The coagulase test is a diagnostic method used to detect the enzyme coagulase produced by certain bacteria by observing plasma for clot formation or agglutination.
Explanation
Coagulase converts fibrinogen to fibrin, so adding bacterial cells to plasma can reveal whether the enzyme is present. Two forms of the test are commonly performed. The slide test detects bound coagulase, or clumping factor. A drop of plasma is mixed with a bacterial colony on a microscope slide; if the cells possess bound coagulase they immediately agglutinate and form visible clumps. Some strains autoagglutinate, so a saline control is included and negative slide tests are confirmed with a tube test. The tube test detects free coagulase. A small volume of bacterial culture is incubated with plasma at 37 °C. During incubation, free coagulase reacts with a coagulase-reacting factor in the plasma to form staphylothrombin, which converts fibrinogen to fibrin. The formation of a clot that remains in place when the tube is gently tilted indicates a positive reaction. The test usually takes a few hours and tubes are often checked at intervals and again after overnight incubation. Rabbit plasma is commonly used as it is sensitive.
Applications and notes
The coagulase test is primarily used to distinguish coagulase-positive Staphylococcus aureus from coagulase-negative species such as S. epidermidis. It may also detect coagulase produced by Yersinia pestis and a few other organisms. Accurate interpretation requires proper controls, fresh plasma and careful observation to avoid false results. Positive coagulase tests indicate that the isolate may be more virulent and should prompt further identification and susceptibility testing.
This simple assay remains a cornerstone of clinical microbiology laboratories for rapid identification of pathogenic staphylococci.
Related Terms: coagulase, Staphylococcus aureus, slide test, tube test, diagnostic test