4T Score Calculator | HIT

Add the four Ts - thrombocytopenia, timing, thrombosis, and other causes - to estimate the pretest probability of heparin-induced thrombocytopenia.

Quick Facts

Scoring
Four categories, 0-2 points each (total 0-8)
The four Ts: Thrombocytopenia, Timing, Thrombosis, and oTher causes.
Interpretation
0-3 low, 4-5 intermediate, 6-8 high probability
A low score has a high negative predictive value for HIT.

Your Results

Calculated
4T Score
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Total points (0-8)
Pretest probability
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Risk category
Estimated HIT risk
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Approximate pooled likelihood
Score breakdown
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Points per T category

Ready

Select each category, then calculate the 4Ts score.

About the 4Ts Score for HIT

The 4Ts score is a bedside clinical prediction tool that estimates the pretest probability of heparin-induced thrombocytopenia (HIT) when a patient's platelet count falls during or after heparin exposure. It was introduced by Warkentin and later validated by Lo and colleagues. This calculator adds the four components and reports the total, the probability category, and an approximate likelihood of HIT. It is intended for educational and informational use; always review the result with a qualified clinician before making any anticoagulation decision.

The four Ts

Each category is scored 0, 1, or 2 points, and the points are added to give a total from 0 to 8:

  • Thrombocytopenia - the size of the platelet count fall and the nadir. A fall greater than 50% with a nadir of at least 20 ×10⁹/L scores 2.
  • Timing - when the platelet count fell relative to heparin. A clear onset between days 5 and 10 (or a fall within one day given heparin in the previous 30 days) scores 2.
  • Thrombosis - new thrombosis or other sequelae. A new confirmed thrombosis, skin necrosis, or acute systemic reaction after an intravenous heparin bolus scores 2.
  • oTher causes - whether another explanation for the low platelets exists. If none is apparent this scores 2, while a definite alternative cause scores 0.

Interpreting the total

  • 0-3 points, low probability. HIT is unlikely; a low score has a high negative predictive value.
  • 4-5 points, intermediate probability. HIT cannot be excluded on the score alone.
  • 6-8 points, high probability. HIT is likely, pending laboratory confirmation.

The approximate pooled likelihoods often cited for these bands are roughly 1.6% for a low score, 14% for an intermediate score, and 64% for a high score. The 4Ts score does not diagnose HIT: an intermediate or high score is typically followed by laboratory testing, such as an immunoassay for anti-PF4/heparin antibodies and, where needed, a functional assay.

Important disclaimer

This tool does not constitute medical advice and does not diagnose or manage HIT. Do not use it to start, stop, or change anticoagulation without consulting a licensed healthcare provider.

Frequently Asked Questions

What is the 4Ts score used for?
It estimates the pretest probability of heparin-induced thrombocytopenia (HIT). Four categories - Thrombocytopenia, Timing, Thrombosis, and oTher causes - are each scored 0 to 2 and summed to a total of 0 to 8, which maps to low, intermediate, or high probability.
How is the total score interpreted?
A total of 0 to 3 is low probability, 4 to 5 is intermediate, and 6 to 8 is high. Approximate pooled likelihoods of HIT are about 1.6% for a low score, 14% for an intermediate score, and 64% for a high score.
Does a low 4Ts score rule out HIT?
A low score of 0 to 3 has a high negative predictive value, so HIT is unlikely, but the score is a clinical estimate rather than a diagnostic test. Confirmatory laboratory testing is used when suspicion remains or the score is intermediate or high.
Why does having no other cause score the most points?
The fourth T, oTher causes, asks whether something besides heparin could explain the low platelets. When no alternative cause is apparent, HIT is more likely, so that answer scores 2 points, while a definite alternative cause scores 0.

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