The coagulase test
The single most important test used to distinguish S.aureus from other staphylococci demonstrates production of coagulase. S.aureus produces two forms of coagulase. Bound coagulase, otherwise known as "clumping factor", can be detected by carrying out a slide coagulase test, and free coagulase can be detected using a tube coagulase test.
The slide coagulase test is traditionally carried out with a drop of plasma (rabbit plasma anticoagulated with EDTA is recommended) but often are used commercial latex slide agglutination tests (in the picture drySPOT™, Oxoid). Blue latex particles are coated with porcine fibrinogen, rabbit IgG and polyclonal antibodies raised against capsular polysaccharide of S.aureus. The test is able to detect the presence of bound coagulase, Protein A (a protein found on the cell surface of about 95% of human strains of S.aureus; it has the ability to bind the Fc portion of immunoglobulin G (IgG) and capsular polysaccharide (present in certain methicillin-resistant strains of S.aureus). The capsule can mask both Protein A and the clumping factor thereby preventing agglutination.
Approximately 97% of human strains of S.aureus possess both bound coagulase and extracellular staphylocoagulase.
S.hyicus, S.intermedius, S.lugdunensis, S.xylosus, S.schleiferi and S.haemolyticus may give positive results in coagulase tests (or rapid latex procedures). S.aureus and S.hyicus are PYRase negative. All the other strains named above will be positive. S.hyicus and S.intermedius are rarely encountered in the clinical laboratory.