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OBJECTIVES: To examine how lifestyle, hormonal, and other factors influence the sensitivity and specificity of mammography. METHODS: Women recruited into the Million Women Study completed a questionnaire about various personal factors before routine mammographic screening. A sample of 122,355 women aged 50-64 years were followed for outcome of screening and incident breast cancer in the next 12 months. Sensitivity and specificity were calculated by using standard definitions, with adjustment for potential confounding factors. RESULTS: Breast cancer was diagnosed in 726 (0.6%) women, 629 in screen positive and 97 in screen negative women; 3885 (3.2%) were screen positive but had no subsequent diagnosis of breast cancer. Overall sensitivity was 86.6% and specificity was 96.8%. Three factors had an adverse effect on both measures: use of hormone replacement therapy (sensitivity: 83.0% (95% confidence interval 77.4% to 87.6%), 84.7% (73.9% to 91.6%), and 92.1% (87.6% to 95.0%); specificity: 96.8% (96.6% to 97.0%), 97.8% (97.5% to 98.0%), and 98.1% (98.0% to 98.2%), respectively, for current, past, and never use); previous breast surgery v no previous breast surgery (sensitivity: 83.5% (75.7% to 89.1%) v 89.4% (86.5% to 91.8%); specificity: 96.2% (95.8% to 96.5%) v 97.4% (97.3% to 97.5%), respectively); and body mass index < 25 v > or = 25 (sensitivity: 85.7% (81.2% to 89.3%) v 91.0% (87.5% to 93.6%); specificity: 97.2% (97.0% to 97.3%) v 97.4% (97.3% to 97.6%), respectively). Neither sensitivity nor specificity varied significantly according to age, family history of breast cancer, parity, past oral contraceptive use, tubal ligation, physical activity, smoking, or alcohol consumption. CONCLUSIONS: The efficiency, and possibly the effectiveness, of mammographic screening is lower in users of hormone replacement therapy, in women with previous breast surgery, and in thin women compared with other women.

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Body Mass Index, Breast Neoplasms, Cohort Studies, Female, Hormone Replacement Therapy, Humans, Life Style, Mammography, Mass Screening, Menopause, Middle Aged, Postoperative Complications, Prognosis, Sensitivity and Specificity