Heparin and the Agglutination of Platelets in Vitro.

Heparin inhibits both the coagulation of the blood and the agglutination of platelets. A much higher concentration, however, is required for the latter. Solandt and Best 1 have shown that over 300 mg per kg of body weight must be injected intravenously to prevent the intravascular clumping of platelets. The object of the present study was to determine the minimum amount of heparin that is required for the prevention of platelet agglutination in vitro.

Blood was drawn into a dry sterile syringe, and immediately distributed in 1 cc amounts to a series of small test tubes containing 0.1 cc of heparin solution (Roche Organon Liquaemin∗) of varying concentrations as recorded in the table. After thorough mixing, the blood was drawn to the 0.5 mark in a red blood cell pipette and the pipette filled with 0.85% sodium chloride solution. The platelet count was made in the usual manner.

A concentration of more than 0.1 mg (or 10 Toronto units) per cc of human blood is required to prevent clumping of platelets. No decrease in the platelet count by heparin as reported by Copley and Robb 2 was observed. Why so much more heparin is required for the prevention of platelet agglutination than for inhibiting coagulation is not clear. Nevertheless, it is well recognized that a definite relationship exists between these two processes. An additional observation in support of this may be cited. The platelets of blood obtained from animals whose prothrombin has been drastically reduced by feeding dicumarol show no tendency to agglutinate in the absence of an anticoagulant. A typical experiment was as follows: Dicumarol was given to a rabbit daily until the prothrombin time was 7 minutes.