Pre-test clinical score for HIT

The pretest clinical scoring system, the ‘4 T’s’ provides high-negative predictive value in assessing patients with suspected HIT when a low score is obtained.

Beforehand following data is to be collected ie

  • Baseline PLT count (before starting heparin) to know % fall and nadir PLT count during episode
  • Timing of PLT count fall after-heparin exposure and any prior history of heparin exposure
  • Any thrombotic episode (Skin lesions at heparin injection sites are whether erythematous or necrotic, Any progression of thrombosis refers to objectively documented increase in thrombus size eg, extension of deep-vein thrombosis by ultrasonography or recurrence of thrombosis refers to newly formed thromboembolus in previously affected region, apart from this any new onset thrombosis is documented or not)


(Select any one valid option for each of the 4 category and submit)

1. Characteristic of thrombocytopenia severity (based on % fall of PLT count from baseline)

2. Timing of platelet count fall post-heparin exposure

3. Thrombo­sis or other sequelae

4. Other cause for thrombo­cyto­penia


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Heparin-induced thrombocytopenia diagnostic criteria:- The pretest clinical scoring system, the ‘4 T’s’ (based on study published in J Thromb Haemost. 2006 Apr;4(4):759-65.) Link of original article. 

Factor 0 points 1 point 2 points
Thrombo­cyto­penia PLT count fall < 30%, OR PLT nadir < 10 x109/L PLT count fall 30-50%, OR PLT nadir 10-19×109/L PLT count fall > 50%, AND
PLT nadir ≥ 20 x109/L
Timing of PLT count fall post-heparin exposure Platelet count fall < 4 days without prior ex­posure in past 100 days Onset after day 10, OR
fall ≤ 1 day with prior heparin ex­posure 30-100 days ago.
Clear onset bet­ween days 5-10, OR fall ≤ 1 day with prior hep­arin ex­posure < 30 days ago
Thrombo­sis or other sequelae None Pro­gressive or re­current thrombo­sis, OR
Non-necro­tizing (erythem­atous) skin lesions, OR
Sus­pected thrombo­sis (not proven)
New thrombo­sis (con­firmed), OR
skin necrosis, OR
acute sys­temic reaction post-intra­venous un­fraction­ated heparin bolus
Other cause for thrombo­cyto­penia Definite Possible None

Total score derived from above is used to determine pre-test probability of detecting HIT antibodies

Pretest probability Score
High 6 – 8
Intermediate 4 – 5
Low ≤ 3