It is relatively common for doctors to diagnose someone with multiple sclerosis when the patient doesn't have the disease -- a misdiagnosis that not only causes patients potential harm but costs the U.S. health care system untold millions of dollars a year, according to a study recently published online in the journal Neurology.

The study is based on a survey of 122 multiple sclerosis specialists nationwide and was conducted by researchers at Oregon Health & Science University and the Portland Veterans Affairs Medical Center. Neurology is the medical journal of the American Academy of Neurology.

The survey found that all but six of the multiple sclerosis specialists -- more than 95 percent -- had seen at least one patient within the past year who had been diagnosed with MS by another medical provider, but the MS specialist "strongly felt [the patient] did not in fact have MS."

Almost three-quarters of the MS specialists said they had seen at least three patients within the past year who they believe had been misdiagnosed. More than one-third of respondents said they had seen six or more patients within the past year who had been misdiagnosed. In total, the study estimated that the 122 MS specialists had seen almost 600 patients within the past year who had been misdiagnosed with MS.

Many of the MS specialists said a significant percentage of these misdiagnosed patients had already begun disease-modifying therapy for MS, which carries potentially serious side effects and can be very expensive, often at least $40,000 per patient per year. Based on the responses from the MS specialists, the study estimated that the 122 MS specialists had seen approximately 280 patients who had been misdiagnosed and were receiving MS treatment -- costing the health system at least $11 million per year in unnecessary and inappropriate treatment for that group of patients alone.

"What we found was that the misdiagnosis of MS was common -- perhaps more so than previously thought. This has significant consequences for patients and for our health care system as a whole," said Andrew Solomon, M.D., the lead author of the study.

Solomon worked on the study while he was a post-doctoral fellow in multiple sclerosis at OHSU and at the Portland Veterans Affairs Medical Center. Solomon is now an assistant professor at the University of Vermont College of Medicine and is a member of the University of Vermont Medical Group Neurology Service at Fletcher Allen Health Care.

Dennis Bourdette, M.D., the senior author of the study and director of the OHSU Multiple Sclerosis Center, said the misdiagnoses not only meant patients were getting expensive and potentially harmful treatments they didn't need, but they were also not getting the appropriate treatment for the diseases they may have had.

"These patients were getting the wrong treatment -- and missing out on the correct treatment," Bourdette said.

The survey also detailed the emotional and ethical challenges of informing a patient of a misdiagnosis. More than two-thirds of the MS specialists said that informing a patient with a diagnosis of MS that they likely did not have the disease was more challenging than informing a patient of a new diagnosis of MS. And, in an especially surprising finding, about one in seven of the MS specialists said they had sometimes chosen not to inform a patient of their suspected misdiagnosis, citing among their reasons the fact that the patients were not receiving MS treatment, or the potential psychological harm in changing a diagnosis.

The study underlines a significant but underappreciated problem within the U.S. health care system: the dangers, costs and physician challenges associated with misdiagnosed diseases.

In recent years, medicine has begun paying more attention to medical errors and adverse medical events -- giving a patient the wrong drug or too much of it, for example, or not preventing avoidable infections. But less attention has been paid to the rate of diagnostic errors -- which experts estimate average about 10 percent across a wide variety of medical conditions.

Often, these diagnostic errors happen with diseases, like MS, "that don't have a definitive test," said Eran Klein, M.D., Ph.D., the third co-author of the study and an assistant professor in OHSU's Department of Neurology. "These diseases instead require well-honed skills of a professionally trained clinician who is knowledgeable about the disease, can study a patient's medical history, perform a detailed physical examination and evaluate additional medical information to make the proper diagnosis. This study sheds light on the importance of clinical expertise in recognizing and correcting diagnostic error."

The study was funded by the Department of Veterans Affairs, the National Institutes of Health and the Partners MS Fellowship Award.