Paper I
Question
Coagulase test.
Answer
The coagulase test detects the enzyme coagulase, which converts fibrinogen to fibrin, causing plasma to clot — the single most important test distinguishing pathogenic Staphylococcus aureus (coagulase-positive) from the generally less pathogenic coagulase-negative staphylococci.
Types of coagulase and corresponding tests:
- Free (extracellular) coagulase — detected by the tube test: bacterial growth is emulsified in citrated/EDTA rabbit or human plasma and incubated at 37°C, examined for clot formation up to 4 hours (and again at 18–24 hours, since some strains clot only later, or an early clot may lyse if left too long) — visible gel/clot formation is a positive result.
- Bound coagulase (clumping factor) — detected by the rapid slide test: a bacterial suspension is mixed directly with a drop of plasma on a slide; visible clumping within seconds indicates a positive result. The slide test is faster but less sensitive, so a negative result should be confirmed by the tube test before reporting an isolate as coagulase-negative.
Clinical significance: coagulase positivity correlates with virulence (the fibrin coat helps the organism resist phagocytosis) and is the key laboratory criterion for identifying S. aureus in routine diagnostic microbiology, directly guiding both clinical diagnosis and infection-control decisions.