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For a wide range of diseases, the diagnosis comes later in life for women than for men, according to a large Danish study.

Researchers don't know whether the later diagnoses are due to genetics, the environment, possible biases in the healthcare system - or some combination of reasons.

The study of health data from 6.9 million Danish people found that across hundreds of diseases, women on average were diagnosed when they were about four years older than the age at which the conditions were recognized in men.

"We're not just looking at one disease here, we're looking at all diseases and we are looking at an entire population, from cradle to grave," lead author Søren Brunak from the University of Copenhagen said.

On average, women received cancer diagnoses 2.5 years after men. They received diagnoses for metabolic diseases like diabetes 4.5 years later.

"(This) actually surprised us quite a lot," Brunak said. "Men generally have a tendency to get to the doctor later... So presumably the difference in onset is even larger."

Brunak and his team considered incidence rates of diseases in the 18 broad categories of the ICD-10 diagnosis system managed by the World Health Organization.

The study wasn't designed to explain the causes of the differences. Another limitation is that researchers only looked at diagnoses made in hospitalized patients.

Dr. Noel Bairey Merz, director of the Barbra Streisand Women's Heart Center at the Cedars-Sinai Smidt Heart Institute, who was not involved in the study, pointed out to Reuters Health that the study, therefore, lacks information on age at diagnosis for people who didn't require hospitalization.

"On the other hand," she said, "being hospitalized is a sign of a serious illness, so (that) adds significance to the diagnosis and supports that disease onset may be later in women."

Brunak's study, published in Nature Communications, showed that the bone-thinning disease osteoporosis was a notable exception to the trend. Here, women were typically diagnosed before they suffered a fracture, while the opposite was true for men.

"I am fascinated by this study, which generally confirms all that I present in my Stanford course on Sex and Gender in Human Physiology and Disease," said Marcia Stefanick, Director of Stanford University's Women's Health and Sex Differences in Medicine Center.

"When men get diseases that most healthcare professionals consider 'women's diseases,' they are diagnosed at later, more serious stages, and vice versa," Stefanick, who was not involved with the study, told Reuters Health in an email.

"For example, women are diagnosed later for heart disease, not only because it is still largely considered a 'man's disease', but also because our diagnostic tests are male-biased, in terms in terms of 'typical' being the male presentation. All medical schools and healthcare training should emphasize both biological sex differences and gender biases so healthcare professionals are aware of unconscious biases."

Bairey Merz agreed, adding that more research is needed to determine if the gender differences in age at diagnosis are "real" and whether they are linked to gender bias, actual biological sex differences or random error associations.