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The ability to coagulate human or rabbit blood plasma is considered to be the single most reliable indication of pathogenicity among Staphylococcus strains. The presence of the enzyme coagulase binds plasma fibrinogen, causing the plasma to clot. Two methods are available for detecting coagulation: the slide test, which depends upon cell-bound coagulase (sometimes referred to as "clumping factor") and the tube test, which is brought about by free coagulase liberated from the bacterial cells into the surrounding medium. The two coagulases are antigenically distinct, but approximately 99% of S. aureus strains produce both forms. When properly controlled, the speed and accuracy of the slide test make it the method of choice: however, a negative reaction should be confirmed through the use of the tube method since occasional strains fail to produce the bound coagulase. Coagulase discs are made from rabbit plasma.
