The normal blood clotting time in the light of experience with the two-syringe technique.

The surgeon who requests the pathological laboratory to determine a patient's blood clotting time expects a result which will indicate if surgery might be dangerous. If he follows the teaching of Cummine and Lyons (1948) or of Dutton (1954) he may wish to use the test to anticipate thromboembolic episodes. The physician may request the test for various reasons. It is, accordingly, very important to know the upper limit of normal, which is stated by Wintrobe (1951) to be 15 minutes using the method of Lee and White (1913). Other textbooks give lower figures. Friedman, Schwartz, and Vincenti (1952), using a two-tube Lee-and-White technique, made the range of normal 7 to 25 minutes. They regarded the test as affording "little if any useful function in evaluating the clotting mechanism, especially in relation to the bleeding and clotting tendencies." The work of Allen and Attyah (1953) suggested a possible explanation of these inconsistencies. These workers claimed that, even when a vein was entered with minimal trauma, blood withdrawn into the first syringe clotted sooner than that withdrawn subsequently into a second syringe. Normal medical students were used in their investigations. With large-calibre short tubes (internal diameter 9.5 mm., height 40 mm.) and 10 ml. syringes with 20-gauge needles, they showed that the coagulation time of the blood in the second syringe was significantly longer than that in the first. Each of 10 tubes was filled from the bottom with 1 ml. using an 18-gauge needle, and the whole test was performed in a water-bath at 370 C. The end-point was taken at the time for the blood in the last tube to clot, each being completely inverted at intervals of five minutes. The tubes were examined in the order in which they had been filled. In 96% the clotting time was 2035 minutes; it was never under 15 or over 40 and the average was 26 minutes. Method of Investigation