People with rheumatoid arthritis (RA) are almost 5 percent more likely than other people to develop reduced kidney function, a sign of chronic kidney disease, according to a study published recently in the American Journal of Kidney Diseases

Although that figure may not sound high, senior study author Eric Matteson, MD, chair of the rheumatology department at Mayo Clinic in Rochester, Minn., says it’s significant. “It’s a big deal when you think there’s 1.5 million patients with RA, and how frequent kidney disease is becoming in the population.”

Chronic kidney disease is the gradual loss of kidney function, which can progress to kidney failure. Chronic kidney disease, in general, can be caused by a number of conditions, including diabetes, high blood pressure, autoimmune processes and inflammation within the kidney.

Previous studies have suggested an increased risk of kidney disease in people with RA due to chronic inflammation and exposure to certain medications. “There was kind of a suspicion about that, and we knew that there were certain types of kidney problems that people with RA can get, but no one studied in a systematic way what that risk is,” says Dr. Matteson.

This study looked at the health records of 1,623 people – 813 people who were diagnosed with rhuematoid arthritis between 1980 and 2007 and 813 people without RA who were similar in terms of age, gender, weight and other factors. To gauge their kidney function, the researchers looked at their recorded levels of creatinine, a waste molecule that the kidneys filter from the blood. They then used a formula to calculate the estimated glomerular filtration rate (eGFR), or how much blood moves through the glomeruli – tiny filters in the kidneys – each minute. A low glomerular filtration rate indicates poor kidney function 

Among those who had normal kidney function when they entered the study, 25 percent of those with RA and 20 percent of those without RA developed mildly or moderately reduced kidney function over 20 years. There was no difference in the rate of severe kidney dysfunction.

RA patients who had high levels of inflammation in the first year after RA diagnosis (as measured by the blood test known as erythrocyte sedimentation rate, or “sed rate”) were more likely than others to develop reduced kidney function. “The general conclusion from that is that inflammation is a risk factor for renal dysfunction,” says Dr. Matteson.