Objective: To investigate whether the risk of interstitial cystitis increases among the patients with systemic lupus erythematosus. Methods: This was a nationwide population-based cohort study. Data were obtained from the National Health Insurance Research Database in Taiwan. Women aged >18 years newly diagnosed as systemic lupus erythematosus during 2001–2008 were identified as the control group. The comparison included individuals randomly selected from the National Health Insurance Research Database in the year of 2000, by matching one systemic lupus erythematosus participant with eight non-systemic lupus erythematosus participants with sex and age. These participants were followed up until being diagnosed as interstitial cystitis, or the end of 2011. Women diagnosed with lupus cystitis were excluded from this study. Results: This study included 7240 women with systemic lupus erythematosus and 57 920 women without systemic lupus erythematosus as controls. The incidence rate of interstitial cystitis was significantly higher in the systemic lupus erythematosus group, with an incidence rate ratio of 2.26 (95% confidence interval 1.57–3.27, P < 0.0001). After adjustment, the risk increased by 2.45-fold (adjusted hazard ratio 2.45, 95% confidence interval 1.57–3.27, P < 0.05). Age as a factor increases incidence rate ratios among all age groups, 2.12-, 3.32- and 4.65-fold. Age ≥45 years had an increased adjusted hazard ratio (2.07, 95% confidence interval 1.37–3.13, P < 0.05). Comorbidities, for example, hypertension, diabetes mellitus, dyslipidemia and renal disease, were insignificant. Conclusions: This is the first population-based cohort study showing a higher incidence of interstitial cystitis among patients with systemic lupus erythematosus. These findings support the concordance of interstitial cystitis with autoimmune diseases, and the temporal relationship to develop interstitial cystitis in patients with systemic lupus erythematosus.
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