What other tests can be done, aside from whole blood clotting time, in cases of vasculotoxic snake bites?

Alternative Tests for Vasculotoxic Snake Bite Coagulopathy

Several specialized coagulation tests beyond whole blood clotting time (WBCT20) can be used to diagnose and monitor coagulopathy in vasculotoxic snake bites, including prothrombin time/INR, D-dimer, thromboelastography, and specific venom-based assays.

Key Diagnostic Tests for Vasculotoxic Snake Envenomation

Standard Laboratory Tests

• Prothrombin Time (PT)/International Normalized Ratio (INR): The most useful investigation in venom-induced consumption coagulopathy (VICC) 1
• D-dimer: Assists in early diagnosis of VICC 1
• Fibrinogen levels: May be measured but often add little clinical value in the acute setting 1
• Complete coagulation screen: To detect disseminated intravascular coagulation (DIC) pattern 2

Advanced Coagulation Tests

• Thromboelastography (TEG): Provides whole blood assessment of all portions of coagulation including initiation, propagation, and clot maintenance; may reveal fibrinolytic processes earlier than traditional monitoring 3
• Russell's viper venom time (RVVT): Uses Russell's viper venom as an activator to initiate coagulation and assess the clotting pathway 4
• Assays based on FX activation: Using snake venom (Russell's viper venom) to activate Factor X and initiate coagulation 4

Field Tests When Laboratory Testing Unavailable

• Syringe method: Using a plain syringe to observe clot formation within 20 minutes (sensitivity 88.9%, specificity 82.4%) 5
• Bottle method: Using a ceftriaxone bottle to observe clot formation within 20 minutes (sensitivity 83.3%, specificity 90.0%) 5

Limitations of WBCT20 and Need for Alternative Tests

The 20-minute whole blood clotting test (WBCT20) has limitations:

• Sensitivity of only 82% in detecting VICC in Russell's viper bites 6
• Misses approximately one-fifth of cases where antivenom might be indicated 6
• Point-of-care testing INR and whole blood clotting tests have been shown to be unreliable in VICC 1

Specialized Tests Using Snake Venoms

Several diagnostic tests utilize snake venoms as reagents:

• Noscarin and RVV-V assay: Uses Australian eastern tiger snake venom (Notechis scutatus) and Russell's viper venom to activate Factor V 4
• FXa-based assay: Uses purified Factor Xa with phospholipids to assess coagulation 4
• Southern copperhead venom test: Uses Agkistrodon contortrix contortrix venom to activate endogenous protein C 4

Practical Approach to Testing

1. Initial assessment: Perform PT/INR and D-dimer if laboratory facilities available
2. If laboratory unavailable: Use bedside clotting tests (syringe or bottle method)
3. For comprehensive assessment: Consider thromboelastography if available
4. For monitoring recovery: Serial PT/INR measurements

Common Pitfalls and Caveats

• Bedside tests should be performed by trained personnel to ensure accuracy 6
• Use standardized glass tubes (borosilicate) with specific dimensions (5 ml with 10 mm internal diameter) for WBCT20 6
• Anticoagulant interference may affect test results; proper sample preparation is essential 7
• Repeat testing may be necessary as coagulopathy can develop or worsen over time

The choice of test depends on available resources, clinical setting, and the specific snake species involved in the envenomation. In resource-limited settings, bedside methods provide reasonable accuracy when performed correctly, while comprehensive laboratory testing should be utilized when available.