What labs should be monitored in a patient with a snake bite?

Laboratory Monitoring in Snake Envenomation

In snake envenomation cases, comprehensive laboratory monitoring should include complete blood count, coagulation studies (PT/INR, aPTT, fibrinogen), renal function tests, and electrolytes to assess for hemolysis, coagulopathy, and organ dysfunction. 1

Initial Laboratory Assessment

• Complete Blood Count (CBC):

  • Hemoglobin/hematocrit: To assess for anemia from hemolysis
  • Platelet count: To monitor for thrombocytopenia
  • White blood cell count: To evaluate inflammatory response

• Coagulation Studies:

  • Prothrombin Time (PT)/International Normalized Ratio (INR)
  • Activated Partial Thromboplastin Time (aPTT)
  • Fibrinogen levels: Often markedly decreased in envenomation
  • 20-minute Whole Blood Clotting Test (20WBCT): Bedside test recommended by WHO for diagnosis of significant coagulopathy 2

• Chemistry Panel:

  • Electrolytes
  • Blood Urea Nitrogen (BUN) and Creatinine: To assess renal function
  • Liver function tests (AST, ALT)
  • Creatine kinase: To evaluate for rhabdomyolysis

Timing and Frequency of Monitoring

1. Initial labs: Immediately upon arrival at emergency department
2. Follow-up labs: Every 6 hours for the first 24 hours 3
3. Extended monitoring: Continue monitoring until laboratory values normalize

Specific Tests Based on Clinical Presentation

For Coagulopathy Assessment

• Fibrin degradation products (FDPs): May peak at extremely high levels (mean of 7,680 μg/ml reported in Eastern diamondback rattlesnake bites) 3
• D-dimer: To assess for fibrinolysis
• Thrombin time: Values ≥120 seconds indicate incoagulable blood 3

For Hemolysis Evaluation

• Peripheral blood smear: To look for schistocytes (>1.0% indicates thrombotic microangiopathy) 4
• Lactate dehydrogenase (LDH)
• Haptoglobin
• Free hemoglobin

For Renal Function Assessment

• Urinalysis: To check for hematuria, myoglobinuria
• Serum creatinine and BUN: Serial measurements to monitor for acute kidney injury

Diagnostic Accuracy of Coagulation Tests

• INR >1.155 has a sensitivity of 78.5% and specificity of 90.3% for detecting hypofibrinogenemia 2
• 20WBCT has sensitivity of 81.0% and specificity of 90.3% for detecting hypofibrinogenemia 2
• Both 20WBCT and INR are superior to aPTT for assessing coagulation abnormalities in viper bites 2, 5

Monitoring for Specific Complications

Thrombotic Microangiopathy

• Diagnostic criteria: Anemia with >1.0% schistocytes on blood film, plus thrombocytopenia (<150 × 10⁹/L) or relative decrease in platelet count >25% from baseline 4
• Monitor for acute kidney injury which may require dialysis

Venom-Induced Consumption Coagulopathy

• Characterized by prolonged PT/INR, decreased fibrinogen
• May not show classic DIC pattern (platelet counts and antithrombin III levels may remain relatively normal) 3

Common Pitfalls and Considerations

1. Anticoagulant Interference: Heparin in samples may affect coagulation tests; use heparin neutralizers when appropriate 6

2. Timing Considerations: The magnitude and duration of coagulopathy are proportional to the clinical severity of envenomation 3

3. Antivenom Effect: Treatment with antivenom can blunt the coagulopathy, which should be considered when interpreting laboratory results 3

4. Long-term Monitoring: Patients are at risk for chronic kidney disease following thrombotic microangiopathy, requiring long-term follow-up 4

By systematically monitoring these laboratory parameters, clinicians can effectively assess the severity of envenomation, guide antivenom administration, and detect complications early to improve patient outcomes.