Karolinska study adds new insight into risk of thromboembolism in patients with RA

Earlier studies have shown that people with rheumatoid arthritis (RA) are more likely to develop blood clots in the legs or lungs - venous thromboembolism - especially in conjunction with hospitalisation. It has therefore been suggested that people with RA be given routine anticoagulants when admitted to hospital. Researchers at Karolinska Institutet have now shown in a large-scale registry study that although this patient group is indeed more likely to develop thromboembolisms, the risk is different to what was previously feared.

The study, which is published in the scientific journal JAMA, renews medical science's understanding of how the risk of thromboembolism - or the lodging of a blood clot in a blood vessel - changes with time for RA patients.

"Some previous studies have only included people who are in hospital for RA," says Dr Marie Holmqvist, researcher at the Department of Medicine in Solna. "But today it's not so common amongst these patients, which means that studies are done on people who are more than averagely sick. It's also hard to make out if the increased risk of venous thromboembolism is attributable to the RA itself or to a generally higher risk connected with being in hospital."

For this present study, the researchers therefore examined, on the one hand, the risk of thromboembolism amongst patients with RA who had not been identified on the basis of hospitalisation, and the risk of thromboembolism amongst hospital patients on the other. All patients were compared with controls from the normal population, matched for variables such as age and sex.

The study included some 40,000 RA patients in specialist outpatient care, as identified by the National Board of Health and Welfare's patient register, and over 8,000 patients from the quality register for rheumatology, which includes newly diagnosed patients. The team also used various registries to find out which patients had been in hospital during the study period and which had experienced a venous thromboembolism.

The team, which comprises researchers from Karolinska Institutet, Umeå University and Lund University, was able to confirm that RA per se is a risk factor for thromboembolism. During a median follow-up period of six years, they found that 2.8 per cent of the patients with RA had developed a clot in the leg or lung, as opposed to 1.7 per cent of the normal population.

However, the study gave a different picture of the risk trend for RA patients. The risk of thromboembolism was increased already one year after diagnosis, but then remained relatively stable for the remainder of the follow-up period. The risk of thromboembolism associated with a hospital stay was no higher in the RA patients than in the normal population.

"It's essential that we fully understand why and when people with RA run a higher risk of thromboembolism, since an erroneous conclusion can, at worst, give rise to treatment recommendations associated with an increased bleeding risk," says Dr Holmqvist. "This is particularly serious when it comes to RA patients who frequently already are on medication which affect the blood's clotting properties, such as certain anti-inflammatory drugs or pain killers."

The study was financed by grants from the Swedish Research Council, the Swedish research programme "Controlling Chronic Inflammatory Diseases with Combined Efforts" (COMBINE), and the Swedish Foundation for Strategic Research (SSF).