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Category Medicine & Tests

T₁ Thrombocytopenia

2 pts Platelet fall >50% AND nadir ≥20 ×10⁹/L 1 pt Platelet fall 30–50% OR nadir 10–19 ×10⁹/L 0 pts Platelet fall <30% OR nadir <10 ×10⁹/L

T₂ Timing of Platelet Count Fall

2 pts Clear onset days 5–10; or ≤1 day if prior heparin within 30 days 1 pt Consistent with days 5–10 but unclear; onset after day 10; or ≤1 day with prior heparin 30–100 days ago 0 pts Fall <4 days without recent heparin exposure

T₃ Thrombosis or Other Sequelae

2 pts Confirmed new thrombosis; skin necrosis; acute systemic reaction post IV heparin bolus 1 pt Progressive or recurrent thrombosis; non-necrotizing skin lesions; suspected but unconfirmed thrombosis 0 pts None

T₄ Other Causes of Thrombocytopenia

2 pts No other apparent cause 1 pt Possible other cause present 0 pts Definite other cause present

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About

The 4T scoring system quantifies pretest probability for Heparin-Induced Thrombocytopenia (HIT), a prothrombotic disorder triggered by platelet-activating antibodies against platelet factor 4/heparin complexes. Misdiagnosis carries direct clinical risk: false negatives leave patients on heparin with ongoing thrombotic danger, while false positives trigger unnecessary discontinuation and costly confirmatory immunoassays. The score evaluates four clinical dimensions - degree of Thrombocytopenia, Timing of platelet fall, Thrombosis or sequelae, and oTher causes of thrombocytopenia - each graded 0, 1, or 2. The total ranges from 0 to 8.

A score of 0 - 3 indicates low pretest probability (negative predictive value ≥ 99.8%, per Cuker et al. 2012). Intermediate (4 - 5) and high (6 - 8) scores warrant immunoassay confirmation. This tool implements the original Lo et al. (2006) criteria. Note: inter-rater reliability is moderate (κ ≈ 0.50); scoring accuracy depends on precise clinical timeline documentation. The calculator does not replace laboratory confirmation via ELISA or serotonin release assay.

Formulas

The 4T score is a simple additive index across four clinical categories:

S_total = T₁ + T₂ + T₃ + T₄

Where each T_i ∈ {0, 1, 2}, giving S_total ∈ [0, 8].

T₁ = Thrombocytopenia severity (degree of platelet count fall and nadir value).

T₂ = Timing of platelet count fall relative to heparin exposure.

T₃ = Thrombosis or other clinical sequelae (new thrombosis, skin necrosis, anaphylactoid reaction).

T₄ = Other causes of thrombocytopenia excluded.

Risk stratification follows a piecewise classification:

{

Low probability if S_total ≤ 3Intermediate if 4 ≤ S_total ≤ 5High probability if S_total ≥ 6

Reference Data

Category	Score 2	Score 1	Score 0
Thrombocytopenia	Platelet fall > 50% AND nadir ≥ 20 ×10⁹/L	Platelet fall 30 - 50% OR nadir 10 - 19 ×10⁹/L	Platelet fall < 30% OR nadir < 10 ×10⁹/L
Timing of platelet fall	Clear onset day 5 - 10 OR ≤ 1 day if prior heparin within 30 days	Consistent with day 5 - 10 but unclear; onset after day 10; OR fall ≤ 1 day with prior heparin 30 - 100 days ago	Fall < 4 days without recent heparin exposure
Thrombosis or other sequelae	New confirmed thrombosis; skin necrosis at injection site; acute systemic reaction post IV heparin bolus	Progressive or recurrent thrombosis; non-necrotizing erythematous skin lesions; suspected but unconfirmed thrombosis	None
Other causes of thrombocytopenia	No other apparent cause	Possible other cause present	Definite other cause present
Score Interpretation
Low probability	0 - 3 points - HIT unlikely (NPV ≥ 99.8%). Consider alternative diagnoses.
Intermediate probability	4 - 5 points - Consider immunoassay (ELISA). Sensitivity ~97%; specificity lower.
High probability	6 - 8 points - Strongly consider stopping heparin. Send immunoassay + functional assay (SRA).
Key Reference Values
HIT incidence (UFH)	0.5 - 5.0% of heparin-exposed patients
HIT incidence (LMWH)	0.1 - 0.8% of heparin-exposed patients
Typical onset	Day 5 - 10 after heparin initiation
Rapid-onset HIT	≤ 24h if prior heparin exposure within 100 days
Anti-PF4/heparin ELISA	Sensitivity ≥ 97%; Specificity ~74%
SRA (functional assay)	Sensitivity ~95%; Specificity > 97% (gold standard)

Frequently Asked Questions

A score of 0 - 3 has a negative predictive value (NPV) exceeding 99.8% for HIT, as validated in meta-analyses by Cuker et al. (Blood, 2012). This means HIT can be effectively ruled out without further immunoassay testing when the 4T score is low.

Patients previously exposed to heparin within 30 - 100 days may have residual anti-PF4/heparin antibodies. Upon re-exposure, these pre-formed antibodies can trigger rapid-onset HIT within 24 hours, bypassing the typical 5 - 10 day seroconversion window. The Timing score of 2 accommodates this rapid-onset variant when prior exposure occurred within 30 days.

Published studies report a Cohen's kappa of approximately 0.50 (moderate agreement) for the 4T score across different clinicians. The "Timing" and "oTher causes" categories introduce the most subjectivity. To mitigate this, document exact heparin start dates and platelet count trends with timestamps. When uncertain between two sub-scores, select the lower value and note the ambiguity for downstream clinicians.

Guidelines from the American Society of Hematology (ASH, 2018) recommend discontinuing heparin and initiating a non-heparin anticoagulant (argatroban, bivalirudin, or fondaparinux) when the 4T score is 6 - 8, even before confirmatory laboratory results. For intermediate scores (4 - 5), clinical judgment determines whether to switch anticoagulation while awaiting ELISA results. The 4T score is a clinical decision aid, not a definitive diagnostic.

The original 4T scoring criteria by Lo et al. do not assign different weights based on heparin type. However, HIT incidence is significantly lower with LMWH (0.1 - 0.8%) versus UFH (0.5 - 5.0%). In surgical patients receiving UFH, the prior probability is higher, so even intermediate 4T scores warrant more aggressive workup compared to medical patients on LMWH.

HIT characteristically produces a relative platelet fall of > 50% from the post-operative or post-exposure peak (not the pre-operative baseline). The nadir rarely drops below 20 ×10⁹/L. If the nadir is < 10 ×10⁹/L, alternative diagnoses such as DIC, sepsis, or drug-induced immune thrombocytopenia become more likely, and the Thrombocytopenia sub-score receives 0 points.