Mayo Clinic researchers say they’ve identified a way to determine the average lifetime risk of getting an inflammatory autoimmune disease such as lupus or rheumatoid arthritis. According to their calculations, the lifetime risk for women is 8.4 percent, or 1 in 12. The lifetime risk for men is 5.1 percent, or 1 in 20.

Their findings were recently published in Arthritis & Rheumatism.

To come up with the lifetime risk figures, researchers crunched numbers from the Rochester Epidemiology Project, which followed 1,179 patients diagnosed with inflammatory autoimmune diseases between 1955 and 2007.

They also looked at specific conditions and calculated that the lifetime risk of rheumatoid arthritis, or RA, for women is 3.6 percent, or 1 in 28. For men it’s 1.7 percent, or 1 in 59.

The lifetime risk of polymyalgia rheumatica is 1 in 41 for women, 1 in 60 for men. And the risk of lupus is smaller still: 1 in 110 for women, 1 in 476 for men.

“We’re adding up the risk across the lifetime to say – somewhere in your life, will you get it? Nobody’s ever done it for RA [and these other conditions],” says Cynthia Crowson, the study’s lead author and an assistant professor in biostatistics at the Mayo Clinic in Rochester, Minn. “Knowing what your lifetime risk is helps you if you want to make any lifestyle changes. The biggest [controllable] risk factor for RA that comes to mind is smoking,” Crowson says.

Risk vs. Prevalence

Lifetime risk is an individual measure of risk. In other words, it’s something you can apply to one person. That’s different from prevalence rates, which are an estimate of how many people currently have the disease. The prevalence rate for RA, for example, is quoted often. It’s 1 in 100.

“The CDC [Centers for Disease Control and Prevention] has done prevalence because that’s what people use for health care policy. That drives what the costs will be to take care of RA patients – you need to know how many you have,” Crowson says. “Lifetime risk is something that can help a person understand their individual risk.”

But researchers stress that risk changes depending on age. “We know RA is more common in older ages. Most of the risk doesn’t occur until they’re over 50,” Crowson says. Your risk also can increase or decrease based on family history or certain lifestyle factors like smoking.

What It Means for Patients

Crowson says these new figures can be helpful to patients in general because they allow them to compare the lifetime risks of these conditions to the lifetime risks of other well known diseases. She says that may help people decide what if any lifestyle changes they should
consider to reduce their lifetime risk. She says it also puts into context how serious these diseases are compared to other major diseases like:

Coronary heart disease – 1 in 3 women; 1 in 2 men

Stroke – 1 in 5 women; 1 in 6 men

Prostate cancer – 1 in 6 men

Breast cancer – 1 in 8 women

Alzheimer’s disease – 1 in 5 women; 1 in 10 men

Parkinson’s disease – 1 in 75 women; 1 in 50 men

“These figures do help us to understand for the first time just how common these diseases are when taken together.  If we say that,  for example, the risk a woman has of getting breast cancer in her life is 1 in 8, then you can see that the risk a woman has for getting a rheumatic disease is just somewhat lower,” says Eric Matteson, MD, a professor of medicine in the division of rheumatology at the Mayo Clinic in Minnesota who also worked on the study. “Taken together, likelihood of getting one of these diseases is not quite as high as some common cancers, but [it’s] certainly not low. Rheumatoid arthritis is the most common of these diseases, and here the likelihood of getting the disease is for women is about a third of the lifetime risk of getting breast cancer, for example.”

Emily Somers, PhD, is an epidemiologist and assistant professor of medicine and environmental health in the division of rheumatology at the University of Michigan in Ann Arbor. She agrees the new lifetime risk estimates can be very helpful for patients, as long as the numbers are properly explained to them.

“Based on experience with risk communication from other disease settings, such as breast cancer, a common misconception with lifetime risk is that patients may think it is their risk of developing disease within the next year, which clearly could cause undue anxiety,” Somers says. “It is important to recognize such issues when communicating with patients, even when terminology is seemingly obvious to health professionals.”

Imperfect Science

Although the study offers new and interesting numbers, Somers says it’s not without flaws. She points out that the data comes from a relatively homogeneous population that is mostly white.

“Therefore, the risk estimates from this study are most applicable to persons from similar demographic backgrounds,” Somers says. “In order to better estimate risk for the broader U.S. population, it would be important to have data from populations with greater racial, ethnic and socioeconomic diversity. The ideal would be to generate estimates specific for various racial and ethnic groups.”  

The authors mention that in the paper as a limitation, and say that going forward, they would like to get more specific with lifetime estimates by breaking them down by things like risk factors.