Table 1.

The 5 phases of HIT

Phase	Platelet count	Functional assay	Immunoassay
Suspected HIT	Decreased	?	?
Acute HIT	Decreased	+	+
Subacute HIT A	Normal	+	+
Subacute HIT B	Normal	−	+
Remote HIT	Normal	−	−

Patients with suspected HIT are those who are thought to have HIT on clinical grounds but for whom confirmatory laboratory test results are not yet available. Once the diagnosis is confirmed, the patient is labeled as having acute HIT, a highly prothrombotic phase that persists until platelet count recovery. Subacute HIT A is the phase after platelet count recovery but before the functional assay becomes negative. Subacute HIT B is the interval after the functional assay becomes negative but before the immunoassay becomes negative. Finally, once anti-PF4 or anti-heparin antibodies are no longer detectable by immunoassay, the patient is said to have remote HIT. Adapted from Cuker.⁸