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Antivenom Dosing Algorithm

Calculators  Toxicology
The Antivenom Dosing Algorithm is a dosing tool intended to assit with calculation and administration of antivenom to patients with snake envenomation by crotaline snakes (rattlesnake, cottonmouth, copperhead)
Is there a symptom of envenomation?
Inflammation, tenderness, ecchymosis, redness, or blister at a bite site; lower fibrinogen or platelets, elevated protime; systemic symptoms (low blood pressure, bleeding beyond puncture site, angioedema, neurotoxicity, refractory vomiting, diarrhea)
No 0
Yes 1
Is there an indication for antivenom?
Increasing swelling; reduce fibrinogen or platelets, elevated protime ; any systemic indications
No 0
Yes 1


Measured Factor
Antivenom administration dosing instructions
Measured Factor Disease
  • envenomation
Measured Factor Detail
The Antivenom Dosing Algorithm is a dosing tool recommended for patients with known or suspected crotalid envenomation. The algorithm consits of two components' the signs of envenomation (swelling, tenderness, redness, ecchymosis, or blebs at bite site, elevated protime, decreased fibrinogen or platelets and systemic signs) and indications for antivenom (progressive swelling, elevated protime, decreased fibrinogen or platelets, and any systemic signs). If a patient has signs of envenomation and indication for antivenom the recommendation is to mix 4-6 vials of crotaline Fab antivenom in 250 mL normal saline (NS) infused over one hour. If the patient has no signs of envenomation or indication for antivenom then the use of antivenom is not recommended. Recommendations for no signs of envenomation are to observe patients for ≥8 hours and to repeat labs prior to discharge. Recommendations for apparent minor envenomation without indications for antivenom are to observe patients for 12-24 hours and to repeat labs at 4-6 hours and prior to discharge.
General Physician
Body System
Measured Factor Low Impact
  • No antivenom administration
Measured Factor High Impact
  • Administration of antivenom is recommended

Result Interpretation

Ranges Ranges
  • Normal: No antivenom
  • Normal Adult Male: No antivenom
  • Normal Adult Female: No antivenom
  • Normal Pediatric: No antivenom
  • Normal Geriatric Male: No antivenom
  • Normal Geriatric Female: No antivenom
Result High Conditions
  • Envenomation
Study Additional 1
Multicenter, observational case series aimed to determine the use of Crotalidae polyvalent immune Fab (ovine) (CroFab; FabAV) therapy for severe crotaline snake envenomation and the short-term outcomes associated with its use. The primary endpoint measure was the change in overall envenomation severity and incidence of hypersensitivity reactions after administration of FabAV. To determine response to therapy a 7-point scale system was used that incorporated local, systemic and hematologic venom effects, the scale was abbreviated from the Snakebite Severity Score to avoid bias due to the retrospective aspect of the data available. The modified score consisted of a range of possible scores from 0 to 6; depending on the factors present patients were assigned severe (scores of 5 or 6), moderate (Scores of 3 or 4) or mild (Scores of 2 or less) envenomation. The study population consisted of 209 patients, of whom 28 patients had severe envenomation. Patients who had severe envenomation had a median severity score of 5 (interquartile range, IQR 5 to 5) before FabAV administration and a median severity score of 1 (IQR 1 to 2) after administration. Additionally, after receiving FabAV, patients experienced improved pain and swelling, cardiovascular, respiratory, neurologic and gastrointestinal effects, coagulopathy and thrombocytopenia . Among 247 patients who were observed for incidence of hypersensitivity reactions, 6.1% of them experienced immediate hypersensitivity reactions and 5% of them experienced serum sickness during a minimum of 6 days of follow-up after the last dose of FabAV. Overall, FabAV therapy was associated with a clinical improvement in patients with severe crotaline snake envenomation. Incidence of hypersensitivity reactions may be less than shown in the FabAV prescribing information.
References: 2
Study Additional 2
Comprehensive review article aimed at retrospectively assessing the effectiveness of Crotalidae Polyvalent Immune Fab (Ovine) (FabAV) therapy in patients with severe crotaline snake envenomation. Two trained reviewers extracted case-level data related to the administration of FabAV to patients with crotaline snake envenomation and used descriptive statistics to analyze the data. To determine severity of envenomation, the Snakebite Severity Score (SSS) was used with the cutoff for severe envenomation being an SSS ≥ 7. Review consisted of 24 cases of severe human envenomation treated with FabAV, 12 of which achieved initial control of all venom effects (50%). The median dose of FabAV used to obtain initial control was 6 vials (range of 4-18 vials). Of the 24 patients, 9 patients had severe venom effects that did not improve after FabAV therapy. Overall, FabAV administration appeared to be effective in managing severe crotaline snake envenomation based on the articles and cases that were reviewed.
References: 3
Study Additional 3
Retrospective review article reviewing the effectiveness of treating envenomations caused by copperhead snake envenomations with Crotalidae polyvalent immune Fab (FabAV). Article looked at data collected between October 2000 and October 2002. Charts were reviewed retrospectively for all patients that presented with copperhead snake envenomations and were treated with FabAV. Data measured included progression of limb swelling, coagulopathy and hemodynamic status before and after FabAV administration. Additionally, adverse effects of therapy and recurrence were also recorded. Results were calculated from approximately 400 cases, of which 32 received FabAV and were thus included. Most of the patients had moderate envenomations and the median time to FabAV administration was 4 hours. A total of 4 cases (13%, 95% CI 1%-24%) were considered treatment failures and 6 cases (19%, 95% CI 5% - 32%) had recurrent swelling. The rest of the cases had a median time to achieve initial control of 1 hour, with a median dose of 4 vials. A total of 28 cases achieved cessation of the progression of local tissue injury within 4 hours of receiving FabAV (88%, 95% CI 76%-99%). Data collected demonstrated that administration of FabAV to patients bitten by copperheard snakes helped halt local tissue effects of envenomation, however a controlled trial is needed to define its role for treatment of copperhead envenomation.
References: 4


Eric J. Lavonas, MD, is a professor of Emergency Medicine and Interim Director of Emergency Medicine at the school of medicine in the University of Colorado and Denver Health, respectively. Dr. Lavonas completed his medical degree at Suny Upstate Medical Center in 1992. His main research interest include envenomation and antidote development


  1. Lavonas EJ, Ruha AM, Banner W, Bebarta V, Bernstein JN, Bush SP, et al.; Rocky Mountain Poison and Drug Center, Denver Health and Hospital Authority. Unified treatment algorithm for the management of crotaline snakebite in the United States: results of an evidence-informed consensus workshop. BMC Emerg Med. 2011 Feb 3;11:2.
  2. Lavonas EJ, Kokko J, Schaeffer TH, Mlynarchek SL, Bogdan GM, Dart RC. Short-term outcomes after Fab antivenom therapy for severe crotaline snakebite.Ann Emerg Med. 2011 Feb;57(2):128-137.e3.
  3. Lavonas EJ, Schaeffer TH, Kokko J, Mlynarchek SL, Bogdan GM. Crotaline Fab antivenom appears to be effective in cases of severe North American pit viper envenomation: an integrative review. BMC Emerg Med. 2009 Jun 22;9:13.
  4. Lavonas EJ, Gerardo CJ, O'Malley G, Arnold TC, Bush SP, Banner W Jr, Steffens M, Kerns WP 2nd. Initial experience with Crotalidae polyvalent immune Fab (ovine) antivenom in the treatment of copperhead snakebite. Ann Emerg Med. 2004 Feb;43(2):200-6.

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