A new study reveals that around 12 million people in the US are misdiagnosed in outpatient clinics each year — the equivalent to 1 in every 20 adults. According to the study researchers, the findings pose a “substantial patient safety risk.”

The research team, led by Hardeep Singh of the Veterans Affairs Center for Innovations in Quality, Effectiveness and Safety and the Baylor College of Medicine, both in Houston, TX, says that determining the frequency of diagnostic errors in outpatient clinics is challenging. This is because of varying definitions of error and the fact that data need to be reviewed across an array of multiple health care providers and settings.

In an attempt to reach an estimate, Singh and colleagues revised the data of three studies that included hundreds of medical records of patients who attended outpatient clinics.

The team then confirmed the number of diagnostic errors over all three studies, which were defined as “missed opportunities to make a timely or correct diagnosis based on available evidence.” They note that the criteria for diagnostic errors were comparable across all studies.