Relationship of venom effects to venom antigen and antivenom serum concentrations in a patient with Crotalus atrox envenomation treated with a Fab antivenom

Study objective: To described the association among venom antigenemia, serum antivenom concentrations, and Venom effects in a 53-year-old woman who was bitten by a Western diamondback rattlesnake (Crotalus atrox). Methods: The patient was enrolled in a multicenter trial of an investigational Fab antivenom. Her clinical condition and coagulation parameters were monitored for 2 weeks after the bite. Results: After antivenom administration, the progression of the venom's effects was arrested. The antivenom reversed some local venom effects, caused venom antigens to disappear from the blood, and resolved the patient's profound thrombocytopenia (before antivenom, 12,000/mm(3); 1 hour after antivenom, 227,000/mm(3)). Local venom effects recurred twice in the 24 hours after antivenom administration but were easily managed with additional Fab antivenom. Venom antigenemia was detected on days 5 and 8 after the initial treatment and was accompanied in one instance by the new onset of hypofibrinogenemia (119 mg/L) that resolved spontaneously and in both instances by renewed profound thrombocytopenia. Repeat Fab antivenom doses on days 6 and 9 were followed by increases in platelet count (from 16,000 to 40,000/mm(3) and from 11,000 to 20,000/mm(3), respectively) and by the reduction or disappearance of venom antigenemia. The patient sustained no significant bleeding complications, and all laboratory values had returned to normal 2 weeks after the bite. Conclusion: initial control of local symptoms and coagulopathy was prompt after the administration of Fab antivenom. Repeat doses during the 24 to 36 hours after a bite may be necessary for local control. Recrudescence of coagulopathy was likely due in part to renewed venom antigenemia after clearance of Fab antivenom. The role of Fab antivenom in the treatment of recurrent coagulopathy requires further study.