RN Tools | 4Ts Score for Heparin-Induced Thrombocytopenia

Thrombocytopenia*

Timing of platelet count fall*

Thrombosis or other sequelae*

Other causes for thrombocytopenia*

High Probability of HIT (~64%)

points

Home

4Ts Score for Heparin-Induced Thrombocytopenia

Differentiates patients with HIT from those with other causes of thrombocytopenia.

Critical Care

Emergency Medicine

Hematology and Oncology

Hospitalist Medicine

Formula

The 4TS Score is calculated by the addition of the selected points:

Category	2 points	1 point	0 points
Thrombocytopenia	Platelet count fall >50% AND platelet nadir ≥20 × 10⁹ L⁻¹	Platelet count fall 30%–50% OR platelet nadir 10–19 × 10⁹ L⁻¹	Platelet count fall <30% OR platelet nadir <10 × 10⁹ L⁻¹
Timing of platelet count fall	Clear onset between days 5 and 10 OR platelet fall ≤1 day (prior heparin exposure within 30 days)	Consistent with days 5–10 fall, but not clear (e.g. missing platelet counts) OR onset after day 10 OR fall ≤1 day (prior heparin exposure 30–100 days ago)	Platelet count fall <4 days without recent heparin exposure
Thrombosis or other sequelae	New thrombosis (confirmed) OR skin necrosis at heparin injection sites OR acute systemic reaction after intravenous heparin bolus	Progressive or recurrent thrombosis or nonnecrotizing (erythematous) skin lesions or suspected thrombosis (not proven)	None
Other causes for thrombocytopenia	None apparent	Possible	Definite

More Info

≤3 points: low probability for HIT (≤5% in original study, <1% in meta-analysis).
4-5 points: intermediate probability (~14% probability of HIT).
6-8 points: high probability (~64% probability of HIT).

Why Use

Consider using the 4Ts scoring system as an alternative evaluation tool prior to time-consuming antibody testing for HIT or empiric substitution of heparin for another anti-coagulant.

When to Use

Consider using the 4Ts scoring system to stratify patients’ risk for HIT in patients with thrombocytopenia who are currently or were recently on heparin derived agents.
Patients falling into the low-risk category frequently do not need further testing for HIT.
Consider further laboratory evaluation for HIT or switching to a non-heparin derived anti-coagulant in those patients in the intermediate or high-risk groups.

Pearls / Pitfalls

The 4Ts for the diagnosis of heparin-induced thrombocytopenia (HIT) is a tool developed to help clinicians rule out HIT in patients who develop thrombocytopenia in the clinical setting.

Included patients being evaluated for thrombocytopenia or suspected HIT in two clinical settings: inpatients at Hamilton General Hospital (HGH) in Canada and various clinicians in a variety of healthcare settings in Germany and Austria.
The study prospectively applied previously developed scoring systems using various clinical features of HIT.
Scoring system consisted of four criteria, each of which was worth 0, 1, or 2 points.
Study used ≤3 points to define low probability group (≤5%) for HIT, 4-5 points for intermediate and 6-8 points for high.
Gold standard for diagnosis of HIT was either:
- A platelet serotonin release assay (SRA) and a PF4/polyanion-enzyme immunoassay (EIA) with ≥50% serotonin release and positive EIA, OR
- A positive heparin-induced platelet activation (HIPA) test in at least three of four donor platelets.

Points to keep in mind:

Scoring system criteria for HIT was slightly different between the two main sites.
The study used two different tests to diagnose HIT, depending on the site.
One of the four components of their scoring system, other causes for thrombocytopenia, is subjective.
Results between the two sites in the intermediate and high probability groups were statistically different.

A subsequent review and meta-analysis of the 4Ts scoring system for HIT found that patients in the low-risk group had a negative predictive value of 0.998, irrespective of type of clinician, prevalence of HIT, or patient population.

4Ts Score for Heparin-Induced Thrombocytopenia

Differentiates patients with HIT from those with other causes of thrombocytopenia.

Critical Care

Emergency Medicine

Hematology and Oncology

Hospitalist Medicine

Formula

The 4TS Score is calculated by the addition of the selected points:

Category	2 points	1 point	0 points
Thrombocytopenia	Platelet count fall >50% AND platelet nadir ≥20 × 10⁹ L⁻¹	Platelet count fall 30%–50% OR platelet nadir 10–19 × 10⁹ L⁻¹	Platelet count fall <30% OR platelet nadir <10 × 10⁹ L⁻¹
Timing of platelet count fall	Clear onset between days 5 and 10 OR platelet fall ≤1 day (prior heparin exposure within 30 days)	Consistent with days 5–10 fall, but not clear (e.g. missing platelet counts) OR onset after day 10 OR fall ≤1 day (prior heparin exposure 30–100 days ago)	Platelet count fall <4 days without recent heparin exposure
Thrombosis or other sequelae	New thrombosis (confirmed) OR skin necrosis at heparin injection sites OR acute systemic reaction after intravenous heparin bolus	Progressive or recurrent thrombosis or nonnecrotizing (erythematous) skin lesions or suspected thrombosis (not proven)	None
Other causes for thrombocytopenia	None apparent	Possible	Definite

More Info

≤3 points: low probability for HIT (≤5% in original study, <1% in meta-analysis).
4-5 points: intermediate probability (~14% probability of HIT).
6-8 points: high probability (~64% probability of HIT).

Why Use

Consider using the 4Ts scoring system as an alternative evaluation tool prior to time-consuming antibody testing for HIT or empiric substitution of heparin for another anti-coagulant.

When to Use

Consider using the 4Ts scoring system to stratify patients’ risk for HIT in patients with thrombocytopenia who are currently or were recently on heparin derived agents.
Patients falling into the low-risk category frequently do not need further testing for HIT.
Consider further laboratory evaluation for HIT or switching to a non-heparin derived anti-coagulant in those patients in the intermediate or high-risk groups.

Pearls / Pitfalls

The 4Ts for the diagnosis of heparin-induced thrombocytopenia (HIT) is a tool developed to help clinicians rule out HIT in patients who develop thrombocytopenia in the clinical setting.

Included patients being evaluated for thrombocytopenia or suspected HIT in two clinical settings: inpatients at Hamilton General Hospital (HGH) in Canada and various clinicians in a variety of healthcare settings in Germany and Austria.
The study prospectively applied previously developed scoring systems using various clinical features of HIT.
Scoring system consisted of four criteria, each of which was worth 0, 1, or 2 points.
Study used ≤3 points to define low probability group (≤5%) for HIT, 4-5 points for intermediate and 6-8 points for high.
Gold standard for diagnosis of HIT was either:
- A platelet serotonin release assay (SRA) and a PF4/polyanion-enzyme immunoassay (EIA) with ≥50% serotonin release and positive EIA, OR
- A positive heparin-induced platelet activation (HIPA) test in at least three of four donor platelets.

Points to keep in mind:

Scoring system criteria for HIT was slightly different between the two main sites.
The study used two different tests to diagnose HIT, depending on the site.
One of the four components of their scoring system, other causes for thrombocytopenia, is subjective.
Results between the two sites in the intermediate and high probability groups were statistically different.

4Ts Score for Heparin-Induced Thrombocytopenia

Formula

More Info

Why Use

When to Use

Pearls / Pitfalls

Advice

4Ts Score for Heparin-Induced Thrombocytopenia

Formula

More Info

Why Use

When to Use

Pearls / Pitfalls

Advice