Slide Coagulase Test

Detects clumping factor (formerly referred as cell-bound coagulase)

• Clumping factor directly coverts fibrinogen to fibrin causing agglutination
• Heavy suspension of organism is made on glass slide and mixed with drop of plasma
• Agglutination indicates a positive test:
  • ƒ Indicates Staphylococcus aureus
  • ƒ Some species of Coagulase negative staphylococcus can be positive
• Negative results should be confirmed

Slide Coagulase Test Procedure

1. Emulsify a staphylococcal colony in a drop of water on a clean and grease free glass slide with a minimum of spreading. (If the isolate does not form a smooth, milky suspension, do not proceed with the test).
2. Make similar suspensions of control positive and negative strains to confirm the proper reactivity of the plasma.
3. Dip a flamed and cooled straight inoculating wire into the undiluted plasma at room temperature, withdraw, and stir the adhering traces of plasma (not a loopful) into the staphylococcal suspension on the slide. Flame the wire and repeat for the control suspensions.
4. Read as positive a coarse clumping of cocci visible to the naked eye within 10 seconds. Read as negative the absence of clumping or any reaction taking more than 10 seconds to develop, but re-examine any slow reacting strains by the tube coagulase test.

Observation

1. Coagulase Positive: Macroscopic clumping in 10 seconds or less in coagulated plasma drop and no clumping in saline or water drop.
2. Coagulase Negative: No clumping in either drop

Interpretation: Slide coagulase test is the main method used to identify S. aureus in clinical laboratories but it has some limitations.

1. About 15% of ordinary strains of S. aureus and many more of MRSA give negative reactions.
2. Few species of coagulase negative staphylococci give positive reactions.

Note: All coagulase negative slides must be confirmed using tube coagulase test as the definitive test for S. aureus.