Heparin 20 days earlier+2 timing points
−20 days
Prior exposureDay 0
Restart≤1 day
Platelet fall
Prior exposureDay 0
Restart≤1 day
Platelet fall
Exposure within the preceding 30 days can support rapid-onset timing.
4Ts Score Calculator for HIT
Four clinical domains. One transparent score. A structured starting point for HIT assessment.
For clinician use · not a diagnosis. Interpret with the patient’s full history and local protocol. Patients should not stop or change anticoagulants themselves. No completed medical review of this tool is claimed.
4Ts Clinical Assessment
0 of 4 answeredSelect one description in each domain. Start with the documented clinical facts.
Pre-test Probability Summary
4Ts · 0–84Ts CLINICAL SCORE
— / 8Complete all four domainsUnanswered findings are not scored as zero.
Domains answered0 / 4
Assessment statusIncomplete
Possible total0–8
Where the score sits
Pre-test categoriesLowIntermediateHigh
Select the four domains to classify the assessment.
See the platelet decline
Teaching examplePeak 240 → nadir 90 ×10⁹/L · a 62.5% fall
Score the onset, not the lowest-count day.Here, timing starts on day 7. The nadir occurs later, on day 9.
Illustrative measurements only · not a real patient record.
Your score, domain by domain
| Clinical domain | Points |
|---|---|
| Thrombocytopenia | — |
| Timing | — |
| Thrombosis / sequelae | — |
| Other explanations | — |
| Total | Pending |
Clinical next steps
ASH guidanceConfirm the relevant history before using a risk category. Missing information can change both the score and its interpretation.
Clinician-directed care only. Drug choice, dose and intensity require individual bleeding-risk, organ-function and treatment assessment.
Read the full ASH recommendations →Verify the uncertain information. A reassuring low category should not be used to dismiss HIT when key facts are missing or unreliable.
What do published probability figures mean?
A 2012 meta-analysis pooled 13 studies (3,068 people): low-score NPV 99.8% (95% CI 97–100%); intermediate-score PPV 14% (9–22%); high-score PPV 64% (40–82%). These are pooled study estimates, not a personalized probability. A high score alone does not confirm HIT.
Cuker et al., Blood, 2012
Platelet fall helper
Optional · does not auto-score×10⁹/L
×10⁹/L
RELATIVE PLATELET DECLINE—
100 × (peak − nadir) / peak
Choose the right comparison.
Review the platelet peak around the relevant heparin course, the subsequent nadir and the onset of the decline. A substantial fall can occur while the count remains in the normal range. Scoring notes
Same rapid fall. Different exposure history.
Timing examples · not whole scoresFor a platelet fall within 1 day, previous heparin exposure changes the timing-domain assessment.
Heparin 60 days earlier+1 timing point
−60 days
Prior exposureDay 0
Restart≤1 day
Platelet fall
Prior exposureDay 0
Restart≤1 day
Platelet fall
A rapid fall with exposure more than 30 to 100 days earlier receives 1 point.
No recent heparin exposure0 timing points
No recent
exposureDay 0
Start≤1 day
Platelet fall
exposureDay 0
Start≤1 day
Platelet fall
An early fall by day 4 without recent exposure receives 0 timing points.
Qualifying exposure history must be verified. The other three domains are still required.Timeline diagrams are schematic, not to scale.
Medical illustration · not a microscope image · not to scaleFewer platelets can coexist with more clotting.
Immune HIT involves platelet activation triggered by antibodies against PF4–heparin complexes. Platelet consumption can lower the count while thrombotic complications develop.
PF4–heparin interactionImmune platelet activationPlatelet fall + thrombosis risk
The score estimates clinical probability; laboratory evaluation helps establish the diagnosis.
A practical diagnostic pathway
Overview · clinician interpretationEstablish clinical probability
Document the four domains and check the exposure timeline.
Use targeted testing
Intermediate/high probability generally prompts an anti-PF4/heparin immunoassay.
Interpret in context
A positive immunoassay often needs a functional assay such as SRA, when available.
High clinical suspicion, assay strength and test availability may alter the sequence. This overview does not replace the full guideline.
Formula & Worked Example
4Ts = T₁ + T₂ + T₃ + T₄
Each domain contributes 0, 1 or 2 points.
Each domain contributes 0, 1 or 2 points.
1
Platelet fall >50%, nadir ≥20: 2 points.
Clear onset on day 7: 2 points.
2
Progressive thrombosis: 1 point.
Another cause is possible: 1 point.
3
2 + 2 + 1 + 1 = 6 / 8
High pre-test probability; not a confirmed diagnosis.
Example data are illustrative and should not be applied to a real patient.
Important scoring pitfalls
Check heparin exposures in the previous 100 days, including prior admissions. Evaluate the start of a sustained decline, rather than only the day the count is lowest. Consider recent surgery, infection, disseminated intravascular coagulation, medications and mechanical support as competing explanations.
After cardiac surgery, a fall followed by recovery and a later second decline requires careful review. Recalculate the score when the clinical picture changes.
Frequently Asked Questions
Does a score of 6–8 diagnose HIT?
No. It indicates a high pre-test category and calls for prompt clinician assessment and appropriate laboratory evaluation.
Can a normal platelet count still score points?
Yes. The relative change matters. A large fall from a high peak can meet the decline criterion even when the current count remains within the reference range.
Why is there no default zero score?
An unanswered question is not evidence that a finding is absent. The tool leaves the classification incomplete until all four domains have a documented selection.
Which timing convention is used?
This tool uses the conventional day 5–10 criteria in Cuker et al. (2012), including rapid-onset cases with recent exposure and early falls by day 4 without recent exposure. Some local tools use a different timing window; follow the protocol adopted by your service.
What if the timing or another finding is uncertain?
Seek the missing records and reassess. The “review needed” option is not scored as zero. If a provisional choice is necessary, mark the assessment uncertain and use clinical judgment.
Should a patient stop heparin based on this tool?
No. Patients should contact the treating team. Any change to anticoagulation requires clinician supervision and consideration of bleeding and clotting risk.
Is this score a bleeding-risk assessment?
No. It estimates the clinical probability of HIT. Bleeding risk and anticoagulant management are separate clinical assessments.
Sources & References
ASH · HIT clinical guidelinesDiagnostic and initial management recommendations.May & Cuker · Practical Guide, 2024Scoring nuances and practical diagnostic context.
Cuker et al. · Blood, 20124Ts definitions and predictive-value meta-analysis.Independent references · Logos identify sources, not institutional endorsement. No specialist approval is claimed. Content basis checked 19 October 2026.
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Four clinical domains. One transparent score. A structured starting point for HIT assessment.
