Table 1.

Diagnosis of heparin-induced thrombocytopenia

Consider HIT whenever a patient exposed to heparin (or has reasonably supposition of exposure):

Has a significant fall in platelet count and/or

Has new blood clot

Formulate clinical probability estimate (4Ts score awards 0-2 points for each of the following parameters):

Thrombocytopenia, is it typical (at least 30% to 50% fall)

Timing of platelet fall (5-12 d after heparin initiation; consider also alternate temporal scenarios)

Thrombotic complications contemporaneously (consider more strongly if unusual sites)

Other likely explanations for low platelets and/or clots

If moderate or strong suspicion, order serologic test (ELISA):

<0.4, “negative”; 0.4-1.0, <2% to 5% have platelet-activating antibodies

1.0-1.4, 10% to 20% have activating antibodies; 1.4–2.0, 50% have activating antibodies

>2.0, 90% have activating antibodies

If diagnosis is clear on the basis of clinical probability and ELISA (great majority), no need for confirmatory serology.

If clinical probability is low, there is no reason for ordering serologic tests (can lead to harm).

ELISA, enzyme-linked immunosorbent assay; HIT, heparin-induced thrombocytopenia; 4Ts score, thrombocytopenia, timing after beginning heparin, whether accompanying thromboses or other sequelae are present, and whether other explanations are possible or likely.