ObjectiveThis study developed and validated a claims-based algorithm for identifying patients with irritable bowel syndrome (IBS) and investigated the characteristics of patients associated with a correct identification.MethodsUsing a classification tree procedure, a claims-based algorithm was developed and tested against medical chart findings. Patients’ medical charts were reviewed to verify diagnoses. The classification tree separated the patient population into 12 mutually exclusive subgroups and had an overall positive predictive value (PPV) of 70%.Setting and ParticipantsClaims data were obtained for adults aged 18 years or older who were enrolled in a large health maintenance organization in California, USA, during the study period (January 1 1997 to June 30 1999) and who had at least one claim with a primary International Classification of Diseases Ninth Revision Clinical Modification (ICD-9-CM) diagnosis of 564.1 (irritable colon) in 1998. In October 2000 this code description was revised to ‘irritable bowel syndrome’.ResultsThe PPVs of end nodes of the tree varied (18 to 100%) depending on the demographic, type of resource utilized, and diagnoses characterizing the 12 groups. Younger patients (aged <50 years) and those with several claims with a 564.1 ICD-9-CM code were more likely to be patients with IBS. The PPV for each of these 12 subgroups could be used as a covariate in regression models that predict outcomes of IBS and its management.ConclusionsClaims databases provide an efficient and effective tool for conducting health services research on large patient populations. Medical record validation of claims-based algorithms is an important step in improving the reliability and utility of these databases.