Blood Tests: The Differences Between Whole Blood, Serum, and Plasma

     If there is no anti-clotting agent in a tube used to collect blood, the blood will clot (due to the presence of fibrinogen in the blood). Upon centrifugation of the tube, the clear yellow liquid at the top is called serum and contains little or no fibrinogen because the fibrinogen has been used up in the clotting of the blood cells. A sample of the serum can then be removed for analysis of its serum alcohol concentration.

     If there is an anti-clotting agent in the tube, the blood should not clot if properly mixed (tube inverted a few times). The tube can then be either shaken and a sample of the whole blood can be then be removed for analysis of its blood alcohol concentration, or the tube can be centrifuged. Upon centrifugation of the tube, the clear yellow liquid at the top is call plasma and contains about 0.34 grams of fibrinogen per 100 mL of plasma. A sample of the plasma can then be removed for analysis of its plasma alcohol concentration.

     Because serum and plasma only differ by the absence or the presence of a trace amount of the fibrinogen protein, the serum alcohol concentration and the plasma alcohol concentration should be essentially identical. However, due to their higher water content than whole blood, the alcohol concentration in either serum or plasma should be about 18% greater on average than the actual whole blood alcohol concentration.No forensic testing laboratory would try to measure the alcohol concentration in a blood clot which, because of its relatively low water content, would be relatively low in alcohol concentration compared to the whole blood alcohol concentration.

Written by Allen Trapp who is board certified by the National College for DUI Defense and the author of Georgia DUI Survival Guide Visit Website Written by Allen Trapp who is board certified by the National College for DUI Defense and the author of Georgia DUI Survival Guide Visit Website

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