Breast cancer may be harder to detect and exclude among women with certain personal and lifestyle characteristics, according to research published in this week's BMJ.

Women aged 50-64 years, who were taking part in the Million Women Study, completed a questionnaire about various personal factors before routine breast screening. A sample of 122,355 women was then monitored for 12 months to examine how these factors influenced the sensitivity and specificity of screening.

These measures provide an indication of how likely women are to have breast cancer diagnosed between routine breast screening appointments and how likely they are to be recalled for further investigation after attending for screening, without having breast cancer.

Three factors had an adverse event on both measures: use of hormone replacement therapy (HRT), previous breast surgery, and a body mass index of 25 or less. The seven other factors examined, including age, family history of breast cancer, physical activity, smoking or alcohol consumption, had no significant effect on either sensitivity or specificity.

"Our results suggest that mammography may thus be less efficient in users of HRT, in women with previous breast surgery, an in thin women compared with other women," concludes Dr Emily Banks, lead author on the paper. "These women are also more likely than other women to have breast cancer diagnosed between screens."

"While these findings suggest that mammography may be more efficient in some women than others, it remains the best way of detecting breast cancer when it is still at an early stage," adds co-author Dr Gillian Reeves. "These results highlight how important it is that, as well as attending for routine screening, women remain breast aware between the three yearly NHS screening intervals."