Anti-Snake Venom Dosing and Monitoring
Immediate Pre-Hospital Management
Transport the victim to a medical facility immediately, as antivenom is the cornerstone of treatment and is only available in healthcare settings 1. While en route:
- Rest and immobilize the bitten extremity below heart level to minimize systemic venom absorption 1
- Remove all rings, watches, and constricting jewelry before swelling develops to prevent ischemic injury 1
- Minimize patient exertion during transport, as physical activity increases venom absorption through the lymphatic system 1
Do NOT waste time with ineffective interventions: no ice, no suction devices, no tourniquets, no electric shock, and no pressure immobilization bandaging for North American snakebites 1. These measures are either ineffective or potentially harmful with cytotoxic venoms 1.
Hospital-Based Antivenom Dosing
Initial Assessment and Dosing Decision
Administer antivenom to patients who develop pain, swelling, ecchymoses, systemic symptoms, or abnormal laboratory findings within 30 minutes to one hour of the bite 2. Antivenom therapy is most effective when given within four hours of the snakebite 2.
Dosing Protocols
The evidence strongly supports low-dose antivenom protocols over traditional high-dose regimens, with comparable efficacy and lower complication rates 3, 4. Based on the most recent comparative trial:
- Initial dose: 70 mL (approximately 7 vials) of polyvalent ASV 5
- Subsequent doses: 30 mL (approximately 3 vials) every 6 hours 5
- Continue until two consecutive 20-minute Whole Blood Clotting Time (20WBCT) tests are negative 5
The 2020 randomized trial comparing India's National protocol (100 mL initial dose followed by 100 mL every 6 hours) versus the lower-dose protocol (70 mL followed by 30 mL every 6 hours) found no statistical difference in mortality, morbidity, or total ASV required 5. Multiple studies from 2013-2014 demonstrated that low-dose regimens (average 4-7 vials total) achieved comparable outcomes to high-dose protocols with lower costs and fewer adverse effects 3, 4.
Pre-Administration Requirements
Perform hypersensitivity testing before administering antivenom 2. Have resuscitation equipment and medications immediately available for potential anaphylactic reactions 2.
Monitoring Protocol
Laboratory Monitoring
- 20-minute Whole Blood Clotting Time (20WBCT) every 6 hours: This is the primary endpoint for determining when to stop antivenom administration 5
- Baseline and serial complete blood counts, coagulation studies, renal function tests, and electrolytes 2
- Monitor for coagulopathy, thrombocytopenia, and hemolysis 2
Clinical Monitoring
- Measure and mark the leading edge of swelling/edema every 30-60 minutes to assess progression 2
- Continuous cardiac monitoring for hypotension and shock 6
- Neurological assessments every 1-2 hours for signs of neurotoxicity: ptosis, difficulty swallowing, muscle weakness, respiratory distress 6
- Urine output monitoring for acute kidney injury (occurs in 10-20% of cases) 4, 5
Duration of Monitoring
- Minimum 24-48 hours of hospitalization with continuous monitoring, as complications can develop or progress during this period 7
- For coral snake bites specifically, prolonged observation is essential as neurotoxicity onset may be delayed up to several hours despite minimal local findings 6
Special Considerations for Neurotoxic Envenomation
Neurotoxic signs from krait and cobra bites respond slowly and may require continuous ASV administration 8. Recurrent neurotoxicity can occur from continued venom absorption, necessitating higher cumulative doses and prolonged treatment 8. Some cases have required exceptionally high doses for reversal of neurological manifestations 8.
Critical Pitfalls to Avoid
- Do not assume no envenomation based on lack of immediate symptoms—coral snake and sea snake bites cause life-threatening systemic toxicity despite minimal local findings 7, 6
- Do not stop monitoring after initial improvement—in 12% of patients, clotting time remains persistently prolonged despite antivenom administration 5
- Do not delay antivenom administration for extensive testing—time to antivenom is the most important prognostic factor 7
- Failing to provide supportive care (plasma expanders, pain medication, tetanus prophylaxis, antibiotics as indicated) alongside antivenom therapy 2