Background: The 20-min whole blood clotting test (WBCT20) is widely used for the identification of coagulopathy in snake envenoming, but its performance in practice has not been evaluated. Aim: We aimed to investigate the diagnostic utility of the WBCT20 for coagulopathy in Russell's viper envenoming. Design: Prospective observational study. Methods: Adult patients with snake envenoming were recruited. Age, sex, bite information, clinical effects, serial WBCT20 and antivenom treatment were recorded. Definite Russell's viper envenoming was confirmed with venom specific enzyme immunoassay. We assessed sensitivity of admission WBCT20 to coagulopathy (international normalized ratio, INR > 1.5) in Russell's viper envenoming, the specificityof negative WBCT20 in non-envenomed patients and directly compared paired WBCT20 and INR. Results: Admission WBCT20 was done in 140 Russell's viper bites with coagulopathy and was positive in 56/140 [sensitivity 40% (95% confidence interval (CI): 32-49%)]. A negative WBCT20 led to delayed antivenom administration [WBCT20-ve tests: median delay, 1.78 h (interquartile range(IQR): 0.83-3.7 h) vs. WBCT20 + ve tests: median delay, 0.82 h (IQR: 0.58-1.48 h); P = 0.0007]. Delays to antivenom were largely a consequence of further WBCT20 being performed and more common if the first test was negative (41/84 vs. 12/56). Initial WBCT20 was negative in 9 nonenvenomed patients and 48 non-venomous snakebites [specificity: 100% (95% CI: 94-100%)]. In 221 paired tests with INR > 1.5, the WBCT20 was positive in 91(41%). The proportion of positive WBCT20 only increased slightly with higher INR. Conclusions: In clinical practice, the WBCT20 has low sensitivity for detecting coagulopathy in snake envenoming and should not over-ride clinical assessment-based decisions about antivenom administration. There is an urgent need to develop a simple bedside test for coagulopathy in snake envenoming.