Snake envenomation: is the 20 min whole blood clotting test (WBCT20) the optimum test for management?

The 20 min whole blood clotting test (WBCT20) is a simple bedside test recommended by World Health Organization (WHO) to assess hemotoxic envenomation and guide administration of polyvalent anti-snake venom (ASV). However, reliability and validity of this test has not been well documented in literature.

Sixty consecutive patients with history of snake bite were prospectively evaluated at a teaching hospital in India over 2 years. Envenomation was established by clinical and laboratory criteria. WBCT20 was done at 0, 4 and 12 h using standardized protocol. Prothrombin time (PT) with international normalized ratio (INR) was estimated at similar intervals to detect venom-induced consumption coagulopathy. Sensitivity, specificity and likelihood ratios (LR) were determined for WBCT20 using envenomation criteria as gold standard. WBCT20 was compared with PT/INR at cutoff values of ≥1.4 and ≥1.2. Two observers performed test-retest correlation to determine inter-observer variability of WBCT20.

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Seventeen of 60 patients had evidence of hemotoxic envenomation. Four patients had combined neurotoxicity and hemotoxicity. Sensitivity and specificity of WBCT20 were 94 and 76%; positive and negative LR were 3.9 and 0.08, respectively. No inter-observer variability was noted.

WBCT20 is a highly sensitive test with excellent reliability for detecting envenomation. However, the false positive rate in this study was 24%. Asymptomatic snake bite patients with a positive WBCT20 but no corresponding clinical signs of envenomation should be tested using PT/INR before receiving ASV to prevent unnecessary waste of anti-venom.