MDCalc

Antivenom Dosing Algorithm

Doses antivenom (CroFab only, not Anavip) for pit viper snakebites.

This dosing tool is intended to assist with calculation, not to provide comprehensive or definitive drug information. Always double-check dosing of any drug and consult a pharmacist when necessary.

Use only in cases of symptomatic crotalid snake envenomation (pit vipers including rattlesnakes, cottonmouths, and copperheads) in the US, for CroFab administration. Do not use in cases of coral snake envenomation or snakes not indigenous to the US. Do not use for Anavip administration, as it will result in incorrect dosing. Report all cases of suspected/confirmed envenomation to poison control (1-800-222-1222).

Are there signs of envenomation?
Swelling, tenderness, redness, ecchymosis, or blebs at bite site; elevated protime, decreased fibrinogen or platelets; systemic signs (hypotension, bleeding beyond puncture site, refractory vomiting, diarrhea, angioedema, neurotoxicity)

Result:

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Advice
  • Leading edge of swelling and tenderness should be marked every 15-30 minutes.
  • Elevate and immobilize extremity, treat pain aggressively with IV opioids, and update tetanus status as needed.
Management

Maintenance therapy:

  • 2 vials of antivenom every 6 hours x 3 doses (given 6, 12, and 18 hours after initial control).
  • May not be needed if close observation by physician expert is available.

Follow-up planning:

  • Patient should return for worsening swelling not relieved by elevation, or abnormal bleeding (e.g. melena, gum bleeding, easy bruising).
  • If fever, rash, or muscle/joint pains occur (i.e., suggesting serum sickness), patient should return.
  • Patient should be given bleeding precautions (no contact sports, elective surgery, or dental work for 2 weeks).
  • Follow-up:
    • Patients who did not require antivenom: as needed.
    • Copperhead victims: as needed.
    • Cottonmouth and rattlesnake victims: repeat labs (CBC, PT, fibrinogen) twice (2-3 days and 5-7 days after discharge), then as needed.
Critical Actions
  • Report to poison control (1-800-222-1222) all cases of suspected/confirmed envenomation.
  • Avoid the following:
    • Cutting or suctioning wound.
    • Ice.
    • NSAIDs: avoid in rattlesnake/cottonmouth victims (Pham 2018).
    • Prophylactic antibiotics.
    • Prophylactic fasciotomy.
    • Routine use of blood products.
    • Electrical shock therapy.
    • Steroids, unless allergic phenomena observed.
    • Tourniquets.
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