourniquet typically is used in order to cause short-term venous occlusion, allowing palpationof the veins in the antecubital fossaprior to venipuncture. If the tourniquet is maintained longer than 1 minute, a relative hemoconcentrationoccurs due to fluid changes. Low molecular weight compounds such as potassium leave the capillaries and pool in the interstitial fluid region during hemoconcentration, while proteins tend to be increased in the remaining plasma. Clenching of the fist prior to venipuncture is a common technique. However, the technique should be avoided prior to collection of specimens for clinical laboratory analysis due to the changes that can be initiated. Fist clenching tends to increase lactate, phosphate, and potassium as well as lower pH. Secondarily, ionized calcium increases due to the transient lactic acidosis.