4Ts Score for Heparin-Induced Thrombocytopenia (HIT)
Estimates pre-test probability of heparin-induced thrombocytopenia (HIT).
Creator
Dr. Andreas Greinacher, MDInstructions
To calculate the percent fall in platelets, use the highest platelet count immediately before the potentially HIT-related decline.
To calculate the number of days, consider the first day of heparin exposure as Day 0. Use the day the platelet count begins to fall after Day 0, not the day the platelet nadir is reached.
Magnitude of platelet count fall
4Ts Score for Heparin-Induced Thrombocytopenia (HIT)
What it is
The 4Ts Score is a pretest clinical scoring system for heparin-induced thrombocytopenia (HIT), a rapid, life-threatening adverse drug reaction to heparin exposure. It's designed to be applied before ordering laboratory HIT antibody testing, helping clinicians decide whether a patient's dropping platelet count is likely due to HIT versus one of the many other causes of thrombocytopenia in hospitalized patients (sepsis, recent cardiopulmonary bypass, medications, acute PE, end-stage renal disease, cirrhosis, or indwelling arterial devices).
The Four "Ts"
Each of the four categories is scored 0, 1, or 2 points, for a total possible range of 0–8:
Category | 2 points | 1 point | 0 points |
|---|---|---|---|
Thrombocytopenia | Platelet fall >50% AND nadir ≥20 ×10⁹/L⁻¹ | Platelet fall 30–50% OR nadir 10–19 ×10⁹/L⁻¹ | Platelet fall <30% OR nadir <10 ×10⁹/L⁻¹ |
Timing of platelet fall | Clear onset days 5–10 after heparin start, OR ≤1 day if heparin exposure within past 30 days | Consistent with days 5–10 fall but unclear (missing counts), OR onset ≤1 day if heparin exposure was 31–100 days ago | Platelet fall ≤4 days without recent heparin exposure |
Thrombosis or other sequelae | New thrombosis, skin necrosis, or acute systemic reaction after IV heparin bolus | Progressive/recurrent thrombosis, erythematous skin lesions, or suspected (not yet proven) thrombosis | None |
oTher causes of thrombocytopenia | None evident | Possible | Definite |
Interpreting the Score
Total Score | Pretest Probability | Positive Predictive Value |
|---|---|---|
0–3 | Low | ≤5% PPV (in original study), <1% (in meta-analysis); very reliable rule-out |
4–5 | Intermediate | ~11% PPV (in original study), ~14% PPV (in meta-analysis) |
6–8 | High | ~34% PPV (in original study), ~64% PPV (in meta-analysis) |
Key clinical takeaway:
The score's greatest strength is its negative predictive value — a low-probability score (0–3) has a negative predictive value of essentially 0.998 in the largest meta-analysis (Cuker et al., 2012, Blood), meaning HIT can be confidently ruled out without further testing. Positive predictive value at higher scores is much less impressive, so intermediate/high scores should prompt further lab testing (PF4/heparin antibody assay) rather than being treated as diagnostic on their own.
It is important to note that one of the four components (“other causes for thrombocytopenia”) is subjective. Other causes of thrombocytopenia could include sepsis, recent cardiopulmonary bypass, medications, acute pulmonary embolism, end-stage renal disease, cirrhosis, or indwelling arterial devices.
In 2018, the American Society of Hematology (ASH) guidelines strongly recommended using the 4Ts score, rather than clinical gestalt, to estimate the probability of HIT. They also strongly recommended ordering laboratory testing only if the patient’s HIT probability is intermediate or high, based on moderate-certainty evidence.
Gold standard for the diagnosis of HIT was defined as either:
A platelet serotonin release assay (SRA) with ≥50% serotonin release and a positive PF4/polyanion-enzyme immunoassay (EIA; not a cutoff of >0.40 OD), OR
A positive heparin-induced platelet activation (HIPA) test in at least 3 of 4 donor platelets.
Companion tool worth considering:
The HIT Expert Probability (HEP) Score was developed specifically as an alternative with better interobserver agreement than the 4Ts, from broader (26-expert) consensus.
All questions & possible results
4Ts CriteriaTitle not visible
Thrombocytopenia
Magnitude of platelet count fall
Select one option:
- Platelet count fall >50% AND nadir ≥20
- Platelet count fall 30-50% OR nadir 10-19
- Platelet count fall <30% OR nadir <10
Timing of platelet count fall
Select one option:
- Clear onset between days 5-10 of heparin exposure, OR platelet fall ≤1 day with prior heparin exposure within the last 30 days
- Consistent with fall at days 5-10, but not clear (e.g. missing platelet counts), OR onset after day 10, OR fall ≤1 day with prior heparin exposure 30-100 days ago
- Platelet count fall at <4 days without recent heparin exposure
Thrombosis or other sequelae
Select one option:
- New thrombosis OR skin necrosis; acute systemic reaction after IV heparin bolus
- Progressive OR recurrent thrombosis; non-necrotizing (erythematous) skin lesions; suspected thrombosis not yet proven
- None
Other causes for thrombocytopenia
Select one option:
- None apparent
- Possible
- Definite
Possible results
Low Probability of HIT
Score 0-3 points: low pre-test probability of heparin-induced thrombocytopenia.
≤5% PPV in original study, <1% PPV in meta-analysis
HIT is unlikely:
Consider alternative causes of thrombocytopenia
Consider a hemtology consult if indicated.
Ordering a HIT antibody testing or a functional assay may not be necessary.
Continue or restart heparin if indicated.
Prior to time-consuming HIT antibody testing or empiric substitution of heparin with another anticoagulant, consider using the HIT Expert Probability (HEP) Score in conjunction with this tool as an alternative assessment. The HEP Score was developed from expert consensus (26 HIT experts) and demonstrated better interobserver agreement than the 4Ts score.
Clinical judgement and a complete clinical evaluation ought to guide management.
High Probability of HIT
Score 6-8 points: high pre-test probability of heparin-induced thrombocytopenia.
~34% PPV in original study, ~64% PPV in meta-analysis
Urgent clinical evaluation and intervention may be warranted:
Discontinue heparin products and substitute with non-heparin anticoagulant.
Order HIT antibody testing (ELISA); based on the result, determine if a functional assay (e.g., serotonin release assay) is needed to diagnose HIT.
Assess for thrombosis if HIT is diagnosed.
Consider hematology consultation.
Prior to time-consuming HIT antibody testing or empiric substitution of heparin with another anticoagulant, consider using the HIT Expert Probability (HEP) Score in conjunction with this tool as an alternative assessment. The HEP Score was developed from expert consensus (26 HIT experts) and demonstrated better interobserver agreement than the 4Ts score.
Clinical judgement and a complete clinical evaluation ought to guide management.
Intermediate Probability of HIT
Score 4-5 points: intermediate pre-test probability of heparin-induced thrombocytopenia.
~11% PPV in original study, ~14% PPV in meta-analysis
Consider further evaluation and intervention:
Discontinue heparin products and substitute with a non-heparin anticoagulant.
Order HIT antibody testing (ELISA); based on the result, determine whether a functional assay (e.g., serotonin release assay) is needed to diagnose HIT.
Assess for thrombosis if HIT is diagnosed.
Consider hematology consultation.
Prior to time-consuming HIT antibody testing or empiric substitution of heparin with another anticoagulant, consider using the HIT Expert Probability (HEP) Score in conjunction with this tool as an alternative assessment. The HEP Score was developed from expert consensus (26 HIT experts) and demonstrated better interobserver agreement than the 4Ts score.
Clinical judgement and a complete clinical evaluation ought to guide management.
Citation
Lo GK, Juhl D, Warkentin TE, Sigouin CS, Eichler P, Greinacher A. Evaluation of pretest clinical score (4 T's) for the diagnosis of heparin-induced thrombocytopenia in two clinical settings. J Thromb Haemost. 2006 Apr;4(4):759-65.
Literature
Lo GK, Juhl D, Warkentin TE, Sigouin CS, Eichler P, Greinacher A. Evaluation of pretest clinical score (4 T's) for the diagnosis of heparin-induced thrombocytopenia in two clinical settings. J Thromb Haemost. 2006 Apr;4(4):759-65.
https://pubmed.ncbi.nlm.nih.gov/16634744/Vatanparast R, Lantz S, Ward K, Crilley PA, Styler M. Evaluation of a pretest scoring system (4Ts) for the diagnosis of heparin-induced thrombocytopenia in a university hospital setting. Postgrad Med. 2012 Nov;124(6):36-42.
https://pubmed.ncbi.nlm.nih.gov/23322137/Cuker A, Gimotty PA, Crowther MA, Warkentin TE. Predictive value of the 4Ts scoring system for heparin-induced thrombocytopenia: a systematic review and meta-analysis. Blood. 2012 Nov 15;120(20):4160-7.
https://pubmed.ncbi.nlm.nih.gov/22990018/Nagler M, Fabbro T, Wuillemin WA. Prospective evaluation of the interobserver reliability of the 4Ts score in patients with suspected heparin-induced thrombocytopenia. J Thromb Haemost. 2012 Jan;10(1):151-2.
https://pubmed.ncbi.nlm.nih.gov/22060821/Nagler M, Angelillo-Scherrer A. Diagnostic value of the 4Ts score for heparin-induced thrombocytopenia in the critically ill. J Crit Care. 2014 Dec;29(6):1126-7.
https://pubmed.ncbi.nlm.nih.gov/25113851/Linkins LA, Bates SM, Lee AY, Heddle NM, Wang G, Warkentin TE. Combination of 4Ts score and PF4/H-PaGIA for diagnosis and management of heparin-induced thrombocytopenia: prospective cohort study. Blood. 2015 Jul 30;126(5):597-603.
https://pubmed.ncbi.nlm.nih.gov/25926600/Cuker A, Arepally GM, Chong BH, Cines DB, Greinacher A, Gruel Y, Linkins LA, Rodner SB, Selleng S, Warkentin TE, Wex A, Mustafa RA, Morgan RL, Santesso N. American Society of Hematology 2018 guidelines for management of venous thromboembolism: heparin-induced thrombocytopenia. Blood Adv. 2018 Nov 27;2(22):3360-3392.
https://pubmed.ncbi.nlm.nih.gov/30482768/Larsen EL, Nilius H, Studt JD, Tsakiris DA, Greinacher A, Mendez A, Schmidt A, Wuillemin WA, Gerber B, Vishnu P, Graf L, Kremer Hovinga JA, Goetze JP, Bakchoul T, Nagler M. Accuracy of Diagnosing Heparin-Induced Thrombocytopenia. JAMA Netw Open. 2024 Mar 4;7(3):e243786.
https://pubmed.ncbi.nlm.nih.gov/38530310/
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