Conceptualization of the time course for HIT.

The first indication of the autoimmune disorder is detection of antibodies to PF4 by an immunologic method such as an ELISA. Higher titer antibodies would be detected in a functional assay such as a serotonin release assay. At the same time as the detection of the antibodies by functional methods, patients may have a decrease in the platelet count and possibly thrombosis as well. With prompt discontinuation of heparin and institution of alternative therapy, additional thrombosis is prevented and the platelet count increases toward the normal range. The functional assay then becomes negative, whereas the more sensitive ELISA assay may remain positive for as long as 124 days. Rx indicates treatment.