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)
USE ABOVE COLLECTED DATA TO CALCULATE HIT SCORE
(Select any one valid option for each of the 4 category and submit)
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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
The score ranges from 0 to 8.
- A score of 0 to 3 indicates a low pretest probability for HIT of less than 5%. PF4 antibody testing is not needed.
- A score of 4 to 5 indicates intermediate risk of HIT while a score of 6 to 8 indicates high risk. PF4 antibody testing should be ordered for patients with either intermediate or high risk.
Calculation of the 4Ts score prior to PF4 antibody testing decreases the likelihood of false positive results and eliminates unnecessary testing.