At a rural primary health centre a farmer is brought 3 hours after a suspected viper bite with gum bleeding. No laboratory coagulation facilities are available. The 20-minute whole blood clotting test (20WBCT) is performed by leaving 2 mL of venous blood undisturbed in a clean dry glass tube. Which result confirms systemic envenomation and the need for antivenom?

• A Blood is still unclotted after 20 minutes ✓
• B Blood clots within 5 minutes
• C Clot forms but lyses on gentle shaking
• D Serum separates from the clot within 20 minutes

Explanation

In viper envenomation, venom-induced consumption coagulopathy leaves the blood incoagulable, so failure to clot within 20 minutes in a clean glass tube is a positive test indicating systemic envenomation requiring polyvalent antivenom. Normal blood should clot firmly within minutes. The test is repeated every 6 hours after each antivenom dose to monitor response. Clot formation within the observation window, even if slow, indicates adequate clotting factors and argues against significant envenomation.

Reference: WHO Guidelines for the Management of Snakebites, South-East Asia Region, 2nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.