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Disability claims in the U.S. related to rheumatoid arthritis (RA) were just as common in 2015 as in 1999, a government researcher found, against expectations that the advent of biologic therapy would have led to a steady decrease.

Over that 17-year period, rates of Medicare enrollments for disability rose and fell, ending at the exact same level in the last year as it did in the first, which was 26.0 per million population, according to Michael M. Ward, MD, MPH, of the National Institute of Arthritis and Musculoskeletal and Skin Diseases in Bethesda, Maryland.

In fact, there was not a single year in which the rate fell lower than 26.0 per million, Ward .

"These findings suggest that recent treatment advances have not yet had a major impact on permanent work disability associated with RA at the population level in the U.S.," he concluded.

The results are also unexpected since, as Ward observed, these new therapies -- nearly all of which are biologics or other targeted drugs -- have been demonstrated over and over to reduce disease activity, improve function, and help patients avoid surgery relative to treatment with old-line agents, and are now widely used. Infliximab (Remicade) was approved for RA in 1999; adalimumab (Humira) followed in 2002.

have even found that tumor necrosis factor inhibitors, the longest established type of biologic drug for RA, have helped people avoid short-term loss of work, Ward noted. In other countries, RA has become less common as a reason for permanent disability for work.

However, "[s]imilar national studies have not been reported in the U.S.," Ward wrote.

He drew on Medicare data covering all fee-for-service beneficiaries during 1999-2015, focusing on those ages 20 to 59 who joined Medicare after qualifying for Social Security disability benefits. (The latter require that individuals be unable to work for at least a year because of a medical condition considered unlikely to improve.) There were roughly 4,000 to 7,500 such people every year.

Rates per million fluctuated considerably, reaching about 34 in 2003 before dropping back again, then rising to 40.0 in 2011, after which it declined again to the 1999 baseline. Thus, it's possible that this late trend is a genuine signal for a population-level decrease in RA disability rates that is now underway. Of course, the answer will have to await more data.

Not surprisingly, the likelihood of a disability claim increased with age: lowest in the 20-24 group and highest for those ages 55-59. Men were about two-thirds less likely to have claims than women.

In a sensitivity analysis, Ward also examined a subset of about 76,000 people who had filed at least three Medicare claims for RA in the first year of disability, thinking that patients with more active disease would have been more likely to benefit from the newer treatments. He did the same, and with the same rationale, for enrollees younger than 40.

In both cases, their rates were consistently lower than for others in the sample, but the trend over time was the same -- a pair of up-and-down waves ending where they began.

Ward offered several observations on these data. One was that the findings might reflect suboptimal access to newer therapies, which are also vastly more expensive than old-line drugs such as methotrexate. "Low-income nonprofessional workers ... are most at risk for work disability related to RA," he noted.

He also listed a number of factors that enter into the decision to seek a disability claim: availability of caregiver support, financial resources, and opportunities for less physically demanding work, among others. Examining their potential role in the study's findings would require more data than are available in Medicare records. Impacts of wider economic trends (the study did span the so-called Great Recession) could also only be guessed at.

Another limitation was Ward's assumption that overall RA prevalence remained the same over time -- which is likely true, but not 100% certain.

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