Fibromyalgia is considered to mostly affect women, with estimates showing they account for more than 90% of reported cases.

However, according to a new study, this percentage is not supported by unbiased studies, and when gender bias is eliminated, the proportion of women with fibromyalgia is lowered to 60%.

The study, “Fibromyalgia diagnosis and biased assessment: Sex, prevalence and bias,” was published in the journal Plos One.

Fibromyalgia is chronic widespread pain in muscles and tendons. There are two types of fibromyalgia: one that is diagnosed based on a set of published criteria by the American College of Rheumatology (CritFM) and another based on clinical diagnosis (ClinFM).

Epidemiological studies estimate the majority of fibromyalgia cases to be in women, but this finding could be biased due to different criteria and methodology used in making diagnoses.

Moreover, “different criteria sets and methodology, as well as bias, have led to widely varying (0.4%->11%) estimates of prevalence and female predominance (>90% to <61%),” the researchers wrote.

Using the 2016 American College of Rheumatology fibromyalgia criteria and a rheumatic disease databank, the researchers analyzed fibromyalgia prevalence and gender distribution.

They used two data sets in their analysis. The first group, referred to as the selection biased group, consisted of 1,761 patients who were specifically diagnosed with fibromyalgia by a rheumatologist. A second set, called the unbiased group, included 4,342 rheumatoid arthritis patients without a fibromyalgia diagnosis.

In the selection-biased group, 90% of the study population were women. However, in the second group, fibromyalgia symptoms were detected in only 58.7%.

Generalized pain was reported by 36.8% of the women compared with 32.4% of men. Similarly, the magnitude and severity of fibromyalgia symptoms were also higher in women, the authors noted.

The team also assessed fibromyalgia prevalence and gender bias in a previously reported German population study with 2,435 patients using the American College of Rheumatology’s 2016 criteria.

In this study, 59.2% of the patients were women, the team reported.

“The results of this study show that in a general population survey or in a sample unbiased by fibromyalgia selection by physicians, 60% or fewer subjects will be women,” the researchers said. “We recommend the use of 2016 fibromyalgia criteria [CritFM] for clinical diagnosis and epidemiology because of its updated scoring and generalized pain requirement.”

“If women are over-diagnosed with fibromyalgia and men are under-diagnosed, then statistics relating to symptoms, prevalence, costs, comorbidity and clinical outcomes will be inaccurate. ClinFM can never provide valid and reliable measures of such outcomes,” they said.