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.