A case-control investigation of adenomyosis: impact of control group selection on risk factor strength

Using a medical record abstraction-based case-control study with two control groups, we evaluated adenomyosis risk factors and investigated differences related to comparison group selection.

Medical records of all female 18- to 49-year-old Group Health (GH) enrollees with ICD-9 code 617.0 were abstracted using a standard data collection form. Cases were enrollees diagnosed with adenomyosis (n = 174) between April 1996 and September 2001. For comparison, medical records of two control groups were selected from the GH population: An age-matched sample of female enrollees (population-based controls; n = 149) and all female 18- to 49-year-old enrollees undergoing a hysterectomy (hysterectomy controls; n = 106) during the same time without adenomyosis. Odds ratios (OR) and 95% confidence intervals (CI) were calculated using logistic regression, adjusted for identified covariates.

Compared with normal and underweight women, overweight and obese women had increased adenomyosis risk using hysterectomy controls (OR, 2.2, 95% CI, 1.0-4.5; obese: OR, 2.2; 95% CI, 1.1-4.3) and population controls (overweight: OR, 2.1; 95% CI, 1.2-4.0; obese: OR, 3.8; 95% CI, 2.0-7.0). Using population controls, women with at least one live birth were more likely to have adenomyosis than nulliparous women (OR, 3.4; 95% CI, 1.9-6.2).

Although some risk factors persisted in analyses using either control group, divergent results in relation to other risk factors for adenomyosis suggest that results of investigations of this disease may be affected by the choice of the comparison population.