Coagulase test

Principle

S. aureus produces bound coagulase (clumping factor), detected by the slide test, and free coagulase, which clots plasma, detected by the tube test.

Slide test

• Emulsify a colony in saline on a slide; add a drop of rabbit plasma and mix.
• Clumping within 10 s = positive. A saline-only drop checks for auto-agglutination.

Tube test

• Add a few colonies to 0.5 mL rabbit plasma (EDTA); incubate at 35–37 °C.
• Examine for a clot at 1–4 h; if negative, re-read at 18–24 h at room temperature.

Interpretation

• Positive: S. aureus (and a few animal species)
• A negative slide test must be confirmed by the tube test.

Controls

S. aureus (positive), S. epidermidis (negative).