Coagulase Test

The coagulase test identifies whether an organism produces the exoenzyme coagulase, which causes the fibrin of blood plasma to clot. Organisms that produce catalase can form protective barriers of fibrin around themselves, making themselves highly resistant to phagocytosis, other immune responses, and some other antimicrobial agents.

The coagulase slide test is used to identify the presence of bound coagulase or clumping factor, which is attached to the cell walls of the bacteria. Bound coagulase reacts with the fibrinogen in plasma, causing the fibrinogen to precipitate. This causes the cells to agglutinate, or clump together, which creates the “lumpy” look of a positive coagulase slide test.  You may need to place the slide over a light box to observe the clumping of cells in the plasma.

The coagulase tube test is used to identify the presence of either bound coagulase or free coagulase, which is an extracellular enzyme. Free coagulase reacts with a component of plasma called coagulase-reacting factor. The result is to cause the plasma to coagulate. 

The coagulase test is useful for differentiating potentially pathogenic Staphylococci such as Staphylococcus aureus from other Gram positive, catalase-positive cocci.

• The slide test should be read very quickly, as false positives can occur.

• The slide test should not be performed with organisms taken from high-salt media such as Mannitol Salt Agar, as the salt content can create false positives.

• The tube test is more reliable than the slide test.

 The tube of coagulase plasma on the left, which was inoculated with <i>Staphylococcus epidermidis</i>, is still liquid; it is coagulase negative. The tube of coagulase plasma on the right, which was inoculated with Staphylococcus aureus, is solid; it is coagulase positive.