4Ts Score System assessment for Heparin-Induced Thrombocytopenia is an alternative evaluation tool prior to time-consuming antibody testing for HIT or empiric substitution of heparin for another anti-coagulant.

Patient’s score	Range	Risk stratification	Probability of HIT	Recommendation
0	0	0	0	0

Consider using the 4Ts scoring system 4Ts Score for Heparin-Induced Thrombocytopenia for:

• Differential diagnostic HIT and other causes of thrombocytopenia;
• Patients’ risk stratification for HIT in patients with thrombocytopenia who are currently or were recently on heparin derived agents.

Thrombocytopenia	Platelet count fall >50% AND platelet nadir ≥20×109/l	+2
	Platelet count fall 30–50% OR platelet nadir 10–19×109/l	+1
	Platelet count fall <30% OR platelet nadir <10×109/l	0
Timing of platelet count fall	Clear onset between days 5–10 OR platelet fall ≤1 day (prior heparin exposure within 30 days)	+2
	Consistent with days 5–10 fall, but not clear; onset after day 10 OR fall ≤1 day (prior heparin exposure 30–100 days ago)	+1
	Platelet count fall <4 days without recent exposure	0
Thrombosis or other sequelae	New thrombosis OR skin necrosis; acute systemic reaction post-IV heparin bolus	+2
	Progressive OR recurrent thrombosis; non-necrotizing skin lesions; suspected thrombosis (not proven)	+1
	None	0
Other causes for thrombocytopenia	None apparent	+2
	Possible	+1
	Definite	0

Results evaluation:

4Ts Score System for Heparin-Induced Thrombocytopenia consisted of four criteria, each of which was worth 0, 1, or 2 points. To carry out stratification on the scale, it is necessary to summarize the obtained points.

Score	Risk stratification	Probability of HIT
≤3 points	low probability for HIT	≤5% in original study, <1% in meta-analysis
4-5 points	intermediate probability for HIT	~14%
6-8 points	high probability for HIT	~64%

Patients from the low-risk category frequently do not need further testing for HIT based on the assessment with 4Ts Score for Heparin-Induced Thrombocytopenia. Patients from the intermediate or high-risk groups require further laboratory evaluation for HIT or switching to a non-heparin-derived anti-coagulant.

Detailed parameters evaluation:

Category	2 points	1 point	0 points
Thrombocytopenia	Platelet count fall > 50% AND platelet nadir ≥ 20 ×109/l	Platelet count fall 30%–50% OR platelet nadir 10–19 ×109/l	Platelet count fall < 30% OR platelet nadir < 10 ×109/l
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

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.

4Ts Score for Heparin-Induced Thrombocytopenia was developed based on the prospective studies in patients being evaluated for thrombocytopenia or suspected HIT in two clinical settings: inpatients at Hamilton General Hospital in Canada and various clinicians in a variety of healthcare settings in Germany and Austria. A subsequent review and meta-analysis of the 4Ts scoring system for HIT revealed 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. However, one of the four components of their scoring system, other causes for thrombocytopenia, is subjective.

Literature:
Lo GK, Juhl D, Warkentin TE, Sigouin CS, Eichler P, Greinacher A. Evaluation of pretest clinical score (4 Ts) for the diagnosis of heparin-induced thrombocytopenia in two clinical settings. J Thromb Haemost 2006; 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/

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