Background: Electronic claims and medical record databases are increasingly used for observational studies of psoriasis. The purpose of this study was to assess the validity of psoriasis diagnostic codes in an electronic database. Methods: This study was performed in a population-based setting in Olmsted County, Minnesota, where all diagnoses and procedures from all health care providers in a large community are indexed and recorded in an electronic database. The database was searched for patients aged 18 years or older with diagnostic codes consistent with psoriasis for the time period January 1, 1976, to January 1, 2000. The complete medical records of all patients were reviewed manually for validation of psoriasis diagnoses. Results: We reviewed the complete medical records of 2556 patients with at least one diagnostic code consistent with psoriasis. Based on medical record review, 1458 (57.0%) patients were confirmed to have psoriasis, of which the majority (81%) received confirmation by a dermatologist. The most commonly used diagnostic codes for psoriasis were International Classification of Diseases, Ninth Revision 696.1 (psoriasis, not otherwise specified) with a positive predictive value of 68.7% (95% confidence interval: 66.5%, 70.9%). Increasing frequency of codes in a given time window was associated with positive predictive values. However, positive predictive value for only one code in a 5-year time window was still as high as 60% (95% confidence interval: 57%, 63%). Limitations: Differences between individual electronic medical record databases may limit the ability to form a general conclusion from these findings. The remitting, relapsing course of psoriasis and the heterogeneity in clinical presentation create challenges in case ascertainment. Conclusion: Electronic identification of patients with psoriasis by diagnostic codes alone may lead to misclassification in database studies.
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