4Ts Score Calculator for Suspected HIT
Estimate the 4Ts pretest probability category for heparin-induced thrombocytopenia from platelet fall, timing, thrombosis findings, and competing causes.
Clinical inputs
Percentage fall from the relevant baseline or peak platelet count.
Lowest platelet count observed during the episode being assessed.
Choose the best-fitting timing pattern for the platelet fall.
Use the most specific documented thrombosis or related sequelae category.
Judge whether another explanation for thrombocytopenia is absent, possible, or definite.
Live result
4Ts score
7 / 8
High pretest probability category; reference-style estimate ≈64%.
Probability band
High
Reference probability estimate
≈64%
Thrombocytopenia points
2
Remaining 3Ts points
5
This score is a pretest clinical assessment, not a diagnosis. Missing or uncertain clinical information can change the category, and clinical judgment plus appropriate laboratory evaluation remain essential.
4Ts scoring breakdown
| Criterion | Current finding | Points |
|---|---|---|
| Thrombocytopenia | 55% fall with nadir 80 ×10⁹/L | 2 |
| Timing | Clear onset 5 – 10 days, or day 1 with heparin exposure 5 – 30 days before | 2 |
| Thrombosis | New confirmed thrombosis, skin necrosis, anaphylaxis, or adrenal hemorrhage | 2 |
| Other causes | Possible | 1 |
Score bands: 0 – 3 low, 4 – 5 intermediate, and 6 – 8 high pretest probability. The table and Excel workbook are rebuilt from the same current-state model.
How to use this 4Ts score calculator
What this calculator does
The 4Ts score is a structured pretest probability tool used when heparin-induced thrombocytopenia (HIT) is clinically suspected. It adds points from four domains: the magnitude of thrombocytopenia, the timing of the platelet fall, thrombosis or related clinical sequelae, and whether another cause of thrombocytopenia is present. This calculator converts the five controls above into those four component scores, a total from 0 to 8, and the corresponding low, intermediate, or high pretest probability band. It does not diagnose HIT, replace antibody or functional testing, or determine treatment for an individual patient. The American Society of Hematology HIT guideline explains where the 4Ts score fits in the broader diagnostic process.
When to use it
Use the calculator to standardize a suspected-HIT assessment before ordering or interpreting HIT laboratory tests, to document why a patient falls into a particular pretest category, to re-score when platelet trends or thrombosis findings change, or to review a case for education or quality improvement. Because the score depends on clinical context, uncertainty about exposure timing or competing causes should be resolved from the record whenever possible rather than guessed.
How to calculate
- The calculator opens with a complete demonstration case: a 55% platelet fall, nadir 80 ×10⁹/L, a clear 5 – 10 day timing pattern, a qualifying thrombosis/sequelae finding, and a possible alternative cause. The example produces 7/8 and an immediately available Excel workbook.
- Replace each demonstration value with the case being assessed. Results update live; there is no separate Calculate button.
- Read the total score first, then confirm the component points in the scoring breakdown so that a borderline category can be traced to its underlying finding.
- Select Download Excel to export the current validated controls and results as an XLSX workbook. Reset clears the demonstration and all calculated output; after Reset, Download Excel stays disabled until every required field has a complete valid value again.
Input guide
Platelet count fall is required and accepts a percentage from 0 to 100, using a decimal point for fractions; “55” or “55%” are valid examples. It represents the percentage decrease, not the percentage remaining. A larger fall can increase the thrombocytopenia component, but the nadir is evaluated at the same time. Platelet nadir is required in ×10⁹/L and accepts a nonnegative decimal value; 80 is the demonstration value. It is the lowest platelet count in the relevant episode, not the current count if the count has already recovered. A nadir below 10 ×10⁹/L forces the thrombocytopenia component to 0 points under this scoring model.
Timing is required and is a categorical selection. Choose the 2-point pattern for clear onset 5 – 10 days after heparin exposure or a day-1 fall with exposure 5 – 30 days earlier; choose the 1-point pattern for an unclear 5 – 10 day onset, onset after 10 days, or day-1 fall with exposure 31 – 100 days earlier; choose the 0-point pattern for an early fall without recent exposure. Thrombosis is required and assigns 2, 1, or 0 points based on the listed confirmed, possible/recurrent, or absent findings. Other possible causes is required: none apparent scores 2, possible scores 1, and definite scores 0. A common error is treating “possible” as “none” simply because an alternative diagnosis has not yet been proven.
Output guide
4Ts score is the exact sum of the four component scores and ranges from 0 to 8. Probability band maps 0 – 3 to low, 4 – 5 to intermediate, and 6 – 8 to high pretest probability. Reference probability estimate displays approximately 5%, 14%, or 64% to mirror the reference calculator's category estimates; these figures should not be treated as patient-specific diagnostic probabilities because predictive values vary with population and prevalence. Thrombocytopenia points shows the combined contribution from platelet fall and nadir, while Remaining 3Ts points sums timing, thrombosis, and other causes. The 4Ts scoring breakdown table lists each criterion, the current finding, and its points so the total can be audited.
Worked example
With the opening values, a 55% platelet fall plus a nadir of 80 ×10⁹/L meets the 2-point thrombocytopenia rule. The selected clear 5 – 10 day timing contributes 2 points, the selected new confirmed thrombosis or equivalent sequelae category contributes 2, and “Possible” other causes contributes 1. The total is therefore 2 + 2 + 2 + 1 = 7/8, which falls in the High band. This same 7-point result is used in the first rendered result panel, the breakdown table, and the startup XLSX workbook.
Learn more
The ASH pocket guide for diagnosis and management of HIT reproduces the 4Ts criteria and emphasizes obtaining accurate information because missing data can produce an incorrect score. It also notes that the 4Ts is a clinical guide rather than a substitute for judgment.
How the 4Ts scoring rules work
The thrombocytopenia component uses both the percentage fall and the nadir. Two points require a fall greater than 50% with a nadir of at least 20 ×10⁹/L. One point applies when the fall is 30 – 50% or the nadir is 10 – 19.9 ×10⁹/L, provided the case does not already meet a 0-point condition. Zero points applies when the fall is below 30% or the nadir is below 10 ×10⁹/L. Timing, thrombosis/sequelae, and competing causes are selected directly from the scoring categories shown in the controls.
The total is best understood as a screening and triage aid. The systematic review and meta-analysis of the 4Ts score indexed by PubMed reported a very high negative predictive value for a low score, while intermediate and high scores had much weaker positive predictive values and therefore required further evaluation. That distinction is why a high score should not be read as a confirmed diagnosis.
Interpretation and common pitfalls
Two details commonly change the result. First, the platelet fall is a relative decrease, so the same nadir can score differently when the starting platelet count differs. Second, the timing category depends on prior heparin exposure as well as the current episode. Incomplete medication history can therefore shift the timing score. Competing causes deserve the same care: infection, surgery, disseminated intravascular coagulation, other drugs, transfusion-related effects, and immune thrombocytopenia may provide alternative explanations depending on the case.
Recalculate when the clinical picture changes. New confirmation of thrombosis, clarification of an earlier heparin exposure, or a newly established alternative cause can move the score across a band threshold. The American Society of Hematology notes that HIT is a prothrombotic adverse drug reaction and that its guideline recommendations address diagnosis and initial management; consult the current ASH HIT guideline page for the surrounding clinical recommendations rather than using this calculator as a treatment protocol.