Tube Coagulase Test Procedure
Prepare a 1-in-6 dilution of the plasma in saline (0.85% NaCl) and place 1 ml volumes of the diluted plasma in small tubes.
Emulsify several isolated colonies of test organism in 1 ml of diluted rabbit plasma* to give a milky suspension.
Incubate tube at 35oC in ambient air or in water bath for 4 hours.
Examine at 1, 2 and 4 hour for clot formation by tilting the tube through 90°. (Clots may liquefy after their formation)
Leave negative tubes at room temperature overnight and re-examine.
(This step is essential, for some strains of S. aureus, including many MRSA, produce a delayed clot which is rapidly lysed at 37°C by the organism’s staphylokinase.
Observation
Read as positive any degree of clot formation. Often the plasma is converted into stiff gel that remains in place when the tube is tilted or inverted, but sometimes clots are seen floating in the fluid.
Coagulase Test
Coagulase Positive: Clot of any size eg. Staphylococcus aureus
Coagulase Negative: No clot (plasma remains wholly liquid or shows only a flocculent or ropy precipitate). eg. Staphylococcus epidermidis