Lupus May Be Twice as Common as Previous Estimates Suggested

Laird Harrison

February 12, 2014

Systemic lupus erythematosus (SLE) may be twice as common as previously believed, afflicting 128.7 of every 100,000 women, data from 2 new studies show.

"Our data clearly show that this is a major women's health issue, and it's one that doesn't tend to get the attention that other issues get," one of the lead investigators, Emily Somers, PhD, told Medscape Medical News.

Using comprehensive new methods, including a public health exemption to privacy laws, the researchers say they have been able to compile the most accurate picture of the disease ever produced in the United States.

"For the first time, we have hard statistics on the rates of disease in a diverse US population with a large representation in the African-American population," said Dr. Somers, an assistant professor of medicine, environmental health, and obstetrics and gynecology at the University of Michigan in Ann Arbor.

Dr. Somers' group focused on a population in Michigan, whereas a companion study focused on a population in Georgia. Both studies, published online January 27 and in the February issue of Arthritis & Rheumatology, confirmed a previous finding that lupus is much more common and more serious among blacks than whites.

The finding on the overall prevalence of the disease is particularly important, Eric Matteson, MD, chair of rheumatology at the Mayo Clinic in Rochester, Minnesota, told Medscape Medical News.

"What these studies suggest is that the number of people in the United States who have lupus is a lot higher than we thought," he said. "It may even be double what we thought. So I think that's quite significant."

Previous studies have extrapolated the incidence and prevalence of lupus from small population samples, leaving uncertainty about their statistical validity. Prevalence rates have ranged from 19 to 241 cases per 100,000 people.

To address this problem, the Centers for Disease Control and Prevention initiated 5 parallel studies in populations around the United States.

For the Michigan study, Dr. Somers and colleagues undertook a study of lupus in the southern Michigan counties of Wayne and Washtenaw, encompassing 2.4 million people.

The researchers scoured the records of hospitals and other health systems, private practice specialists, Medicaid claims, the US Renal Data System, and commercial laboratories.

They determined whether cases met at least 4 of the 11 American College of Rheumatology (ACR) criteria for SLE. A panel of 6 rheumatologists then reviewed the cases, using their judgment and other definitions, including the Boston weighted criteria and the Systemic Lupus International Collaborating Clinics criteria.

This resulted in 2 sets of numbers, one for the simple ACR criteria and one using the rheumatologists' definitions.

Using US Census data, the authors then calculated "crude" rates of lupus incidence (the number of new diagnoses per year) and prevalence (the number of people with an existing diagnosis per year) per person for the years 2002-2004. Then they adjusted those rates for sex and age.

The Georgia researchers, led by S. Sam Lim, MD, MPH, from Emory University, Atlanta, Georgia, studied people living in Fulton and DeKalb Counties, who numbered about 1.6 million by 2002 census estimates. They drew their cases of lupus from sources similar to those used by the Michigan researchers.

They, too, counted up the number of individuals who met at least 4 of the ACR criteria. However, instead of having rheumatologists review the cases for the study, the Georgia researchers also counted the cases diagnosed by treating rheumatologists and those cases meeting fewer than 4 ACR criteria but involving kidney disease.

This resulted in 2 sets of numbers: one for the simple ACR criteria and one using combined definitions. The 2 studies arrived at very similar rates using the ACR definition but differed in their secondary sets of numbers.

Table. Age-Adjusted Prevalence per 100,000 People of SLE

  Michigan Study (ACR Definition) Michigan Study (Rheumatologist Definition) Georgia Study (ACR Definition) Georgia Study (Combined Definitions)
Overall 72.8 64.6 73.0 92.1
Male 12.8 12.5 14.7 19.6
Female 128.7 113.4 127.6 159.8
Black 111.6 103.0 118.5 147.5
White 47.5 39.8 32.7 43.1

Incidence rates using the ACR definition were also similar for the 2 studies, with an overall age-adjusted rate of 5.5 per 100,000 in the Michigan study and 5.6 in the Georgia study.

"These studies were done in 2 different parts of the country and using slightly different methodologies," said Dr. Matteson, who was not involved in either project. "And they came to very similar conclusions about the incidence and prevalence of lupus. So that gives me a pretty high degree of confidence that what they are reporting is true."

These 2 studies did not have large enough populations to reach conclusions about other ethnic groups, said Dr. Somers, but similar studies are underway in California and New York to provide similar estimates for Hispanics and Asians, as well as in the Indian Health Service for American Indians and Alaska Natives.

The studies produced several other interesting findings, Dr. Somers said. For example, they showed that incidence peaks at a younger age for black women than for white women.

This could give clues about risk factors for the disease. Earlier studies of families show that genes play a role but do not completely control who gets lupus and who does not, Dr. Somers said. "We think environmental risk factors are driving a large proportion of the disease."

Researchers are looking at toxins such as mercury as possible triggers, she said.

The Georgia researchers commented on the disparities in kidney disease among black and white patients with lupus.

Overall, 6.7% of people with the disease in their study had end-stage renal disease. There was no racial difference in the proportion of blacks and whites with newly diagnosed lupus who have end-stage renal disease. However, among those people who are living with lupus, 7 times as many blacks as whites have end-stage renal disease, they found.

"Some of this can be explained by an inherently higher risk of developing nephritis in persons of African descent," they write. "However, it also indicates a strong potential for disparities in health care access and other socioeconomic factors."

The new information can also help clinicians catch lupus cases, Dr. Somers said. "I would not say we should increase screening. I think we should increase vigilance and the knowledge base. I would say clinicians should be aware of the high rates of lupus, and particularly that African Americans have a higher rate of end-stage kidney disease and kidney involvement."

The Georgia study was supported in part by the Centers for Disease Control and Prevention. The Michigan study was supported in part by the Michigan Department of Community Health, the National Institutes of Health, and the Herbert and Carol and Amster Lupus Research Fund. Dr. Somers’ work and the work of 1 coauthor was supported in part by the National Institutes of Health. Dr. Matteson has disclosed no relevant financial relationships.

Arthritis Rheum. 2014;66:357-378. Somers article full text, Lim article full text

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