A new study published online in the Annals of the Rheumatic Diseases adds to the evidence that rheumatoid arthritis (RA) increases the risk of developing venous thromboembolism (VTE), or blood clots in the veins.

VTE is comprised of deep vein thrombosis and pulmonary embolism. Deep vein thrombosis (DVT) is a blood clot that forms in one of the body’s deep veins, usually in the leg, blocking blood flow to the area and causing pain, redness and swelling. Pulmonary embolism (PE), a life-threatening condition, occurs when a clot (usually from a DVT) breaks away and travels to the lungs, blocking blood flow and causing shortness of breath, pain and cough. Traditional risk factors for VTE include older age, prolonged immobility, recent surgery, oral contraceptive use and obesity.

For this recent study, researchers in Taiwan analyzed a database of the country’s entire population of more than 23 million people. Researchers identified 29,238 patients diagnosed with RA between 1998 and 2008 and followed them up through 2010. The RA patients were compared to a control group of almost 117,000 people without RA of similar age and gender.

The researchers found RA patients were more than three times as likely as the controls to develop DVT and twice as likely to develop a PE. The risk was even greater for RA patients younger than age 50: They were almost six times as likely than controls to develop DVT and more than three times as likely to get a PE.

“Before, I think RA and other autoimmune diseases were not considered a traditional risk factor for VTE. Now, I think things are a little more clear,” says Seoyoung C. Kim, MD, an assistant professor of medicine at Harvard Medical School in Boston. Dr. Kim was not involved with the Taiwanese study, but she co-authored a separate study, published earlier this year in Arthritis Care & Research, that had similar findings.

“Most data on the risk of VTE in RA patients came out over the past one or two years, so it’s quite recent,” Dr. Kim says.

Although the magnitude of the increase in risk for RA patients varies by study, the risk is still relatively low – estimates put the average risk of DVT at approximately 1 in 1,000 people per year, so an average RA patient would have a risk of about 3 in 1,000.

Anne R. Bass, MD, a rheumatologist at Hospital for Special Surgery in New York, says the emergence of RA as a risk factor for VTE parallels the emergence of RA as a risk factor for cardiovascular disease.

“This is comparable to the recognition 10 or 15 years ago that RA was associated with a higher risk of cardiac death and congestive heart failure. [Researchers] began to tease out [whether] that was due to the disease or treatment, and they even showed treatment reduced the risk of cardiac death,” says Dr. Bass. “So right now we are beginning to show, yes, there is higher risk of VTE with RA. Studies now need to show if it’s the RA itself or therapies [to treat it]. And if you give good therapies, do you reduce the risk back to what it would be without RA?”

Because surgery (including joint replacement surgery) increases the risk of VTE, doctors often put patients on blood-thinning medications around the time of an operation. Other ways people with (and without) RA can reduce their risk include staying well-hydrated, wearing compression stockings on long flights and car rides, and avoiding long periods of sitting and immobility.