4T Score for HIT
Clinical scoring system (0-8) estimating pretest probability for Heparin-Induced Thrombocytopenia. Four criteria: Thrombocytopenia, Timing, Thrombosis, oTher causes.
Why This Health Metric Matters
Why: HIT is a serious immune reaction to heparin. Early recognition prevents thrombosis and guides anticoagulation.
How: Score each of four criteria 0-2. Sum determines low, intermediate, or high HIT probability.
- ●0-3 low probability
- ●4-5 intermediate
- ●6-8 high probability
Sample Clinical Scenarios
Click on any scenario to load the values and see the calculated 4T Score.
🔴 High Risk Post-Surgical Patient
Post-cardiac surgery patient with significant platelet drop on day 7 of UFH, new DVT, no other causes
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🟡 Intermediate Risk Medical Patient
Medical patient on LMWH with moderate platelet drop on day 8, possible sepsis contribution
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🟢 Low Risk Early Drop
Patient with platelet drop on day 2, clear other cause (sepsis), no thrombosis
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⚡ Rapid Onset Re-exposure
Patient with prior heparin exposure within 30 days showing rapid platelet drop within 24 hours
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⚠️ Skin Necrosis at Injection Site
Patient developing skin necrosis at heparin injection site with moderate platelet drop
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Enter Clinical Data
Required4T Score Criteria
Patient Demographics (Optional)
Heparin Exposure Details
Platelet Count Data
Additional Risk Factors
⚠️For informational purposes only — not medical advice. Consult a healthcare professional before acting on results.
🏥 Health Facts
Score 6-8 = high HIT probability
— Validation studies
Classic onset 5-10 days
— HIT guidelines
What is the 4T Score for HIT?
The 4T Score is a validated clinical scoring system used to estimate the pretest probability of Heparin-Induced Thrombocytopenia (HIT), a serious immune-mediated adverse drug reaction to heparin. HIT occurs when antibodies form against complexes of platelet factor 4 (PF4) and heparin, leading to platelet activation, thrombocytopenia, and paradoxically, an increased risk of thrombosis.
T1: Thrombocytopenia
Evaluates the degree and nadir of platelet count fall. Greater than 50% drop with nadir above 20 is most suggestive of HIT.
Scoring:
- >50% fall, nadir ≥20: 2 points
- 30-50% fall or nadir 10-19: 1 point
- <30% fall or nadir <10: 0 points
T2: Timing
Assesses when platelet fall occurred relative to heparin exposure. Classic HIT onset is 5-10 days after starting heparin.
Scoring:
- Days 5-10 or rapid re-exposure: 2 pts
- Unclear timing: 1 point
- <4 days, no prior exposure: 0 pts
T3: Thrombosis
Evaluates presence of new thrombosis, skin necrosis, or acute systemic reaction after IV heparin bolus.
Scoring:
- New thrombosis/skin necrosis: 2 pts
- Progressive/suspected: 1 point
- None: 0 points
How to Use the 4T Score Calculator
The 4T Score is calculated by evaluating four clinical criteria, each scored from 0 to 2 points. The total score ranges from 0 to 8 and determines the pretest probability of HIT, guiding subsequent management decisions.
Step-by-Step Calculation
Assessment Process
- 1Assess degree of thrombocytopenia (0-2 points)
- 2Determine timing of platelet fall (0-2 points)
- 3Evaluate thrombosis or other sequelae (0-2 points)
- 4Rule out other causes of thrombocytopenia (0-2 points)
- 5Sum all scores for total 4T Score (0-8)
Risk Stratification
When to Use the 4T Score
The 4T Score should be calculated whenever HIT is suspected in a patient receiving or who has recently received heparin therapy. It is particularly important in certain clinical scenarios where the risk of HIT is elevated.
Post-Surgical Patients
Cardiac, orthopedic, and vascular surgery patients on heparin prophylaxis have elevated HIT risk.
Unexpected Platelet Drop
Any unexplained drop in platelet count >30-50% during heparin therapy warrants 4T Score assessment.
New Thrombosis on Heparin
New venous or arterial thrombosis despite adequate anticoagulation is a red flag for HIT.
Skin Reactions
Skin necrosis at injection sites or erythematous lesions during heparin therapy suggest HIT.
Heparin Re-exposure
Rapid platelet drop within 24 hours of re-exposure suggests pre-existing HIT antibodies.
ICU Monitoring
ICU patients on prolonged heparin therapy should have regular platelet monitoring and 4T assessment if drop occurs.
4T Score Formulas Explained
Total 4T Score Formula
Where T1=Thrombocytopenia, T2=Timing, T3=Thrombosis, T4=oTher causes (each 0-2 points)
Platelet Fall Calculation
Used to determine T1 (Thrombocytopenia) score component
HIT Probability by Score
Low: <5%
Intermediate: 14-64%
High: >64%