4Ts Score Calculator (heparin-induced thrombocytopenia pretest probability)

Understanding the 4Ts score calculator — concept and principle

This calculator adds up the 4Ts score, a bedside tool that estimates how likely heparin-induced thrombocytopenia (HIT) is before laboratory testing. It scores four features from 0 to 2 and gives a total of 0 to 8 with a low, intermediate or high pretest probability. Doctors, pharmacists and nurses caring for patients on heparin use it to decide whether HIT testing is needed.

Four T’s, 0–2 points each — with the default inputs: 5 points.

How it is calculated

Each feature scores 0, 1 or 2; the total is their sum.

Thrombocytopenia: platelet fall over 50% with a lowest count of 20 ×10⁹/L or more (2); fall of 30–50% or lowest 10–19 (1); fall under 30% or lowest under 10 (0)
Timing: clear onset on days 5–10, or within 1 day with heparin in the past 30 days (2); likely but unclear, after day 10, or within 1 day with heparin 30–100 days ago (1); fall within 4 days without recent heparin (0)
Thrombosis or other sequelae: new thrombosis, skin necrosis or acute reaction after an IV bolus (2); progressive or suspected thrombosis or red skin lesions (1); none (0)
oTher causes: none apparent (2); possible (1); definite (0)

Bands: 0–3 low, 4–5 intermediate, 6–8 high.

The principle

HIT is an immune reaction in which antibodies against complexes of heparin and platelet factor 4 activate platelets, lowering the platelet count and raising the risk of clots. It typically appears 5 to 10 days after heparin is started, or sooner after recent exposure. Lo and colleagues (2006) combined these typical features into a simple score. A low score makes HIT unlikely; intermediate and high scores are usually followed by an anti-PF4 antibody test and, where needed, a functional test.

Worked example

Defaults: fall over 50% with lowest count at least 20 (2); clear onset on days 5–10 (2); no thrombosis (0); another cause possible (1).

Score = 2 + 2 + 0 + 1 = 5.
5 lies in the 4–5 band.
Pretest probability group: Intermediate probability.

How to read the result

Where does your result fall?

A low score is the most useful result, because studies found it has a high negative predictive value. An intermediate or high score does not confirm HIT; it means laboratory testing and specialist advice are warranted. Decisions about stopping heparin or starting another anticoagulant are clinical and are made by the treating team.

Common mistakes

Counting from the wrong day; timing starts at the first day of the current heparin exposure.
Missing recent heparin exposure in the past 100 days.
Treating the score as a diagnosis.

Keep in mind: This is not a diagnosis and not medical advice. HIT is a medical emergency; only the treating clinicians can interpret the score with laboratory results.

Frequently asked questions

What does a 4Ts score of 5 mean?

Intermediate pretest probability of HIT; laboratory testing is usually recommended.

Can HIT be ruled out with a low score?

A low score (0–3) makes HIT unlikely in most settings, but clinicians still use judgement.

Why is timing so important?

HIT antibodies usually take about 5–10 days to form after first exposure to heparin.

Sources: Lo GK, Juhl D, Warkentin TE, et al., "Evaluation of pretest clinical score (4 T's) for the diagnosis of heparin-induced thrombocytopenia in two clinical settings", J Thromb Haemost 2006;4:759–765 · World J Crit Care Med 2015;4(3):202–212, Table 1

Checked 9 Nov 2026. Lo GK, Juhl D, Warkentin TE, et al., “Evaluation of pretest clinical score (4 T’s) for the diagnosis of heparin-induced thrombocytopenia in two clinical settings”, J Thromb Haemost 2006;4:759–765 — criteria and bands as tabulated in World J Crit Care Med 2015;4(3):202–212, Table 1.

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