Coagulase test

• Used to differentiate Staphylococcus aureus from less pathogenic coagulase-negative staphylococci
• Tests for the ability of the organism to convert fibrinogen into fibrin (that is, to form a clot)
• Slide coagulase
  • Checks for agglutination after a drop of plasma is combined with a suspension of the bacteria on a slide
  • Positive if clumping is apparent within 10 seconds or less, and no clumping is observed in saline or water
  • A few species of coagulase-negative staphylococci may be positive
  • S. aureus is negative in 15% of tests, especially MRSA, so negative results need to be confirmed with tube coagulase
• Tube coagulase
  • Mix a heavy inoculum of bacteria into a tube filled with rabbit plasma
  • Positive if clot is formed within 4 hours when incubated at 37ºC
  • A few species of Coagulase-negative staphylococci may be positive