| Plasma is the natural environment of blood cells but most chemical measurements are done in serum
|
The formed elements of blood are suspended in an aqueous solution that is termed plasma. Plasma is the supernatant obtained by centrifuging a blood sample that has been treated with an anticoagulant to prevent clotting of red cells. Serum is the supernatant obtained if a blood sample is allowed to clot (usually requires 30-45 minutes) and then centrifuged. In laboratory practice, the most common anticoagulants are lithium heparinate and ethylenediamine tetraacetic acid (EDTA). Heparinate prevents clotting by binding to thrombin. EDTA and citrate bind Ca++ and Mg++ thus interfering with the action of calcium/magnesium-dependent enzymes (prothrombin-converting complex, thromboplastin and thrombin) involved in the clotting cascade. When blood is collected for transfusion, citrate is used as an anticoagulant, as this preserves procoagulants and its effects are readily reversible by calcium. During clotting, fibrinogen is converted to fibrin as a result of proteolytic cleavage by thrombin, and so a major difference between plasma and serum is the absence of fibrinogen in serum.
|
|
