Diagnostic error is a type of preventable harm that is particularly difficult to measure. Detecting diagnostic uncertainty is an important precursor to identifying clinical situations at high risk for error. Researchers conducted a retrospective chart review of primary care visits to assess how clinicians described diagnostic uncertainty. They found that diagnostic uncertainty was present in more than two-thirds of visits. Clinicians documented uncertainty in diverse ways—through direct expression in their assessments, by ordering more diagnostic tests, or by scheduling close follow-up. The authors propose that the International Classification of Diseases billing codes include a consistent taxonomy for diagnostic uncertainty. A past PSNet perspective explored advances in diagnostic safety.