Two new studies representing the latest efforts to determine the incidence and prevalence of systemic lupus erythematosus in the United States have revealed the difficulty of ascertaining cases when definitions of the disease vary.

The two population-based registries on which the studies were based – the California Lupus Surveillance Project and the Manhattan Lupus Surveillance Program (MLSP) – confirmed that black women represent the highest risk group for systemic lupus erythematosus (SLE) and also reaffirmed the elevated risk observed in Hispanic and Asian women, compared with white women.

The MLSP researchers applied the updated 1997 American College of Rheumatology (ACR) classification criteria, the Systemic Lupus International Collaborating Clinics (SLICC) classification criteria, or a treating rheumatologist’s diagnosis to records from hospitals, rheumatologists, and administrative databases, and looked at both the prevalence of the disease in 2007 and the incidence from the period of 2007-2009. Using the ACR’s definition of SLE, they found an age-standardized prevalence of 62.2 and an incidence rate of 4.6 per 100,000 person-years (Arthritis Rheumatol. 2017 Sep 11. doi: 10.1002/art.40192).

The prevalence was significantly higher in women than in men (107.4 vs. 12.5 per 100,000 persons), and was highest overall in non-Hispanic black women (210.9), followed by Hispanic women (138.3), non-Hispanic Asian women (91.2), and non-Hispanic white women (64.3).

However, use of the SLICC classification criteria increased the age-standardized prevalence by 17%-19%, to a prevalence of 73.8 per 100,000 person-years and resulted in a 35% increase in incidence, to 6.2 per 100,000 person-years, compared with the ACR rates.

“The small number of cases that met the ACR but not the SLICC case definition is reassuring as it suggests that few cases met ACR criteria for SLE without the presence of autoantibodies,” wrote Peter M. Izmirly, MD, of New York (N.Y.) University, and his coauthors.

“However, given the descriptive nature of the MLSP and the absence of a gold standard test that would unambiguously identify SLE, this project cannot assess which set of classification criteria is more sensitive or specific.”

Meanwhile, the California Lupus Surveillance Project, a registry of people with lupus living in San Francisco County between 2007 and 2009, used the ACR definition to record an age-standardized annual incidence rate of 4.6 per 100,000 person-years, and an average annual prevalence of 84.8 per 100,000 persons (Arthritis Rheumatol. 2017 Sep 11. doi: 10.1002/art.40191).

As with the MLSP, the prevalence was highest in black women (458.1), followed by Hispanic women (177.9), Asian women (149.7) and white women (109.8). The incidence was 30.5 per 100,000 person-years in black women, 8.9 in Hispanic women, 7.2 in Asian women, and 5.3 in white women.

Incidence and prevalence rose slightly when the authors used a combined approach to case definition that encompassed individuals who met the ACR criteria; were diagnosed with SLE by the treating rheumatologist and also met 3 of the 11 ACR criteria “to allow for the possibility of missing data and inability to confirm criteria in the available medical records for prevalent cases with longstanding disease”; or had lupus-related kidney disease defined either by presence of World Health Organization class II-VI lupus nephritis biopsy findings or by the presence in the medical record of SLE along with either dialysis or renal transplantation. The combined definition approach gave an age-standardized incidence of 5.2 per 100,000 person-years and a prevalence of 96.8 per 100,000. The breakdown of prevalence by race and ethnicity also rose to 498.4 in black women, 209.9 in Hispanic women, 171.0 in Asian women, and 130.0 in white women.

Looking at the clinical manifestations of disease, researchers found hematologic was the most common, affecting 84% of patients, while neurologic disorder affected only 8% of the population. Immunologic manifestations were present in 80%, arthritis in 57%, renal disorders in 45%, pleuritis or pericarditis in 41%, and malar rash in 33%.

There were racial variations in manifestations, with renal manifestations being more common in black, Asian/Pacific Islander, and Hispanic patients, compared with whites. Discoid rash was most common among black patients, while it was not evident at all in Hispanic patients.

Maria Dall’Era, MD, of the Russell/Engleman Research Center at the University of California, San Francisco, and her coauthors said that given the changing demographic of the diverse San Francisco County, a reliable estimate of the burden of lupus in different racial and ethnic groups was essential for health care planning.

“Up until the recent completion of the Georgia and Michigan surveillance projects, most previous epidemiologic studies were limited by small geographic areas, homogeneous populations, varying case definitions, and incomplete case ascertainment that relied on administrative codes or patient self-reported diagnosis,” they wrote.

To overcome the previous lack of data in Asian and Hispanic populations, the authors worked with physicians serving these populations and performed extensive case-finding in hospitals and health care clinics.

“Our approach of partnering with the community and engaging culturally and linguistically concordant community members led to successful case ascertainment of these traditionally understudied populations,” they wrote. “Had we not taken these extra steps, we would have missed SLE cases in the Asian and Hispanic populations.”

Both studies noted that racial and ethnic data were determined from medical records, and may not have accurately reflected the patient’s race or ethnicity.

The Manhattan study was supported by the Centers for Disease Control and Prevention and the New York City Department of Health and Mental Hygiene. The San Francisco study was supported by the Centers for Disease Control and Prevention and the Russell/Engleman Rheumatology Research Center at UCSF. No conflict of interest disclosures were available.

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Lack of lupus ‘gold standard’ definition hampers estimates of its incidence, prevalence