The coagulation time of blood as observed in a glass test tube is subject to variables which have to do with the degree of its exposure to thromboplastic influences (tissue juices in the needle, contaminants due to unclean glassware, or platelet thromboplastin from fragmented platelets due to agitation of the blood). These influences can be largely eliminated by standardization of the test in the hands of one person who has sufficient time to perform it properly. Even then, however, different laboratories, owing to minor variations in technique, have reported different normal ranges of coagulation time, and the same laboratory may undergo changes in normal ranges from year to year. In most routine hospital laboratories, furthermore, the devotion to the procedure of one especially trained worker takes valuable time from some other essential test. The temptation to perform the coagulation times hastily, and thus inaccurately, therefore remains great.
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JOHN S. BELKO, RICHARD WARREN. The Recalcification Time of BloodIts Use as a Measure of the Clinical Effect of Heparin. AMA Arch Surg. 1958;76(2):210–218. doi:10.1001/archsurg.1958.01280200032004