title: Nickel Allergy Is a Risk Factor for Endometriosis: An 11-Year Population-Based Nested Case-Control Study.
abstract: 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 January 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.
  
  
  RESULTS
  In total, 4,985 women were selected from the NHIS cohort database and divided into an endometriosis group (997 women) and a control group (3,988 women). The number of patients with nickel allergy in the endometriosis group was eight (0.8%), and that in the control group was thirteen (0.3%). After adjustment for age and socioeconomic status, the rate of nickel allergy in was higher in the endometriosis group than in the control group [odds ratio: 2.474; 95% confidence interval: 1.023~5.988; p = 0.044].
  
  
  CONCLUSIONS
  We found that nickel allergy is a risk factor for endometriosis.
authors.affiliation: Department of Obstetrics and Gynecology, School of Medicine, Eulji University, Daejeon, South Korea; Department of Obstetrics and Gynecology, MizMedi Hospital, Seoul, South Korea.
authors.forename: Jin-Sung
authors.initials: JS
authors.lastname: Yuk
authors.affiliation: Department of Obstetrics and Gynecology, Hyundai UVIS hospital, Incheon, South Korea.
authors.forename: Jong Seung
authors.initials: JS
authors.lastname: Shin
authors.affiliation: Department of Preventive Medicine, School of Medicine, Eulji University, Daejeon, South Korea.
authors.forename: Ji-Yeon
authors.initials: JY
authors.lastname: Shin
authors.affiliation: Department of Internal Medicine, MizMedi Hospital, Seoul, South Korea.
authors.forename: Eunsuk
authors.initials: E
authors.lastname: Oh
authors.affiliation: Department of Family Medicine, MizMedi Hospital, Seoul, South Korea.
authors.forename: Hyunmee
authors.initials: H
authors.lastname: Kim
authors.affiliation: Department of Obstetrics and Gynecology, School of Medicine, Eulji University, Daejeon, South Korea.
authors.forename: Won I
authors.initials: WI
authors.lastname: Park
mesh_terms: D000328:Adult; D000367:Age Factors; D016022:Case-Control Studies; D004715:Endometriosis / Q000453:epidemiology*; D005260:Female; D006801:Humans; D006967:Hypersensitivity / Q000453:epidemiology*; D008875:Middle Aged; D009532:Nickel; D015995:Prevalence; D056910:Republic of Korea / Q000453:epidemiology; D012307:Risk Factors; D012959:Socioeconomic Factors
chemical_list: D009532:Nickel