title: "Linearity assessment methods for sex steroid hormones and carrier proteins among men in the National Health and Nutrition Examination Survey (NHANES III)".
abstract: BACKGROUND
  It has been hypothesized that racial disparities among several diseases are explained by differences in testosterone (T), 17-β estradiol (E), sex hormone binding globulin (SHBG) and albumin (A) levels, yet epidemiologic results have been mixed. Statistical advice regarding appropriate adjustment methods for carrier proteins of sex steroid hormones in the literature is scant. Therefore, we investigated different adjustment methods for carrier proteins.
  
  
  METHODS
  Data for 1496 men, >17 years from the Third National Health and Nutrition Examination Survey (NHANES III) 1988-91 were used to analyze linearity between sex hormones and carrier proteins by examining correlation, plots, and regression models. The statistical importance of age, body mass index (BMI), and race/ethnicity were examined for changes in results by the adjustment method.
  
  
  RESULTS
  T was weakly correlated with SHBG and A (r-squared, 0.25, 0.13, respectively) and E was weakly negatively correlated with A (p<0.0001), but not SHBG (p<0.1799). Based on the model residual plots and r-squared, the categorical model performed better than linear models. Regression coefficients for age, BMI, and race/ethnicity groups using quotient (e.g., T/A and E/A) models differed from continuous and categorical models.
  
  
  CONCLUSIONS
  Choosing an appropriate adjustment for carrier proteins is important to prevent bias in analyses and inconsistency in findings across studies. Linearity between variables should not be assumed when adjusting models, and should be conducted and reported. An independent categorical carrier protein variable is recommended in analysis exploring factors predicting sex hormone levels, although statistical testing should always be employed.
authors.affiliation: University of South Carolina, Department of Epidemiology and Biostatistics, 800 Sumter Street, Columbia, SC 29208, United States; Inter Tribal Council of Arizona, Tribal Epidemiology Center, 2214 N Central Ave., Phoenix, AZ 85004, United States. Electronic address: msritchey@hotmail.com.
authors.forename: Jamie
authors.initials: J
authors.lastname: Ritchey
authors.affiliation: University of South Carolina, Department of Epidemiology and Biostatistics, 800 Sumter Street, Columbia, SC 29208, United States.
authors.forename: Hongmei
authors.initials: H
authors.lastname: Zhang
authors.affiliation: University of South Carolina, Department of Epidemiology and Biostatistics, 800 Sumter Street, Columbia, SC 29208, United States; University of Memphis, School of Public Health, Division of Epidemiology, Biostatistics, and Environmental Health, 301 Robinson Hall, Memphis, TN 38152, United States.
authors.forename: Wilfried
authors.initials: W
authors.lastname: Karmaus
authors.affiliation: University of South Carolina, Department of Epidemiology and Biostatistics, 800 Sumter Street, Columbia, SC 29208, United States.
authors.forename: Susan E
authors.initials: SE
authors.lastname: Steck
authors.affiliation: University of Florida, Department of Environmental and Global Health, PO Box 100188, HPNP Building, Room 2150, Gainesville, FL 32610, United States.
authors.forename: Tara
authors.initials: T
authors.lastname: Sabo-Attwood
mesh_terms: D000293:Adolescent; D000328:Adult; D000418:Albumins / Q000032:analysis*; D004958:Estradiol / Q000031:analogs & derivatives / Q000032:analysis*; D005916:Globulins / Q000032:analysis*; D006801:Humans; D016014:Linear Models; D008297:Male; D009749:Nutrition Surveys; D013739:Testosterone / Q000032:analysis*; D014481:United States; D055815:Young Adult
chemical_list: D000418:Albumins; D005916:Globulins; D013739:Testosterone; D004958:Estradiol
keywords: Adjustment methods; Albumin; Sex hormone binding globulin; Sex steroid hormones