the present invention is a method of detecting reaction in blood caused by the presence of a foreign substance in the blood , comprising the steps of : establishing a potential across a predetermined spatial volume ; passing a first portion of the blood through the predetermined spatial volume ; substantially continuously measuring the potential across the predetermined spatial volume over a first predetermined period of time ; comparing the measured potential with a baseline ; and calculating the total volume of solids in the first portion of the blood as a function of a total absolute deviation of the measured potential from the baseline . the same procedure is then followed with a second portion of the blood , after it has been exposed to the substance whose reaction is being determined . the two calculations are then compared , with a positive reaction being indicated when the two measured solid volumes are measurably different . the baselines are preferably dynamic baselines , and are determined with reference to the starting point of a sharp rise in the measured potential .