20WBCT
20 Minute Whole Blood Clotting Test (20WBCT)
- Place 2 mls of freshly sampled venous blood in a small, new, dry, glass vessel
- Leave undisturbed for 20 minutes at ambient temperature
- Tip the vessel once
- If the blood is still liquid (unclotted) and runs out, the patient has hypofibrinogenaemia (“incoagulable blood”) as a result of venom-induced consumption coagulopathy
- Warning! If the vessel used for the test is not made of ordinary glass, or if it has been cleaned with detergent, its wall may not stimulate clotting of the blood sample and test will be invalid
- If there is any doubt, repeat the test in duplicate, including a healthy “control” (blood from a healthy person such as a relative)
Recommendation
- In the absence of an alternative simple bedside test of blood coagulability available in hospitals in the developing world, the 20WBCT should continue to be used.
- However, every effort should be made to eliminate false positive (non-clotting) results by ensuring that ordinary glass is used, that recycled glass vessels are not cleaned with detergents or other cleansing fluids and that a normal control blood is used for comparison in cases where the 20WBCT result is inconsistent with the patient’s clinical condition.
- Accepting that the 20WBCT may remain negative (clotting) in patients with evolving venom-induced DIC, the test should be repeated frequently and antivenom treatment should not be delayed if there is other evidence of antihaemostatic disturbances (e.g. spontaneous systemic bleeding distant from the bite site)