LAUSR

OBJECTIVE Epidemiologic data for mixed connective tissue disease (MCTD) are limited. Leveraging data from the Manhattan Lupus Surveillance Program (MLSP), a racially/ethnically diverse population-based registry of cases with SLE and related diseases including MCTD, we provide estimates of the prevalence and incidence of MCTD. METHODS MLSP cases were identified from rheumatologists, hospitals, and population databases using a variety of ICD-9 codes. MCTD was defined as one of the following: 1) fulfillment of our modified Alarcon-Segovia and Kahn criteria which required a positive RNP antibody and the presence of synovitis, myositis, and Raynaud's phenomenon, 2) a diagnosis of MCTD and no other diagnosis of another connective tissue disease (CTD), and 3) a diagnosis of MCTD regardless of another CTD diagnosis. RESULTS Overall, 258 (7.7%) of cases met a definition of MCTD. Using our modified Alarcon-Segovia and Kahn criteria for MCTD, the age-adjusted prevalence was 1.28 (95%CI 0.72-2.09) per 100 000. Using our definition of a diagnosis of MCTD and no other diagnosis of another CTD yielded an age-adjusted prevalence and incidence of MCTD of 2.98 (95%CI 2.10-4.11) per 100 000 and 0.39 (95%CI 0.22-0.64) per 100 000, respectively. The age-adjusted prevalence and incidence were highest using a diagnosis of MCTD regardless of other CTD diagnoses and were 16.22 (95%CI 14.00-18.43) per 100 000 and 1.90 (95%CI 1.49-2.39) per 100 000 respectively. CONCLUSIONS The MLSP provided estimates for prevalence and incidence of MCTD in a diverse population. The variation in estimates using different case definitions is reflective of the challenge of defining MCTD in epidemiologic studies.