A Modification for Routine Laboratory Use of Stefanini's Method of Estimating Factor V Activity in Human Oxalated Plasma

Quick's one-stage prothrombin test is done in this laboratorv using 0.2-mI. quantities as described by Owren (1947b). Human brain extract instead of rabbit brain is used as the source of thromboplastin. This powder is stored in a desiccator in a sealed tube and when required 0.32 g. is warmed with 0.6 ml. of 0.850," saline for 15 minutes at 50C. The temperature is not allowed to rise above 50C.. as this would impair the activity of the resulting thromboplastin. The strength of the calcium chloride solution used is 0.02 /O. With the present batch of thrombop!astin the clotting occurs for normal controls between 10 and 11 seconds after adding the calcium chloride solution.