First example of familial posttransfusion purpura in two PlA1-negative sisters.

Posttransfusion purpura (PTP) (platelet count 5000/microliter) was diagnosed in a female patient (never transfused, gravida IV, para IV) 1 week after transfusion for hysterectomy in 1978. She did not respond to pooled random-donor platelets but recovered following a single plasma exchange. Her platelets were PlA1 negative, and her plasma contained potent anti-PlA1. In 1986, her sister (never transfused, gravida III, para III) developed PTP (platelet counts 5-15,000/microliter) following surgery-associated transfusion. She did not respond to pooled random-donor platelets. Platelet-associated IgG was markedly elevated (5365) molecules/platelet; normal, less than 660); her plasma contained a potent platelet antibody with anti-PlA1 specificity. Her platelets were subsequently shown to be PlA1 negative. The platelet count did not rise above 30,000 per microliter, despite 3 days of high-dose methylprednisolone sodium succinate and 2 weeks of prednisone (80 mg/day). Later, her platelet count increased and remained normal after steroids were discontinued. The two sisters proved to be HLA-identical, and each possessed one haplotype carrying the DR3 marker, which has been implicated as a risk factor in neonatal alloimmune thrombocytopenia associated with anti-PlA1.