Identification of staphylococci is based on microscopic examination, colonial morphology and cultural and biochemical characteristics. Staphylococci associated with acute infection (Staphylococcus aureus in humans and S. intermedius and S. hyicus in animals) can clot plasma. The most widely used and generally accepted criterion for identification of these pathogenic organisms is based on the presence of the enzyme coagulase.¹ The ability of Staphylococcus to produce coagulase was first reported by Loeb² in 1903.

Coagulase binds plasma fibrinogen, causing the organisms to agglutinate or plasma to clot. Two different forms of coagulase can be produced, free and bound. Free coagulase is an extracellular enzyme produced when the organism is cultured in broth. Bound coagulase, also known as clumping factor, remains attached to the cell wall of the organism. The tube test can detect the presence of both bound and free coagulase. Isolates that do not produce clumping factor must be tested for the ability to produce extracellular coagulase (free coagulase).

BBL Coagulase Plasma, Rabbit and BBL Coagulase Plasma Rabbit with EDTA are recommended for performing the direct tube test. The inoculum used for testing must be pure because a contaminant may produce false results after prolonged incubation. For the coagulase test, BBL Coagulase Plasma, Rabbit with EDTA is superior to citrated plasma because citrate-utilizing organisms such as Pseudomonas species, Serratia marcescens, Enterococcus faecalis and strains of Streptococcus will clot citrated plasma in 18 h.³