Calcium Factor in Quantitative Determination of Prothrombin

The method developed by the author for the determination of prothrombin has on account of its simplicity won wide clinical interest. Its reliability as an index of the bleeding tendency in jaundice has been repeatedly confirmed. Nevertheless, any test based on the use of biological materials presents well recognized difficulties. In the method as originally described, 1 the thromboplastin was a source of trouble because of its variability in potency, but after the writer 2 had perfected a method for making a preparation which was stable and had a high and uniform potency, satisfactory results have been obtained. It was, therefore, surprising to learn that Stewart and Pohle 3 using the method in its present form on normal bloods found: “that the results were extremely variable.” They ascribe their difficulties to the concentration of the calcium chloride solution. In the test a 0.025 M solution is used. Assuming that blood contains 10 mg of calcium, a 0.0075 M solution would be equivalent to the concentration of sodium oxalate present in the plasma. One can see that a definite excess of calcium is employed. It was recognized by the author 4 that calcium beyond a certain concentration has an inhibitory action on the coagulation time. The reason a 0.025 M solution was chosen was as follows: The prothrombin test is an outgrowth of the method originally developed at the Fifth Avenue Hospital for the determination of the clotting time of recalcified plasma and in this procedure a 0.025 M solution was found most satisfactory. Since this determination also has clinical value it was considered wise for the sake of simplicity to retain the same calcium chloride solution for both methods.