4T Score Calculator | HIT
Assess the probability of Heparin-Induced Thrombocytopenia (HIT) using the 4T scoring system
4T Score Assessment
1. Thrombocytopenia
2. Timing of Platelet Count Fall
3. Thrombosis or Other Sequelae
4. Other Causes of Thrombocytopenia
4T Score Results
Score Breakdown
Clinical Assessment
Low Risk - ~5% probability
HIT is unlikely. Consider alternative diagnoses for thrombocytopenia.
Recommendation: Functional assays for HIT antibodies are not recommended. Consider other causes of thrombocytopenia.
⚠️ Medical Disclaimer: This calculator is for educational purposes only. The 4T score aids in clinical assessment but cannot substitute for clinical judgment. Always consult healthcare professionals and consider laboratory testing for definitive HIT diagnosis.
Clinical Examples
High Risk Case (Score: 7/8)
• Thrombocytopenia: 60% fall, nadir 30×10⁹/L (2 points)
• Timing: Clear onset day 7 of heparin (2 points)
• Thrombosis: New DVT confirmed (2 points)
• Other causes: None evident (2 points) - 1 not selected = 7
Result: High risk (~64% probability) - Stop heparin, consider alternative anticoagulation
Low Risk Case (Score: 2/8)
• Thrombocytopenia: 20% fall, nadir 60×10⁹/L (0 points)
• Timing: Onset day 2 with no prior heparin (0 points)
• Thrombosis: None (0 points)
• Other causes: Recent major surgery (1 point) - 1 not selected = 2
Result: Low risk (~5% probability) - HIT unlikely, consider other causes
HIT Risk Categories
Low Risk (0-3 points)
~5% probability
HIT unlikely
Intermediate Risk (4-5 points)
~14% probability
Consider testing
High Risk (6-8 points)
~64% probability
Stop heparin
What is HIT?
HIT Type 1
Non-immunological, mild thrombocytopenia, good prognosis
HIT Type 2
Immunological, severe thrombocytopenia, risk of thrombosis
Key Features
- • Antibody-mediated platelet activation
- • Paradoxical thrombosis risk
- • Requires heparin cessation
Other Causes of Thrombocytopenia
HIT Testing
Screening Tests
- • ELISA (Anti-PF4/heparin antibodies)
- • High sensitivity, lower specificity
- • First-line screening
Functional Tests
- • Platelet aggregation test
- • Serotonin release assay (SRA)
- • Higher specificity
- • Confirmatory testing
Understanding the 4T Score
4T Score Components
The 4T score is a clinical prediction rule that assesses the probability of Heparin-Induced Thrombocytopenia (HIT) based on four key clinical features, each starting with the letter "T".
Clinical Significance
- •Helps clinicians decide whether to pursue HIT testing
- •Guides decisions about stopping heparin therapy
- •Reduces unnecessary testing in low-probability patients
- •Aids in risk stratification for alternative anticoagulation
The Four T's
1. Thrombocytopenia
Severity and extent of platelet count fall
2. Timing
When the platelet count began to fall
3. Thrombosis
New blood clots or related complications
4. Other causes
Alternative explanations for thrombocytopenia