Coagulase is a protein enzyme produced by several microorganisms that enables the conversion of fibrinogen to fibrin. In the laboratory, it is used to distinguish between different types of Staphylococcus isolates. Importantly, S. aureus is generally coagulase-positive, meaning that a positive coagulase test would indicate the presence of S. aureus or any of the other 11 coagulase-positive Staphylococci. A negative coagulase test would instead show the presence of coagulase-negative organisms such as Staphylococcus epidermidis or Staphylococcus saprophyticus. However, it is now known that not all S. aureus are coagulase-positive. Whereas coagulase-positive staphylococci are usually pathogenic, coagulase-negative staphylococci are more often associated with opportunistic infection.