Summary We prospectively studied presentation biological differences and the response to therapy in patients with thrombotic thrombocytopenic purpura (TTP) associated with, or unrelated to human immunodeficiency virus (HIV) infection. TTP patients underwent standard evaluations and were treated with prednisone 1 mg/kg in addition to infusions of fresh frozen plasma (FFP; 30 ml/kg/d) until normalization of the platelet count. Unresponsive patients were referred for plasma exchange. Compared with HIV− TTP patients (n = 23), in HIV+ subjects (n = 21) microangiopathy was dominant among Black females, who had lower presentation Hb (median 5·8 g/dl; P = 0·03), platelet count (13 × 109/l; P = 0·05) and a CD4 count of 0·096 × 109/l. HIV+ individuals responded to FFP faster than HIV− patients and none of them required apheresis. Ten HIV− TTP patients required apheresis (P = 0·03) and four died. Responses in the HIV+ and HIV− groups occurred after treatment with a median of 33 and 55 units (one unit = 320 ml) of FFP (P = 0·004) respectively. Response to this protocol was seen in 84% (95% response in HIV+ patients). Regression analysis showed that survival was associated with younger age (P = 0·001), rapid platelet (P = 0·001) and Hb (P = 0·0009) recovery, and fewer FFP units to normal lactate dehydrogenase levels (P = 0·006). We conclude that in HIV+ individuals, microangiopathy is highly responsive to plasma infusions. This observation is important particularly when apheresis is not available.

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