A study shows that tuberculosis infection (TB) is 10 times more common among people who have rheumatoid arthritis than it is in the general population, and that risk appears to be elevated by treatment with any immunosuppressive medication, not just biologic drugs.

Previous research had demonstrated a greater risk of TB in people with rheumatoid arthritis who were also taking biologic drugs, such as infliximab (Remicade).

But the new study, by researchers at McGill University Health Center in Montreal, Quebec, Canada, suggests that the risk is increased for anyone with rheumatoid arthritis who is taking a corticosteroid such as prednisone, or a disease modifying anti-rheumatic drug (DMARD), such as methotrexate (Trexall, Rheumatrex) or leflunomide (Arava).

Researchers surveyed 24,282 people with rheumatoid arthritis (RA) who had records in the public health system databases. Within that group, 50 people were diagnosed with TB from 1992-2003, about 10 times as many as were diagnosed in the general population of Quebec.

They then looked at exposure to a variety of medications including corticosteroids and non-biologic DMARD drugs among those who developed TB and those who did not.

Their analysis suggested that the risk of TB infection increases with the number of prescriptions filled, becoming significant for chronic users and those with six or more prescriptions.

Paul Brassard, MD, Associate Professor of Medicine at McGill University led the study, which was published in the March 2009 issue of Arthritis Care & Research. He says the findings were particularly interesting considering that the total number of TB cases in Canada is low.

Dr. Brassard admits there is a downside of studying information from a database. For example, researchers could not determine if any study participants had contact with someone who had TB or had visited a country where it is common. Their findings also could represent the life experience of older subjects, who were more likely to have been exposed to TB in their youth when the disease was more common in Canada.

Richard E. Chaisson, M.D. is a professor of medicine, epidemiology and international health at the Johns Hopkins University School of Medicine and founding director of its Center for Tuberculosis Research. He agrees this study is a nice addition to already existing research, but says there are some serious limitations that detract from the ability to understand where the increase risk is coming from because researchers didn’t look at the effect of tumor necrosis factor alpha or (TNF-a) blocking drugs.

“What they are showing is patients with arthritis in Quebec have an increased risk of tuberculosis but they can’t really explain it all because they don’t have information on the most important risk factor (the TNFa drugs), so that’s a limitation,” he says.

“They speculate the reason probably is use of anti-TNF drugs. But they weren’t able to measure that, which is unfortunate. But it certainly is consistent with what has been reported elsewhere,” Dr. Chaisson explains.

Still, doctors agree that the new study’s findings support published guidelines that recommend TB screening before starting any immunosuppressive therapy. And if screening wasn’t done before starting the medicine, it should be done after. Despite a warning on drug packaging and in advertisements, many say the increased risk is still not well enough known.

“Screening doesn’t always happen and there certainly needs to be more awareness among patients and doctors that prescribe these drugs,” Dr. Chaisson says.