To identify initial laboratory findings useful for the later diagnosis of idiopathic thrombocytopenic purpura (ITP) in adult patients with thrombocytopenia.

We studied 62 consecutive adult patients who had thrombocytopenia and whose peripheral blood film was normal except for thrombocytopenia at presentation. Each patient underwent physical examination and routine laboratory tests and was prospectively followed for 22.5 ± 9.8 months (range, 8 to 41 months). The frequency of antiglycoprotein (GP) IIb/IIIa antibody-producing B cells, the presence of platelet-associated and plasma anti-GPIIb/IIIa antibodies, the percentage of reticulated platelets, and the plasma thrombopoietin level were examined at the first visit. The final diagnosis was based on the clinical history, physical examination, complete blood test, bone marrow findings, and the clinical course at last observation.

Forty-six patients were diagnosed as having ITP and 16 as having another disorder, including myelodysplastic syndrome, aplastic anemia, amegakaryocytic thrombocytopenia, and reduced platelet production, with or without other cytopenias, and without dysplasia or evidence for destruction. Six initial laboratory findings discriminated ITP from other diagnoses: the absence of anemia, absence of leukocytopenia, increased frequency of anti-GPIIb/IIIa antibody-producing B cells, increased platelet-associated anti-GPIIb/IIIa antibodies, elevated percentage of reticulated platelets, and a normal or slightly increased plasma thrombopoietin level. Three or more of these ITP-associated findings were found at presentation in 44 patients (96%) with thrombocytopenia later diagnosed as ITP, compared with only 1 patient (6%) whose disorder was non-ITP.

Initial laboratory findings can well predict future diagnosis of ITP. Further studies prospectively evaluating these same diagnostic criteria on another, independent set of patients are necessary.