Assessing the extent of contamination in the Canadian National Breast Screening Study

To estimate the proportion of Canadian National Breast Screening Study (NBSS) participants who went outside the trial for a mammogram, based on health insurance claims data.

Prospective cohort study linking trial subjects with population-based administrative data.

All NBSS participants enrolled in the Winnipeg screening center who had health insurance claims to Manitoba Health (n = 9,780).

Claims for bilateral mammograms were compared by screening arm allocation and age group at enrollment. Mammograms likely to be "screening" were defined based on prior claim history.

For women aged 40 to 49 at enrollment, 5.3% in the intervention group and 21.8% in the control group had a claim for at least one bilateral mammogram. After excluding nonscreening mammograms these proportions fell to 2.2% and 14.1% (P < 0.0001). For women aged 50 to 59, 4.5% in the intervention group and 16.7% of the control group had at least one claim for a bilateral mammogram. These proportions were 2.1% and 10.5% for screening (P < 0.0001).

Health care utilization data can be used to estimate contamination in a screening trial. The levels of contamination resulting from participants going outside the study for a screening test can have an impact on the power of the study and need to be considered when interpreting results and planning future screening studies.