Three subtypes of HIT:
1. Typical or classical onset HIT. Typically, the platelet count falls gradually starting 5 to 10 days (up to 2 weeks) after the initiation of heparin. Thrombocytopenic levels may not occur until several days later.
2. Early or rapid-onset HIT. In some cases, thrombocytopenia may occur abruptly within 5 days of heparin therapy. The rapid onset suggests exposure to heparin within the past 3 months and the presence of circulating HIT antibodies. Since the antibodies are already present, thrombocytopenia may occur promptly with heparin administration.
3. Delayed-onset HIT. Although rare, thrombocytopenia may occur from several days to more than a month after cessation of heparin. Such patients may have high titers of HIT antibody.
1. Typical or classical onset HIT. Typically, the platelet count falls gradually starting 5 to 10 days (up to 2 weeks) after the initiation of heparin. Thrombocytopenic levels may not occur until several days later.
2. Early or rapid-onset HIT. In some cases, thrombocytopenia may occur abruptly within 5 days of heparin therapy. The rapid onset suggests exposure to heparin within the past 3 months and the presence of circulating HIT antibodies. Since the antibodies are already present, thrombocytopenia may occur promptly with heparin administration.
3. Delayed-onset HIT. Although rare, thrombocytopenia may occur from several days to more than a month after cessation of heparin. Such patients may have high titers of HIT antibody.
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