Rabbit plasma is the fluid part of rabbit blood once the blood cells have been removed. It is used as a reagent for determining whether a microbe produces the enzyme coagulase, which has the ability to denature and crosslink proteins in the plasma to produce an insoluble clot.
Coagulase tests are performed using a slide agglutination test. Fresh colonies of the microbe being tested are mixed with two drops of distilled water placed at opposite ends of a microscope slide. To one of these cell suspensions, a drop of rabbit plasma is added and mixed. The other serves as the control. After several minutes, the cell suspensions are observed for the presence of cell agglutination (clumping), evidence of the presence of the enzyme coagulase.