4 T's - Determining the probability of HIT

An earlier post discussed how to bridge argatroban to warfarin in patients with HIT but didn't cover how to determine the likelihood of HIT when it is suspected.  This post will cover the 4 T's that are used to quantify this probability and guide clinical decision-making.

Both clinical and laboratory evidence need to be considered when there is a suspicion for HIT.  HIT antibody testing is frequently only performed in patients with an intermediate to high probability of HIT due to cost, availability, and the presence of false positive results.  

The best clinical prediction tool to assist in determining the probability of HIT is the 4 T’s score.1  Patients receive points as follows to a maximum of 8 points:

	2 Points	1 Point	Zero Points
Thrombocytopenia	>50% platelet fall AND nadir ≥20 AND no surgery in past 3 days	>50% platelet fall but surgery in past 3 days OR platelet fall/nadir combination that doesn’t fit in 2 or zero points	<30% platelet fall OR nadir <10
Timing	Platelet fall 5-10 days   after heparin OR within 1 day if exposed to heparin in past 5-30 days	Platelet fall after 10 days OR within 1 day if exposed to heparin in past 31-100 days	Platelet fall within 5 days without heparin exposure in last 100 days
Thrombosis	New thrombosis OR skin necrosis OR anaphylactoid reaction to heparin OR adrenal hemorrhage	Suspected thrombosis (awaiting confirmation) OR recurrent venous thrombosis in patient on anticoagulation OR erythema at heparin injection site	Thrombosis suspected
oTher cause for thrombocytopenia	No alternative explanation	Possible other cause (ie. sepsis, ventilator initiation)	Probable other cause (ie. within 72 hour of surgery, bacteremia, fungemia, chemo- or radiation therapy, DIC, posttransfusion)

How to interpret the 4 T's above?

Here is how the score from the above table can be used in a meaningful way.  The percentages of each risk category was the rate of testing positive for clinically relevant HIT antibodies in the study that developed and tested the tool.2

Score 0-3 = low risk = 1.6% 
Score 4-5 = intermediate risk = 28.6%
Score 6-8 = high risk = 100%

Based on the low likelihood of a clinically relevant HIT antibodies in the 'low risk' group (or high negative predictive value), it has been suggested that these patients can generally have the diagnosis of HIT ruled out.2  However, this has not been confirmed in a prospective study.  

The opposite does not hold true for the 'high risk' group; however, in which studies show 24-61% of patients do not have HIT.1,

Take home points:

• A score ≤3 generally rules out the diagnosis of HIT.
• An online calculator to quickly calculate the 4T's score can be found here.

Reference:

1.  Linkins L, Dans AL, Moores LK, et al.  Treatment and prevention of heparin-induced thrombocytopenia: Antithrombotic therapy and prevention of thrombosis 9^th ed.  Chest  2012;141:e495S-e530S.
2.  Lo GK, Juhl D, Warkentin TE, et al.  Evaluation of pretest clinical score (4 T's) for the diagnosis of heparin-induced thrombocytopenia in two clinical settings.  J Thromb Haemost  2006;4(4):759-65.

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