For accurate determination of the coagulation time Addis[7] has defined the following conditions as essential:
1. The blood must always be obtained under the same conditions.
2. Estimates must all be made at the same temperature.
3. The blood must always come in contact with the same amount and kind of foreign material.
4. The end point must be clear and definite and must always indicate the same degree of coagulation.
The precautions taken to fulfill these conditions were as follows:
1. Drawing the blood.—The blood was taken from the femoral artery. The artery (usually the right) was laid bare in the groin and freed from surrounding tissue. A narrow artery clip, with each limb enclosed in soft rubber tubing (to prevent injury of the tissues), and with its spring exerting gentle pressure, was placed on the artery immediately below the deep femoral branch, thus allowing no blood to stagnate above the clip. Between the clip and a ligature applied about 1.5 centimeters below, an opening was made. The blood was carefully milked out of the vessels between a blunt dissector moved beneath, and a small forceps, twisted into a pinch of absorbent cotton, moved above.
The cannula, cleaned in water, alcohol, and ether, was set in the rubber connection of the glass tube; the point of the cannula was then lubricated with vaseline and slipped into the artery. The pressure of the clip on the artery was next very slightly released and blood was allowed to flow into the cannula up to the lower border of the rubber connection. Only a good-sized drop of blood was needed. Sometimes the blood ran one or two millimeters above or below, but without appreciably changing the result. Since the clip was situated on the femoral immediately below a branch in which the circulation persisted, the blood received in the cannula was always fresh from the moving stream. As soon as the clip gripped the artery again, the cannula was slipped out. A helper then promptly milked the vessel in the manner described above, and covered it with a pad of absorbent cotton smeared with vaseline to prevent drying. Thereby blood was not permitted to stagnate; and when a new sample was to be taken, the vessel was clean and ready for use.
The tip of the cannula was at once plugged by plunging it into a flat mound of plasticine about three millimeters high. It was drawn off sidewise lest the plasticine plug be pulled out again. One of the copper wires D was now slid into the tube and cannula, the tube slipped into the U-support, and the wire lifted and hung on the lever. This procedure, from the moment blood began to flow until the wire was hung, consumed usually about twenty seconds.
2. Uniform temperature.—Under the U-support was placed a large water bath, in which the cannula and the tapering part of the tube were submerged. A thermometer was fixed to the U-support so that the bulb came near the cannula in the bath. The water was kept within a degree of 25° C. This temperature was chosen for several reasons: (a) The cannula has room temperature and rapidly cools the small volume of blood that enters it. To heat blood and cannula to body temperature would take time. A bath near room temperature, therefore, seems preferable to one near body temperature. (b) The test of clotting was conveniently made at intervals of a half-minute, and if the clotting process were hastened by higher temperatures, this interval would become relatively less exact. (c) A temperature of 25° C. rather than lower was selected because, as Dale and Laidlaw[8] have shown, the coagulation time is much slower for a given change in temperature below 25° than for the same change above. And with slowing of the process the end point, when the determination depends on supporting a weight, is less likely to be sharp. (d) The researches undertaken with use of this coagulometer were concerned with factors hastening the process. For that reason and for reason (b), a long rather than a short coagulation time for normal conditions was desirable.