Coagulase Test Guide (Slide and Tube)
An original Learn Microbes bench guide for evaluating bound coagulase (slide clumping factor) and free coagulase (tube clotting) to identify Staphylococcus aureus.
Coagulase Reaction Board
Read slide tests immediately for macroscopic clumping. Tilt tube tests to check for a solid fibrin clot.
Rapid macroscopic clumping of bacterial cells in rabbit plasma within 10-15 seconds.
Smooth, homogenous, milky suspension with no visible particles; must confirm with a tube test.
Milky suspension forms a firm, solid fibrin clot that remains at the bottom of the tube when tilted at 45 degrees.
No clot forms; suspension remains entirely liquid and flows freely along the wall of the tilted tube.
Rapid macroscopic clumping of bacterial cells in rabbit plasma within 10-15 seconds.
Smooth, homogenous, milky suspension with no visible particles; must confirm with a tube test.
Milky suspension forms a firm, solid fibrin clot that remains at the bottom of the tube when tilted at 45 degrees.
No clot forms; suspension remains entirely liquid and flows freely along the wall of the tilted tube.
What to look for
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Macroscopic Clumping | Bound coagulase cross-links fibrinogen, agglutinating the cells | Positive slide reaction |
| Homogenous Mixture | No bound coagulase present; cells stay in a uniform suspension | Negative slide reaction (requires tube backup) |
| Firm Fibrin Clot | Free coagulase activates prothrombin/CRF, converting fibrinogen to a solid clot | Positive tube reaction (S. aureus confirmed) |
| Free Flowing Liquid | No free coagulase; plasma remains liquid when the tube is tilted | Negative tube reaction (CNS confirmed) |
Coagulase study readout
| Pattern | What to call it | How to think about it |
|---|---|---|
| Slide (+), Saline (-) | Staphylococcus aureus | Bound coagulase is present. Confirms S. aureus presumptively. |
| Slide (-), Saline (-) | Pending tube test | Cannot rule out S. aureus; run tube test immediately. |
| Tube (+) | Staphylococcus aureus | Free coagulase is present. Solid clot formed; definitive identification. |
| Tube (-) | Coag-Negative Staph (CNS) | No free coagulase. Usually S. epidermidis, S. saprophyticus, etc. |
| Slide (+), Saline (+) | Invalid / Autoagglutination | The strain is autoagglutinating; must use tube test to resolve. |
- Differentiates Staphylococcus aureus (positive) from coagulase-negative staphylococci (CNS like S. epidermidis).
- Slide test detects bound coagulase (clumping factor); tube test detects free extracellular coagulase.
- A saline control is essential on the slide to identify autoagglutinating strains.
A negative slide test is only a presumptive negative. The tube test remains the gold standard referee.