Abstract: The peak value of the first derivative of the optical density time curve is related to individual clotting factor levels, deficient or elevated. The relationship is logarithmic for all factors except fibrinogen and it is linear for fibrinogen. The apparatus and process disclosed herein apply the relationship to determine blood clotting factor levels.

Abstract: A difference amplifier continuously generates a signal representing the difference between the optical densities of a platelet rich and a platelet poor sample of blood plasma. A variable gain amplifier multiplies the difference signal by a factor which is proportional to the initial difference signal. Electronic circuitry automatically determines the multiplying factor by comparing the initial difference signal to a predetermined magnitude. The multiplied difference signal is continuously recorded on a chart recorder and an optional filter circuit is provided for selectively filtering the difference signal.