The Coagulase Test

The Coagulase Test is used to differentiate Staphylococcus aureus from other species of this genus.

To perform the Slide Coagulase Test, place a drop of coagulase reagent (rabbit plasma) onto a clean microscope slide, and then add several colonies of the unknown Staphylococcus. Mix well.

• If fine grains of sand or small clumps are seen, the coagulase test is positive.
• If the mixture remains smooth (milky looking), the coagulase test is negative.

The Slide Coagulase Test detects bound coagulase only. Bound coagulase is an enzyme in the cell wall of Staphylococcus aureus

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If the slide coagulase test is negative, we can only say we have Staphylococcus. We need to perform an additional coagulase test known as the Tube Coagulase Test. The Tube Coagulase Test detects free coagulase which is an enzyme excreted outside the cell wall by the bacteria.

The Tube Coagulase Test consists of putting approximately 1 ml of coagulase reagent (rabbit plasma) in a labeled test tube and incubating it at 34 to 37 degrees Celsius from 4 to 24 hours.

• If the tube has a thick, solid clot within 24 hours, it is Staphylococcus aureus.
• If the tube does not clot, then it is coagulase negative Staphylococcus (CNS).