Whole Blood Clotting Time Procedure for Snake Bite
The 20-minute whole blood clotting test (WBCT20) is performed by drawing 2-5 mL of fresh venous blood into a clean, dry glass tube (or syringe/ceftriaxone bottle), leaving it undisturbed at ambient temperature, and checking at 20 minutes—if the blood remains liquid or only partially clotted, the test is positive indicating coagulopathy. 1, 2
Step-by-Step Procedure
Materials Required
- Clean, dry glass tube (or alternatively, a plastic syringe or empty ceftriaxone bottle can be used as validated alternatives) 2
- 2-5 mL of freshly drawn venous blood 1, 2
- Timer or watch 1
Technique
- Draw 2-5 mL of fresh venous blood into the container without anticoagulant 1, 2
- Leave the tube undisturbed at ambient room temperature (do not refrigerate or agitate) 1
- Wait exactly 20 minutes 1, 3
- Tip the tube once to assess clot formation 1
Interpretation
- Negative (Normal): Blood forms a solid clot that does not flow when the tube is inverted 1, 3
- Positive (Abnormal): Blood remains completely liquid or shows only partial clotting, indicating venom-induced consumption coagulopathy (VICC) 1, 3
Diagnostic Performance
Accuracy Metrics
- Sensitivity: 55-94% depending on methodology, with the standard glass tube method showing 94% sensitivity 4, 3
- Specificity: 76-99.6%, with false positive rates of approximately 24% 2, 3
- Both syringe and bottle methods correctly classify patients 84.7-86.8% of the time compared to laboratory standards 2
Clinical Validation
The WBCT20 has been validated as highly sensitive for detecting coagulopathy, but the notable false positive rate (24%) means that asymptomatic patients with positive WBCT20 but no clinical signs of envenomation should have confirmatory PT/INR testing before receiving antivenom to avoid unnecessary treatment 3. The American College of Medical Toxicology specifically recommends this confirmatory approach 5.
Timing and Monitoring Protocol
Initial Assessment
- Perform WBCT20 immediately upon hospital arrival 5, 3
- Simultaneously draw blood for PT/INR to confirm abnormal results 3
Serial Monitoring
- Repeat at 4 hours and 12 hours after initial presentation to detect delayed coagulopathy in patients with negative initial results 5, 3
- After antivenom administration, repeat WBCT20 at both 20 and 30 minutes, then every 4-6 hours to monitor treatment response 5
Alternative Bedside Method: Modified Lee-White
The Modified Lee-White (MLW) method is more sensitive than WBCT20 (85% vs 55% sensitivity) for detecting coagulopathy, though both have equivalent specificity (99.6%) 4. However, WBCT20 remains the WHO-recommended standard due to its simplicity and widespread validation 3.
Critical Pitfalls to Avoid
Technical Errors
- Do not use plastic tubes without validation—glass tubes are standard, though plastic syringes and ceftriaxone bottles have been specifically validated as alternatives 2
- Do not agitate or move the tube during the 20-minute waiting period, as this can disrupt clot formation 1
- Do not refrigerate the sample—ambient temperature is required 1
Clinical Interpretation Errors
- Do not rely solely on WBCT20 for treatment decisions in asymptomatic patients—the 24% false positive rate necessitates PT/INR confirmation before administering antivenom 3
- Do not assume a negative test rules out envenomation—coagulopathy can develop hours after the bite, requiring serial testing at 4 and 12 hours 5
- Do not use WBCT20 for coral snake bites—these envenomations require different monitoring focused on respiratory compromise rather than coagulopathy 5
Clinical Context
The WBCT20 is particularly valuable in resource-limited settings where sophisticated laboratory testing is unavailable, as it is inexpensive, rapid, and can be performed at the bedside 1. The test specifically detects consumption of clotting factors (particularly Factors V, X, and XIII) and fibrinogen that occurs with venom-induced consumption coagulopathy 6.