Rabbit coagulase plasma detects the coagulase enzyme produced by Staphylococcus aureus, converting plasma fibrinogen to fibrin and producing a visible clot. The slide test (10 seconds) screens for bound coagulase (clumping factor); the tube test (4–24 h) detects free, secreted coagulase and is required to confirm slide-negative isolates. Use EDTA-anticoagulated rabbit plasma, not citrated — citrate-utilizing organisms can clot citrated plasma without producing any coagulase at all, generating false positives.
Key Facts
- Slide test — detects bound coagulase (clumping factor); result in 10 seconds.
- Tube test — detects free coagulase; read at 1, 2, 4 h and 24 h.
- EDTA over citrate — EDTA chelates Ca2+ and eliminates the citrate-utilizer false positive (Enterococcus, Pseudomonas, Serratia).
- ~10–15% of S. aureus lack clumping factor — slide-negative isolates must be confirmed by tube test.
- MRSA is still coagulase-positive — the test identifies species, not resistance.
- Pro-Lab PL.850 — lyophilized rabbit plasma with EDTA, 5 × 3 mL vials, CE Marked IVD.
Why the Coagulase Test Still Matters
Coagulase is a 60 kDa secreted enzyme that binds prothrombin to form the active complex staphylothrombin, which then cleaves fibrinogen into fibrin. The clinical importance of this reaction is twofold: it is a pathogenicity marker for S. aureus, and it is the simplest test in clinical microbiology for distinguishing S. aureus from the coagulase-negative staphylococci (CoNS) that dominate skin flora. A coagulase-positive cocci-in-clusters isolate from a blood culture, surgical wound, or deep abscess is treated very differently from a CoNS contaminant — the difference is sepsis workup versus repeat culture.
Although MALDI-TOF and molecular methods are now widespread, the coagulase test remains the workhorse confirmatory test in any lab that does not have continuous mass-spectrometry coverage, and it is still the reference method against which newer rapid tests are validated.
Bound vs. Free Coagulase: Two Different Molecules, Two Different Tests
S. aureus produces two distinct fibrinogen-binding factors:
- Bound coagulase (clumping factor A/B) is anchored to the bacterial cell wall and cross-links fibrinogen between adjacent cells. This is what the slide test detects — the visible clumping is bacteria agglutinated by fibrinogen bridges.
- Free coagulase is the soluble, secreted enzyme that activates prothrombin. This is what the tube test detects via formation of a visible fibrin clot in liquid plasma.
Approximately 10–15% of S. aureus isolates do not express clumping factor and will appear slide-negative; the tube test will pick these up. The corollary is critical: all slide-negative results must be confirmed by the tube test before reporting an isolate as coagulase-negative.
The EDTA vs. Citrate Question
Historically, both EDTA and sodium citrate have been used as anticoagulants in commercial rabbit coagulase plasma. Citrated plasma carries a well-documented failure mode: any organism capable of metabolizing citrate — including a number of Enterococcus faecalis, Pseudomonas aeruginosa, and Serratia marcescens strains — can consume the citrate anticoagulant during incubation. Once citrate is depleted, free calcium re-enters solution, and the plasma clots through the normal extrinsic coagulation cascade. The technologist sees a clot and reports a positive result; the organism is not actually coagulase-positive at all.
EDTA prevents this. EDTA chelates calcium with such high affinity that no calcium becomes available to drive the coagulation cascade even if the anticoagulant is partially degraded. The only way an EDTA plasma tube can clot is via the calcium-independent staphylocoagulase / staphylothrombin pathway — which is genuinely diagnostic for S. aureus. CLSI guidance and every major clinical microbiology reference text now specify EDTA-anticoagulated rabbit plasma for coagulase testing.
science Order Now Rabbit Coagulase Plasma EDTA PL.850 — from $46.60 Lyophilized rabbit plasma with EDTA, 5 × 3 mL vials. CE Marked IVD. Direct from Pro-Lab Diagnostics, Georgetown TX. arrow_forwardBench Protocol — Slide Method (Screening)
- Place a drop of sterile 0.85% saline on a clean glass slide.
- Emulsify 2–3 well-isolated colonies of an 18–24 h pure culture in the saline. The suspension should be smooth and dense, with no auto-agglutination.
- Add one drop of rehydrated rabbit coagulase plasma (EDTA) and mix.
- Rock the slide gently for up to 10 seconds. Visible clumping within 10 seconds = positive. Any reaction after 10 seconds, or in the saline-only control, is non-specific and must be ignored.
- All slide-negative isolates — and any borderline reactions — proceed to the tube test for confirmation.
Bench Protocol — Tube Method (Confirmatory)
- Dispense 0.5 mL of rehydrated rabbit coagulase plasma (EDTA) into a sterile 12 × 75 mm test tube.
- Emulsify 2–3 colonies of the suspect organism in the plasma to give a turbid suspension. Run a known coagulase-positive (e.g. S. aureus ATCC 25923) and a known coagulase-negative (e.g. S. epidermidis ATCC 12228) control in parallel.
- Incubate at 35–37°C. Examine at 1, 2, and 4 hours, tilting the tube gently (do not shake — shaking can break a fragile early clot).
- Any degree of clotting that does not flow when the tube is inverted is positive. Read partial clots and solid clots both as positive.
- If negative at 4 hours, hold at room temperature and read again at 24 hours. Some isolates clot slowly; others produce staphylokinase that lyses an early clot, so a 4-hour-positive tube can revert to liquid by 24 hours — once positive, always positive.
Quality Control and Common Pitfalls
The single most common error is reading slide results past the 10-second window. Non-specific protein interactions will clump any dense bacterial suspension given enough time. The second most common error is shaking the tube test to "check" for a clot — this physically disrupts the fibrin lattice and converts a true positive into a reported negative.
Other pitfalls worth flagging in SOPs: do not use colonies from selective media containing high salt (e.g. mannitol salt agar) without sub-culturing first — salt can cause auto-agglutination on the slide. Do not run the test on colonies older than 24 hours; bound coagulase expression decreases as cultures age. And confirm any positive identification from a sterile site with a second method — S. lugdunensis, S. schleiferi subsp. coagulans, and S. intermedius can also be coagulase-positive, and at least S. lugdunensis is clinically important enough that the species call matters.
How It Fits Into a Modern Staph Workflow
In most clinical labs the slide coagulase test is the second-line confirmation after a rapid latex test such as Prolex Staph Latex, which detects clumping factor, protein A, and capsular polysaccharide in a single 30-second reaction. Coagulase tube testing remains the tie-breaker when the latex is equivocal, when a sterile-site isolate is being worked up, or when a teaching lab is demonstrating the classical biochemistry. Many labs also pair coagulase testing with TestOxidase for Gram-negative workups on the same plate, and archive every confirmed isolate on Microbank cryopreservation beads for downstream susceptibility or epidemiology work.
Frequently Asked Questions
What does the coagulase test detect?
The enzyme coagulase, produced almost exclusively by Staphylococcus aureus. Coagulase converts fibrinogen into fibrin, producing a visible clot. A positive result strongly supports identification of S. aureus and differentiates it from coagulase-negative staphylococci (CoNS) such as S. epidermidis.
What is the difference between the slide and tube coagulase tests?
The slide test detects bound coagulase (clumping factor) attached to the bacterial cell wall and gives a result in 10 seconds. The tube test detects free, secreted coagulase by clot formation in liquid plasma, read at 4 and 24 hours. The slide test is a rapid screen; all negative or equivocal slide results must be confirmed by the tube test, because roughly 10–15% of S. aureus isolates lack clumping factor.
Why use EDTA plasma instead of citrated plasma?
Citrated rabbit plasma can yield false-positive tube coagulase results when the organism under test is a citrate-utilizing species — notably some Enterococcus, Pseudomonas, and Serratia strains. These bacteria metabolize the citrate anticoagulant, freeing calcium ions that allow plasma to clot via the normal coagulation cascade rather than through bacterial coagulase. EDTA chelates calcium so that any clot formed must come from genuine staphylococcal coagulase activity.
Is a positive coagulase test enough to identify S. aureus?
For routine clinical workflows, a positive coagulase test on a Gram-positive, catalase-positive cocci-in-clusters isolate is generally accepted as S. aureus. However, S. lugdunensis, S. schleiferi subsp. coagulans, and S. intermedius can also be coagulase-positive. For sterile-site isolates, confirm by latex agglutination, tube coagulase, or MALDI-TOF.
Does coagulase testing detect MRSA?
No. The coagulase test confirms the species (S. aureus) but says nothing about methicillin resistance. MRSA isolates are coagulase-positive — they are still S. aureus. Methicillin resistance is detected separately by cefoxitin disk diffusion, PBP2a latex testing, or mecA/mecC PCR.
How long is rabbit coagulase plasma stable after rehydration?
Lyophilized rabbit coagulase plasma (EDTA) is stable to the labeled expiry when stored at 2–8°C. Once rehydrated with sterile distilled water, working plasma is stable for 72 hours at 2–8°C, or for 12 weeks if aliquoted and frozen at -20°C. Avoid repeated freeze-thaw cycles.
For more information about Rabbit Coagulase Plasma EDTA (PL.850), contact info@pro-lab.us or visit the Rabbit Coagulase product page to order online.