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Laboratory tests 1

Laboratory tests are often very helpful in determining the extent of envenoming, but this does not mean only a large hospital with a well-equipped laboratory can manage a snakebite. It is also vital to repeat tests, even if initially normal. If the patient’s initial examination and tests are normal, indicating no current envenoming, the tests should be repeated at about 2 hours and then a further 3 hours later, to ensure late development of envenoming, particularly coagulopathy, is not missed. In some situations, particularly some viper bites causing coagulopathy (notably Malayan pit viper bite), this may persist, develop late, or recur after antivenom therapy, so retesting over several days may be required. Earlier testing is indicated if symptoms or signs develop.

The range of tests likely to be available will vary from small rural hospitals to large city teaching hospitals.

A small or rural hospital
Many snakebites will initially present to a small rural hospital with no laboratories on-site. However, there are some very useful tests that can easily be performed:

  • Urinalysis looking for haemoglobin/myoglobin (both test positive for blood). Simple microscopy looking for red cells or casts may help differentiate between haemoglobinuria and myoglobinuria.
  • Whole blood clotting time. This simple but effective test requires at least two standard glass test tubes or similar glass containers. A small quantity of the patient’s blood is placed in one tube, a similar amount of blood from a normal control (relative or staff member) is placed in another tube and the time taken to clot is measured. If normal, the blood should clot in 5-10 minutes. If the only patient’s blood fails to clot or develops only a weak clot by 15 minutes, then there is probably a coagulopathy. If neither the patient nor control clot then there is a problem with the test system. It has been suggested that this test can be simplified, by only checking for clots 20 minutes after taking the sample; a normal clot indicates no coagulopathy, while a weak clot or no clot indicates coagulopathy.
  • Only in Australia; snake venom detection on the bite site swab.

In a smaller Australian country hospital, with no rapid access to a medical laboratory, the following are valuable:

  • Venom detection using the SVDK.
  • Urine dipstick testing (for haematuria/myoglobinuria).
  • Whole blood clotting time (perform in a GLASS test tube, simultaneously with blood from a normal control).
  • Though untested by clinical trial in snakebite, the recently available d-dimer test kit for screening for DVT (Simplify D-dimer; AGEN Biomedical Ltd.) may offer a further useful test for country hospitals. However, its applicability to snakebite has yet to be confirmed.

If initial tests are normal, repeat 2-3 hours and 5-6 hours later, or earlier if symptoms of envenoming develop. If initial tests are abnormal and antivenom therapy is commenced, re-test at 3 hours after completion of antivenom, to decide if more antivenom is needed (if there is a coagulopathy present).