At least one in every 20 adults who seek medical care in an emergency room or community health clinic in the U.S. are given the wrong diagnosis, according to a study published by BMJ Quality & Safety on April 16.

Though diagnostic error has been previously studied, research methods have been limited and inconsistent in determining the frequency of diagnostic error in outpatient medical practice. Lead author Hardeep Singh, MD, of the Veterans Affairs Center for Innovations in Quality, Effectiveness and Safety at the Michael E. DeBakey VA Medical Center and Baylor College of Medicine in Houston, and colleagues calculated population-based estimates of diagnostic error to better patient safety efforts.

“Although diagnostic errors are difficult to define and measure, there is mounting evidence of their importance and frequency across several types of conditions,” wrote the authors.

Singh et al analyzed and synthesized data from three previous studies of clinic-based populations that used similar definitions of diagnostic error to estimate the frequency of outpatient diagnostic errors in the U.S. adult population. Two of the studies utilized electronic triggers to find unusual patterns of return visits or lack of follow-up of abnormal clinical findings related to colorectal cancer. The third study investigated consecutive cases of lung cancer.

Once the researchers combined estimates from the three studies, they discovered an outpatient diagnostic error rate of 5.08 percent. This number is equivalent to approximately 12 million U.S. adults each year, wrote Singh and colleagues. Using findings from previous work, they estimated that about half of these errors are potentially harmful to patients. Though this estimation does not necessarily imply harm, about six million people could be negatively affected by misdiagnosis.

“This population-based estimate should provide a foundation for policymakers, healthcare organizations and researchers to strengthen efforts to measure and reduce diagnostic errors,” Singh and colleagues advised.