Coagulation is detected in a fluid sample contained in either a gas flow cartridge or a plunger sensor cartridge, either one of which may be inserted in an actuator mechanism which has the capability of acting on either type of cartridge. Coagulation is detected in the gas flow cartridge by optically sensing the accumulation of coagulating liquid in a foam member at the top of the reaction chamber. In the plunger sensor cartridge, a plunger assembly is raised and released to descend through the sample of liquid in the bottom of the reaction chamber. The plunger assembly descends more slowly upon an increase in the viscosity of the fluid sample due to coagulation. Both cartridges include a lower reagent chamber, an upper reaction chamber, and means initially sealing and thereafter communicating the contents from the reagent chamber to the reaction chamber. The actuator mechanism includes means for delivering the flow of gas to the gas flow cartridge and means for reciprocating the plunger assembly in the plunger sensor cartridge.