Association between the 20-minute whole blood clotting test and fibrinogen concentrations in green pit viper envenomations in Bangkok

Background The 20-minute whole blood clotting test (20WBCT) is widely used in patients with viper envenomation in resource-limited settings. The unclotted result guides the need for antivenom administration. Confusion has arisen on how to interpret “partially clotted” for the test due to a paucity of data. This study’s primary aim was to evaluate the association between fibrinogen concentrations (FC) and states of clotting from the 20WBCT in green pit viper (GPV) envenomation. Methods Patients aged ≥18 years who presented to our hospital with GPV bites were enrolled between September 2022 and November 2023. All 20WBCT were done by laboratory technicians and the results were video recorded. Corresponding blood samples were quantified for FC. Two investigators, blinded to clinical/laboratory data, interpreted clotting results from recordings. A third investigator resolved conflicts. Results Thirty-nine patients contributed 188 blood samples. 20WBCT results were categorized as completely clotted, partially clotted (subdivided into mostly or minimally clotted), or unclotted, with median FC of 259.1, 223.5, 226.1, and 48.7 mg/dL, respectively. FC in the unclotted group were significantly lower than those in the other groups (completely: p = 0.001; mostly: p < 0.001; minimally: p = 0.002), with no differences among the completely clotted and two subgroups of the partially clotted. The unclotted 20WBCT had 28.6% sensitivity and 99.4% specificity for detecting FC < 100 mg/dL. Further analysis found that a FC < 70 mg/dL best predicted the unclotted result (the area under the receiver operating characteristic curve was 0.99 (0.99-1.0) and the accuracy was 98.9%). Conclusions Compared with the partially clotted samples (either mostly or minimally clotted), FC were significantly lower in the unclotted, but similar to the completely clotted samples. The partially clotted samples should be interpreted as completely clotted.