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.