Haematology HIT Guidance | North Bristol NHS Trust

At North Bristol NHS Trust the test used to investigate a potential clinical diagnosis of HIT is an ELISA for heparin/PF4 antibodies.

A pretest probability should be calculated and the need for investigation discussed with a haematologist. The assay will not be performed without a 4Ts score.

Estimating the pretest probability of HIT: the ‘four Ts’

	Points (0, 1 or 2 for each of four categories: maximum possible score =8)
	2	1	0
Thrombocytopenia	> 50% fall or platelet nadir 20–100x 109 per l	30–50% fall or platelet nadir 10–19x 109 per l	fall <30% or platelet nadir <10x 109 per l
*Timing of platelet count fall or other sequelae**	Clear onset between days 5 and 10; or less than 1 d (if heparin exposure within past 100 d)	Consistent with immunisation but not clear (e.g. missing platelet counts) or onset of thrombocytopenia after day 10	Platelet count fall too early (without recent heparin exposure)
Thrombosis or other sequelae (e.g. injection site lesions)	New thrombosis; skin necrosis; post heparin bolus acute systemic reaction	Progressive or recurrent thrombosis; erythematous skin lesions; suspected thrombosis not yet proven	None
Other causes for thrombocytopenia not evident	No other cause for platelet count fall is evident	Possible other cause is evident	Definite other cause is present
Pretest probability score: 6–8 = high; 4–5 = intermediate; 0–3 = low. *First day of immunising heparin exposure is considered day 0; the day the platelet count begins to fall is considered the day of onset of thrombocytopenia (it generally takes 1–3 d more until an arbitrary threshold that defines thrombocytopenia is passed).

If the pretest probability is low consider whether a HIT heparin/PF4 antibody assay should be performed. If the pretest probability is high, heparin should be stopped and an alternative anticoagulant given whilst laboratory tests are performed.

The immunological tests have high sensitivity, 80–100%, for heparin/PF4 antibodies but the diagnostic specificity for the clinical syndrome of HIT is low. A strongly positive test indicates a much greater likelihood of HIT than a weakly positive test.