Estimating the incidence of systemic lupus erythematosus in a tropical region (Natal, Brazil)

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Abstract
The objective of the study was to evaluate the incidence of Systemic Lupus Erythematosus (SLE) in a tropical urban community (Natal city, Brazil). Only patients living in Natal, a city in the northeastern area of Brazil, older than 15 years, and who fulfilled at least four of the American College of Rheumatology criteria between 1 January 2000 and 31 December 2000, were included. Four sources were used to identify new cases of SLE: (1) the University Hospital; (2) ‘health units’ and hospitals of the public health network; (3) specialists at private hospitals and outpatient clinics; and (4) three laboratories performing antinuclear antibody (ANA) determination. Census data from 2000 for Brazilian population was used to calculate incidence rate. The standardized mortality ratio (SMR) method and 95% confidence intervals (95% CI) were calculated. Forty-three patients were diagnosed as new SLE cases in 2000. The calculated incidence was 8.7/100 000/year (95% CI 6.3–11.7). Thirty-eight patients were female 14.1/100000/year (95% CI 10.0–19.3) and five were male 2.2/100 000/year (95% CI 0.7–5.2). The mean age of new SLE cases was 31.8 years old. (95% CI 27.8–35.8). The mean age for females was 31.4 and for males was 35.0 years old. The median of disease duration (time between onset of the first ACR criterion for SLE and diagnosis) was 10 months (1–72 months). This study demonstrated a high incidence of SLE in Natal, apparently higher than reported in other places. The mean age at diagnosis seems lower than referred by other studies. The observed differences may be due to ethnic and/or environmental factors.