What does a Bleeding Time Clotting Test (BT CT) indicate in a patient with a snake bite?

Bleeding Time Clotting Test (BT CT) in Snake Bite

The 20-minute Whole Blood Clotting Test (WBCT20) is a critical bedside diagnostic tool that detects venom-induced consumption coagulopathy (VICC) in snake bite patients, with a sensitivity of 82-94% and specificity of 76-98%, and should be performed immediately upon presentation to guide antivenom administration decisions. 1, 2

What the Test Indicates

The WBCT20 directly assesses whether snake venom has caused consumption coagulopathy by observing whether whole blood forms a clot within 20 minutes. 3, 1

• Positive test (no clot formation at 20 minutes): Indicates venom-induced consumption coagulopathy requiring antivenom administration 1, 2
• Negative test (normal clot formation): Suggests intact coagulation, though does NOT completely rule out envenomation 1, 2

Clinical Performance Characteristics

The test demonstrates robust diagnostic utility when performed correctly:

• Sensitivity ranges from 82-94%, meaning it correctly identifies most patients with coagulopathy but misses approximately 6-18% of cases 1, 2
• Specificity ranges from 76-98%, with false positive rates of 2-24% depending on the study population 1, 2
• The test is most reliable for pit viper envenomations (rattlesnakes, copperheads, cottonmouths) which cause cytotoxic effects including coagulopathy 4

Critical Testing Protocol

Proper technique is essential for accuracy:

• Use exactly 1 mL of fresh whole blood in a clean, dry 5 mL borosilicate glass tube with 10 mm internal diameter 1
• Perform the test at bedside immediately upon patient arrival 3, 5
• Read results at BOTH 20 and 30 minutes for enhanced sensitivity—discrepancies between readings occur in 82% of coagulopathic patients and improve detection of evolving or resolving coagulopathy 6
• Alternative containers (syringe or ceftriaxone bottle) show 84.7-86.8% accuracy when laboratory testing is unavailable 5

Clinical Decision-Making Algorithm

For patients with positive WBCT20 AND clinical signs of envenomation (swelling, pain, ecchymosis, hypotension):

• Administer antivenom immediately—this is the definitive treatment 4, 7

For asymptomatic patients with positive WBCT20 but NO clinical signs:

• Confirm with PT/INR (cutoff ≥1.4) before administering antivenom to avoid unnecessary treatment, as false positive rate can reach 24% 2

For patients with negative WBCT20:

• Do NOT assume no envenomation—the test misses up to 18% of coagulopathy cases 1
• Continue clinical monitoring for at least 12-24 hours, as neurotoxic symptoms can be delayed up to 13 hours despite minimal local findings 8
• Repeat WBCT20 at 4 and 12 hours to detect delayed coagulopathy 2

Serial Monitoring Strategy

After antivenom administration:

• Repeat WBCT20 at both 20 and 30 minutes every 4-6 hours to monitor treatment response 6
• Discrepancies between 20 and 30-minute readings most commonly occur during three critical phases: initial normalization after antivenom, detection of secondary coagulopathy resumption, or final restoration of hemostasis 6
• A positive test after initial normalization indicates need for additional antivenom doses 6, 2

Critical Pitfalls to Avoid

Never delay antivenom administration in clinically evident envenomation while waiting for WBCT20 results—the test guides therapy but should not delay life-saving treatment when clinical signs are obvious 4

Do not rely on WBCT20 alone for coral snake bites—these neurotoxic envenomations cause minimal tissue injury and coagulopathy, requiring different monitoring for respiratory compromise 4, 8

Avoid using WBCT20 as the sole criterion for antivenom in asymptomatic patients—confirm with PT/INR to prevent unnecessary antivenom administration given the 24% false positive rate 2

Practical Advantages

The WBCT20 is particularly valuable in resource-limited settings:

• Requires only a clean glass tube and venous blood—no laboratory equipment needed 3, 5
• Results available within 20-30 minutes at bedside 3, 1
• Inexpensive and deployable in isolated hospitals where 95% of snakebites occur 3, 6
• Demonstrates excellent inter-observer reliability with no significant variability between trained observers 2