More than 20 percent of patients who sought a second opinion at one of the nation’s premier medical institutions had been misdiagnosed by their primary care providers, according to new research published in April. The study examined a group of cases that were sent to the Mayo Clinic for second opinions.

In 62 cases (21 percent), the second diagnosis was “distinctly different” from the first, the researchers reported. In 36 cases (12 percent), the diagnoses were the same. In the remaining 188 cases, the diagnoses were at least partly correct but were “better defined/refined” by the second opinion, according to the study.

“Diagnosis is extremely hard,” said Mark L. Graber, a senior fellow at the research institute RTI International and founder of the Society to Improve Diagnosis in Medicine. “There are 10,000 diseases and only 200 to 300 symptoms.”

Historically, diagnostic error has been both under-measured and under-studied. As a result, we have not focused enough on tools to identify or correct the problem, which can have serious consequences. According to previous research cited in the new study, diagnostic errors “contribute to approximately 10 percent of patient deaths” and “account for 6 to 17 percent of adverse events in hospitals.” Graber estimates that the rate of misdiagnosis, although difficult to determine, occurs in 10 percent to 20 percent of cases.

Doctors are human and diagnosis is inherently difficult, so mistakes will continue to occur. The key is utilizing all of the available tools and information to minimize the risk of misdiagnosis. Should there be a misdiagnosis, doctors should either be able to step back and explore other options, or send the case out to get a fresh perspective. “If you are given a serious diagnosis, or you’re not responding the way you should [to medication], a second opinion is a very good idea. Fresh eyes catch mistakes,” said Graber.