Incidence rates of ankylosing spondylitis (AS) among males versus females are poorly understood. Results of prior research have been mixed, including findings of a 3:1 incidence ratio for males vs. females, but increasing AS rates among females.
We estimated the incidence of AS in a retrospective cohort study of diverse, working-age US military service members during March 2014 - June 2017 (N = 728,556) who underwent clinical practice guideline-directed screening for chronic back pain. Incident AS cases were identified using diagnostic codes from electronic medical and administrative records.
In contrast to some prior studies, AS incidence was similar among males and females (incidence rate ratio: 1.16, p = 0.23; adjusted odds ratio [aOR] = 0.79, 95% confidence interval: 0.61 - 1.02; p = 0.072). AS rates increased approximately monotonically with age. Consistent with prior research, the AS incidence rate was greater in the White population than the Black population (aOR = 1.39, 95% CI 1.01 - 1.66, p = 0.04).
In this study population, the incidence of AS was similar for the sexes. Previous observations of male predominance have typically been derived from clinic populations that are less representative of the US race/ethnicity distribution and based on disease ascertainment tools that may have identified subjects later in their disease course. Our study population also differed in being subject to organized screenings for musculoskeletal complaints. Our findings suggest that sex may not predict AS incidence in the US population.