The objective of this study is to establish a method to identify patients with primary immune thrombocytopenia (ITP) utilizing administrative data from diverse data sources that would be appropriate for epidemiologic studies of ITP, regardless of patients' age and source of health care. Medical records of the Oklahoma University Medical Center, 1995–2004, were reviewed to document the accuracy of the administrative code ICD-9-CM 287.3 for identifying children and adults with ITP, using novel, explicit levels of evidence to identify patients with a definite diagnosis. The proportion of patients diagnosed by hematologists compared to non-hematologists and the proportion of patients diagnosed as outpatients compared to inpatients were determined. For children, age <16 years, 323 outpatient medical records were reviewed; 225 adult outpatient medical records were reviewed. The positive predictive value for the administrative code for identifying patients with a definite diagnosis of ITP by a hematologist was 0.72 in children and 0.69 in adults. In 98% of children and 92% of adults seen as outpatients, the definite diagnosis of ITP was established by a hematologist. One hundred eighteen child and 141 adult inpatient medical records were reviewed. In 95% of children and 83% of adults, the definite diagnosis of ITP by a hematologist was established as an outpatient. This study confirmed the previously reported positive predictive value for the administrative code for identifying patients with ITP. Additionally, it was determined that analysis of hematologists' outpatient administrative codes identified most children and adults with ITP. Am. J. Hematol. 2012. © 2012 Wiley Periodicals, Inc.