Baker, Robert W.

The skin bleeding time (BT)(Mielke, Ivy, Duke) is an important part of the diagnostic workup of bleeding disorders. Because of variable vascularity of the skin from one location to another, results may vary widely with the site and type of incision and the thickness of the skin. In some patients bleeding is difficult to stop. The discomfort of the test and the possibility of inducing scarring limit the frequency with which it can be repeated. A standardized BT that could be performed on the patient’s blood in the laboratory could avoid these problems. We have developed such a test, basing it on our previously described method in which a hole is made in a Teflon arteriovenous shunt in rats, and modified from the method of Blakely et al. A cylindrical hole of 75 μ diameter is made in a 50 cm length of 457 μ I.D. Teflon tubing with the aid of a needle, micromanipulator and dissecting microscope. Blood (2 U heparin/ml) from normal human subjects displaces buffer from the tubing at a rate (.6 ml/min) determined by a syringe pump. Pressure is controlled by raising the distal end of the tubing 5 cm above the hole. BT is recorded by an electronic drop rate counter (Payton BTR 400). Under these conditions, BT is 150 ± 47 sec, and bleeding rate from the hole, shear rate and shear stress on the wall of the hole are .11 ± .02 ml/ min, 40,700 ± 8,100 sec-1 and 1,830 ± 360 dynes/cm2 respectively. The BT can be varied from 0 to >15 min by lowering or raising pressure or flow in the system or by varying hole diameter. By phase microscopy the hole is seen to be occluded by a mass of aggregated platelets and occasional PMN. These observations show that hemostasis at small incisions is controlled by rheologic factors and platelets and does not require physical or chemical properties of a blood vessel. This new method may be useful in the diagnosis of bleeding disorders and in studying the effects of rheologic factors and drugs in hemostasis.