A plasma and blood coagulation time detector is described. The instrument utilizes a conventional glass probe thermistor mounted in a vibratory mixer as the detection element. Both the initiation and the end point of the coagulation process are registered on a strip chart recorder. A precision of 2 to 4% in end point time has been obtained in both the normal and therapeutic prothrombin time ranges. The end point as determined by this method has a correlation coefficient of 0.98 at the 99.5% confidence interval with the Fibrometer method.