There may not be envenomation in each case of snake bite. Amount of venom injected to the body depend upon several factors. The symptoms and signs of envenomation may be as follows:
|Symptoms & Signs|
Sometimes, it becomes difficult on the part of the clinician to effectively distinguish between a poisonous snake bite from a non-poisonous snake bite in the early hours; whether to start treatment with Anti-Snake Venin (ASV) or not.
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Abnormality is blood coagulation is a distinguished feature of viper envenomation, though it may also be seen to some extent in other poisonous snake bites.
(In West Papua and the Maluku Islands, envenoming by Australasian elapids can cause incoagulable blood)
If, laboratory service is not available for better tests like Prothrombin Time (PT) and INR (International Normalised Ratio) to determine the abnormality in coagulation, an useful and informative bedside test requiring very little skill and only one piece of apparatus – a new, clean, dry, glass vessel (tube or bottle) may be helpful.
That is 20-minute whole blood clotting test (WBCT20); can be performed as described below:
Place 2 mls of freshly sampled venous blood in a small, new or heat cleaned,dry, glass vessel.
Leave undisturbed for 20 minutes at ambient temperature.
Tip the vessel once after 20 minutes.
If, after 20 minutes:
The blood is still liquid (unclotted) and runs out, the patient has hypofibrinogenaemia (“incoagulable blood”) as a result of venom-induced consumption coagulopathy.
(In the South-East Asia region, incoagulable blood is diagnostic of a viper bite and rules out an elapid bite)
Note: 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 (surface activation of factor XI – Hageman factor) and test will be invalid.
If there is any doubt, repeat the test in duplicate, including a “control” (blood from a healthy person such as a relative)
Note of Caution: This test may not detect low level of envenomation, hence, cannot be relied upon fully to start the Anti-Snake Venin therapy; may result in delayed decision.