Coagulase

Coagulase is a protein enzyme produced by several microorganisms that enables the conversion of fibrinogen to fibrin.

[1] A negative coagulase test would instead show the presence of coagulase-negative organisms such as S. epidermidis or S. saprophyticus.

The resulting complex is called staphylothrombin, which enables the enzyme to act as a protease to convert fibrinogen, a plasma protein produced by the liver, to fibrin.

Tube test is not performed each institutions but most of the result depends on blood cultures from lab.

The tube test uses rabbit plasma that has been inoculated with a staphylococcal colony (i.e., Gram-positive cocci which are catalase positive).

S. aureus uses coagulase to form a fibrin coat from fibrinogen present in the bloodstream. This helps the bacteria evade detection and phagocytosis by the immune system.
A fibrin clot formed in a test tube by the coagulase reaction