Background Electronic claims and medical record databases are important sources of information for medical research. However, potential sources of error and bias, including inaccurate diagnoses, incomplete data, incorrect data entry, and misclassification bias, necessitate studies that assess the validity of these databases. Objective To assess the validity of the diagnostic code for hidradenitis suppurativa (HS), which is an increasingly studied disease. Methods In this retrospective study, the medical records of 1,168 patients in the Massachusetts General Hospital database who had received at least two International Classification of Disease, Ninth Revision 705.83 codes were manually screened. Results Of the screened patients, 1,046 (89.6%) were confirmed as having HS. Mean age (standard deviation) was 44.0 (15.7) years, median age was 43.0 years, and 748 (71.5%) were female. The majority was white (66.7%), while a significant minority was black (13.9%) or Hispanic (13.4%). An increasing total number of codes and specific terms used to describe HS in the medical record, including “hydradenitis,” “boil,” “draining,” “abscess,” “fistula,” “cyst,” and “nodule,” could be used to improve the positive predictive value of the search. Conclusions Our results highlight the importance of establishing the validity of diagnostic codes in electronic databases and allow for refinements of appropriate ways to design future searches. Given the potential for misclassification of HS patients, establishing the validity of diagnostic codes and searching strategies in electronic databases represents a crucial step for subsequent studies utilizing these databases.

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