4Ts score — HIT pretest probability

Sum four domains (each 0–2): Thrombocytopenia, Timing, Thrombosis or sequelae, and oTher causes. Total 0–8. Educational aid—not a substitute for diagnosis, serology, or specialist management.

T — Thrombocytopenia

Compare current platelet count to baseline before heparin (or the highest count on heparin if no pre-heparin value). Use the option that best matches magnitude of fall and nadir.

Platelet count fall >50% and platelet nadir ≥20 × 10⁹/L (≥20,000/µL) (+2)Platelet count fall 30–50% or platelet nadir 10–19 × 10⁹/L (10,000–19,000/µL) (+1)Platelet count fall <30% or platelet nadir <10 × 10⁹/L (<10,000/µL) (0)

T — Timing of platelet count fall

Day 0 is first heparin exposure. “Recent” prior exposure refers to the window described in each option.

Clear onset between days 5–10 after heparin, or platelet fall ≤1 day with prior heparin exposure within 30 days (+2)Consistent with days 5–10 but not clear (e.g. missing counts); onset after day 10; or fall ≤1 day with prior heparin 30–100 days ago (+1)Platelet count fall before day 5 without recent heparin exposure (0)

T — Thrombosis or other sequelae

Include proven or suspected thrombosis, characteristic skin findings, and acute systemic reactions temporally related to IV unfractionated heparin bolus.

New thrombosis (confirmed); skin necrosis; or acute systemic reaction after intravenous unfractionated heparin bolus (+2)Progressive or recurrent thrombosis; non-necrotizing (erythematous) skin lesions; or suspected but unproven thrombosis (+1)None (0)

T — Other causes of thrombocytopenia

Judge whether alternative explanations (sepsis, drugs, DIC, TTP/HUS, etc.) are absent, possible, or definite.

No other cause for thrombocytopenia apparent (+2)Possible other cause (+1)Definite other cause (0)

Disclaimer: This calculator applies the published 4Ts system for pretest probability of HIT. It does not replace clinical judgment, laboratory testing, or institutional protocols. Reference: Lo et al., J Thromb Haemost 2006; summarized in Blood 2012 (Table 1).

4Ts scoring explained

What is the 4Ts score?

The 4Ts is a clinical prediction rule for the pretest probability of heparin-induced thrombocytopenia (HIT). Each of four domains is scored 0, 1, or 2; the sum ranges from 0 to 8. The mnemonic refers to Thrombocytopenia, Timing of the platelet fall, Thrombosis or other sequelae, and oTher causes of thrombocytopenia.

Published category definitions (0 / 1 / 2)

Category	2 points	1 point	0 points
Thrombocytopenia	Fall >50% and nadir ≥20 × 10⁹/L	Fall 30–50% or nadir 10–19 × 10⁹/L	Fall <30% or nadir <10 × 10⁹/L
Timing	Clear onset days 5–10, or fall ≤1 day with heparin within prior 30 days	Consistent with days 5–10 but unclear; onset after day 10; or fall ≤1 day with heparin 30–100 days ago	Fall before day 5 without recent heparin exposure
Thrombosis / sequelae	New confirmed thrombosis; skin necrosis; acute systemic reaction after IV UFH bolus	Progressive/recurrent thrombosis; non-necrotizing skin lesions; suspected unproven thrombosis	None
Other causes	None apparent	Possible	Definite

Wording aligned with Table 1, Blood 2012 (10.1182/blood-2012-07-443051), summarizing the Lo et al. 4Ts system.

Interpretation bands

Total 0–3

Low pretest probability of HIT in derivation/validation cohorts.

Total 4–5

Intermediate probability — usually prompts further testing and avoidance of heparin while evaluated.

Total 6–8

High probability — manage as suspected HIT pending workup.

Probabilities are population-dependent; always integrate serology (immunoassay ± functional assay), bleeding and thrombosis risk, and whether heparin exposure is truly temporally related.

Clinical caveats

• Postoperative platelet counts often fall physiologically; interpret timing and magnitude in context.
• Delayed-onset and spontaneous HIT presentations may not fit classic day 5–10 windows.
• Low-molecular-weight heparin and unfractionated heparin both carry risk; score heparin exposure as a class per your reference.

Primary reference: Lo GK 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. Summary table: Blood 2012;120(20):4160–4167.

Understanding heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is an immune-mediated prothrombotic disorder triggered by heparin exposure. In classic presentations, IgG antibodies form against platelet factor 4 (PF4)–heparin complexes, activating platelets and promoting a markedly increased risk of arterial and venous thrombosis despite thrombocytopenia. Because HIT is uncommon but potentially catastrophic when missed, clinicians need structured ways to estimate pretest probability before ordering and interpreting laboratory assays.

The 4Ts score is one of the most widely used bedside frameworks for this purpose. It does not diagnose HIT; it organizes the history, examination, and laboratory trajectory into a transparent estimate of how likely HIT is in a given patient, so that testing intensity, anticoagulant choices, and monitoring can align with risk.

What the 4Ts score measures

The score sums four domains, each graded 0, 1, or 2, for a total between 0 and 8. The mnemonic highlights the four themes:

Thrombocytopenia — how large the platelet fall is and how low the nadir becomes.
Timing — whether the platelet count trajectory fits the expected immunologic window after heparin.
Thrombosis or other sequelae — clinically evident thrombosis, characteristic skin lesions, or acute systemic reactions tied to heparin.
oTher causes — whether alternative explanations for thrombocytopenia are absent, possible, or definite.

In validation work, aggregate scores have been grouped into low, intermediate, and high pretest probability strata. Exact positive predictive values vary by setting (surgical versus medical patients, ICU versus ward, prevalence of HIT), so the score should always be read alongside local epidemiology and expert pathways.

Domain 1: Thrombocytopenia

This domain rewards a pattern that is both substantial in relative terms and not so severe that HIT becomes improbable on platelet count alone. HIT typically produces a meaningful drop from the patient’s baseline (or from the peak count observed during heparin therapy when a pre-heparin baseline is unavailable). The scoring tiers contrast:

A large proportional fall with a nadir that remains in a moderate range (consistent with typical HIT platelet nadirs in many cohorts) receives the highest domain score.
An intermediate fall or a lower but not profoundly reduced nadir receives partial credit.
A small relative fall or a very low nadir receives no points in this domain, reflecting patterns less characteristic of classic HIT or more suggestive of alternative diagnoses.

Practical tips: use the same unit system consistently (for example, ×10⁹/L or per µL), document the baseline clearly, and remember that perioperative platelet changes, sepsis, intra-aortic balloon pumps, and massive transfusion can all distort the apparent magnitude of fall.

Domain 2: Timing of the platelet count fall

HIT is classically described as a delayed-onset immune reaction, with platelet counts often beginning to fall roughly five to ten days after heparin is started in a patient without recent exposure. The timing domain therefore asks whether the observed fall aligns with that window in a clear versus plausible but uncertain way, or whether it is too early without recent heparin to fit the usual story.

Important exceptions raise the score even when the calendar does not read “day 7”:

Recent heparin within roughly the prior month can lead to rapid falls on re-exposure because of preformed antibodies; the scoring system allows strong timing points when a fall occurs very soon after heparin in that setting.
When platelet data are sparse or missing, clinicians may only be able to assign partial credit: the trajectory is compatible with days 5–10 but not provably so.
Onset after day 10 is handled as intermediate timing in the published table, reflecting that delayed or atypical courses occur and should not be dismissed solely because they are late.

Electronic health record artifacts (wrong heparin start time, undocumented flushes, line locks, or peri-procedural boluses) are a frequent source of error—reconstructing exposure carefully often changes the assigned timing tier.

Domain 3: Thrombosis or other sequelae

HIT is not “just low platelets.” Many high-risk presentations include new thrombosis (often unusual sites), progressive or recurrent events, or skin findings at injection sites. The highest domain score is reserved for well-documented thrombosis, clear skin necrosis, or a dramatic acute systemic reaction following an intravenous unfractionated heparin bolus—phenomena that strongly raise concern for immune-mediated platelet activation in the heparin context.

Intermediate credit applies when thrombosis is suspected but not yet proven, when lesions are erythematous but not necrotizing, or when thrombotic events appear progressive or recurrent. Absence of these features scores zero in this domain, which does not exclude HIT (some patients have isolated thrombocytopenia) but lowers pretest probability in the aggregate model.

Domain 4: Other causes of thrombocytopenia

This domain forces explicit consideration of competing diagnoses: sepsis, disseminated intravascular coagulation, thrombotic microangiopathies, drug-induced immune thrombocytopenia, alcohol-related marrow suppression, splenic sequestration, hemodilution, major surgery, and many others. When another cause is definite and sufficient, the domain scores zero. When an alternative remains possible but not established, partial credit applies. When no convincing alternative exists, the domain contributes the maximum points.

Intellectual honesty matters here. Labeling a cause “definite” solely to lower the score without a real diagnostic workup undermines the tool’s purpose. Conversely, anchoring on HIT while ignoring a clear septic shock picture misallocates risk.

Interpreting the total score

After summing the four domains, totals are commonly mapped to three bands:

Total 4Ts	Typical interpretation	Practical implications (pathway-dependent)
0–3	Low pretest probability in many published cohorts	Often allows deferral or very selective testing; still revisit if new thrombosis, skin lesions, or clearer timing emerges
4–5	Intermediate probability	Usually warrants immunoassay and careful avoidance of further heparin while results and clinical evolution are weighed; functional testing may be added per local algorithms
6–8	High probability	Manage as suspected HIT pending workup: stop heparin from all routes, choose a non-heparin anticoagulation strategy when indicated, and coordinate with hematology or anticoagulation services

These bands describe probability strata, not certainties. A low score does not absolutely exclude HIT in an unusual case; a high score still requires laboratory confirmation in most systems before long-term labeling.

How the 4Ts score fits with laboratory testing

Modern evaluation usually pairs clinical scoring with anti-PF4/heparin immunoassays (enzyme immunoassay or chemiluminescence platforms, depending on the hospital). Strong immunoassay positivity in a high pretest probability patient is highly suggestive; weak positivity or negativity may require functional assays (serotonin-release assay or heparin-induced platelet activation assays where available) interpreted in expert hands.

Because immunoassays can remain positive after HIT resolves, serologic results must be read in light of the clinical time course and heparin exposure, not as isolated binary answers.

Limitations and situations that test the score

Postoperative and critically ill patients have high baseline rates of thrombocytopenia and thrombosis from non-HIT mechanisms, which can inflate or deflate apparent pretest probability depending on chart details.
Delayed-onset HIT and spontaneous HIT–like syndromes may not follow textbook timing; rigid application without clinical judgment can mislead.
Low-molecular-weight heparin and unfractionated heparin both pose risk; scoring should reflect actual exposures, including low-dose prophylaxis and catheter flushes.
Scores are only as accurate as the data entered—missing platelet counts, uncertain heparin start times, and undocumented boluses are common failure modes.

Using this calculator responsibly

The CalcMD 4Ts tool applies the published domain definitions to help learners and clinicians compute a transparent total. It is an educational aid. It does not replace institutional protocols, direct patient assessment, or specialist consultation. Any decision to start, stop, or switch anticoagulation must be individualized to bleeding risk, thrombosis risk, renal function, procedural plans, and local formularies.

CalcMD.

4Ts Score — Heparin-Induced Thrombocytopenia (HIT)

Learn the 4Ts score: thrombocytopenia, timing, thrombosis or sequelae, and other causes (each 0–2; total 0–8). Pretest probability bands, clinical pearls, limitations, and how scoring connects to HIT testing—educational overview for the CalcMD calculator.