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Table 2 Definitions of Envenomation Severity from the FabAV Prescribing Information[3]

From: Crotaline Fab antivenom appears to be effective in cases of severe North American pit viper envenomation: An integrative review

Envenomation Category Definition
Minimal Swelling, pain, and ecchymosis limited to the immediate bite site;
Systemic signs and symptoms absent;
Coagulation parameters normal with no clinical evidence of bleeding.
Moderate Swelling, pain, and ecchymosis involving less than a full extremity or, if bite was sustained on the trunk, head or neck, extending less than 50 cm;
Systemic signs and symptoms may be present but not life threatening, including but not limited to nausea, vomiting, oral paresthesia or unusual tastes, mild hypotension (systolic blood pressure >90 mmHg), mild tachycardia (heart rate <150), and tachypnea;
Coagulation parameters may be abnormal, but no clinical evidence of bleeding present. Minor hematuria, gum bleeding and nosebleeds are allowed if they are not considered severe in the investigator's judgment.
Severe Swelling, pain, and ecchymosis involving more than an entire extremity or threatening the airway;
Systemic signs and symptoms are markedly abnormal, including severe alteration of mental status, severe hypotension, severe tachycardia, tachypnea, or respiratory insufficiency;
Coagulation parameters are abnormal, with serious bleeding or severe threat of bleeding.