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

TL;DR

This study clarifies how to interpret a 20-minute blood clotting test for green pit viper bites, showing that 'partially clotted' results should be treated as 'clotted' to guide antivenom use.

Contribution

The study provides a clear interpretation framework for the 20WBCT in green pit viper envenomation, improving antivenom decision-making in resource-limited settings.

Findings

Unclotted 20WBCT results correlate with very low fibrinogen concentrations (<70 mg/dL).

Partially clotted samples have fibrinogen levels similar to completely clotted samples.

Reclassifying partially clotted as clotted improves the test's reliability for antivenom decisions.

Abstract

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.

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.

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%).

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.

Green pit viper envenomation can cause coagulopathy. The 20-minute whole blood clotting test (20WBCT) is used to guide antivenom treatment in a large number of hospitals in developing nations. However, it is unclear how to interpret a “partially clotted” result. We studied patients with green pit viper bites, comparing the 20WBCT results with their fibrinogen concentrations. We found that “unclotted” samples had very low fibrinogen concentrations, while the concentrations in “partially clotted” blood were similar to those in “completely clotted” samples. These findings suggest that “partially clotted” results should be interpreted as “clotted.” While fibrinogen assays are the “reference standard” for diagnosing venom-induced consumptive coagulopathy, their reliance on specialized equipment and cold-chain reagents limits their utility in rural areas where most viper envenomations occur. The 20WBCT remains the primary diagnostic tool in these resource-limited settings. By refining the interpretation of this test—specifically by simplifying the binary ‘clot versus no-clot’ decision—our study directly addresses a critical gap in public health infrastructure, providing clinicians with a more reliable method to make life-saving antivenom decisions at the bedside.

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