A new study from Sweden suggests that patients with rheumatoid arthritis may be at higher risk for blood clots in the first ten years after diagnosis compared to the general population.

But while admission to hospital was also a risk factor for blood clots in such patients, Marie Holmqvist of the Karolinska Institute in Stockholm, and colleagues, found this to be no greater than it was for the general population.

This second finding is important because it was generally believed that hospitalization increased the risk of a blood clot or venous thromboembolism (VTE) for a patient with rheumatoid arthritis because the chronic inflammation can result in the blood tending to coagulate more.

Such a belief could lead scientists to wrong conclusions about the underlying biology of VTE in patients with rheumatoid arthritis, such as the timing of risk development, which in turn could result in inappropriate treatments.

As far as the authors know, this is the first study to look at the risk for VTE in patients with rheumatoid arthritis separately from risks tied to hospitalization.

They write about their findings in the 3 October issue of Journal of the American Medical Association, JAMA.

For their study, Holmqvist and colleagues used two cohorts, both Swedish. One cohort was from the National Patient Register and included 37,856 rheumatoid arthritis patients diagnosed between 2005 and 2009, plus 169,921 matched controls from the general population.

The other cohort was from a national rheumatology register and included 7,904 patients and 37,350 controls enrolled at time of diagnosis between 1997 and 2009. The researchers used data from this cohort to analyze risk of VTE relative to disease onset. The results showed that:

  • Patients with rheumatoid arthritis were at greater risk for VTE than the general population (5.9% elevated risk versus 2.7% per 1,000 person-years respectively).

  • By the time of rheumatoid arthritis symptom onset, there was no statistically significant link between a history of VTE and rheumatoid arthritis.

  • Patients with rheumatoid arthritis have an increased risk of VTE in the first year after diagnosis compared with controls (3.8% versus 2.4% per 1,000 person-years respectively), but this risk does not increase further during the first decade.

  • Although rates for VTE after hospitalization were higher for rheumatoid arthritis patients than for the general population, the rate for VTE per 1,000 person-years in the first year after hospital discharge was not.

The researchers conclude: “Compared with the general population, Swedish patients with RA [rheumatoid arthritis] had an elevated risk for VTE that was stable over the first 10 years after diagnosis.”

“Although hospitalization was a risk factor for VTE the first year after discharge, the excess risk was not greater in patients with RA than in the general population,” they add.

Rheumatoid arthritis is a chronic, progressive and disabling auto-immune disease that causes inflammation, pain, and swelling of joints. Persistent inflammation over time can damage affected joints. The extent of disability varies depending on severity, but eventually it affects a person’s ability to carry out everyday tasks.

The disease can progress very fast, and can affect any joint, but most commonly affects hands, feet and wrists. Because it is a systemic disease it can also affect internal organs like the lungs, heart and eyes, although this is not the case for all patients.

Women are three times more likely to develop the disease than men, and it more commonly strikes between the ages of 40 and 60, although it can occur at any age. In the UK there are over 600,000 people living with rheumatoid arthritis, or about 1% of the population.

There is currently no cure for the disease, and the earlier it is diagnosed the bigger the effect treatment has on its course and the chances of improving quality of life and reducing joint damage.

Written by Catharine Paddock PhD