Nested Case-Control Study

Background A cross-sectional study has reported that nickel allergy is associated with endometriosis. However, causal studies of this association are limited. Objective The objective of this study was to compare the prevalence of nickel allergy in women with and without endometriosis. Methods We used a National Health Insurance Service (NHIS) sample cohort dataset that included approximately 1 million individuals from South Korea; the data were obtained between Jan-uary 01, 2002, and December 31, 2013. We selected the endometriosis group according to diagnosis code (N80.X), surgery codes, and drug codes during the years 2009~2013. The controls were randomly matched to the endometriosis patients at a ratio of 4:1 by age and socioeconomic status. Patients with nickel allergy were defined in the cohort dataset as those with a simultaneous diagnosis code (L23.0) and patch test code during 2002~2008.