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PURPOSE: Even when a screening study has demonstrated a mortality reduction, the degree of pre-testing and contamination is of importance as it can dilute the "true" effect of screening. Our object was to describe the level of pre-testing and contamination in the Göteborg-1 prostate cancer screening trial.

MATERIALS AND METHODS: A total of 20,000 men, 50-64 years old, were invited in 1994 and randomized to either a screening group (offered prostate specific antigen testing every 2 years) or to a control group. Follow-up was through December 31, 2014. Outcome measurement was overall testing in the screening group and control group. A positive prostate specific antigen test was defined as a prostate specific antigen ≥3 ng/ml.

RESULTS: In the study, 4.2% in the screening group and 4.6% men in the control group were tested before study start. During follow-up, 72% in the control group took at least 1 prostate specific antigen test (contamination) compared to 87% of men in the screening group. Of all prostate specific antigens, 24% in the screening group and 39% in the control group were above threshold. In total, 66% of the men underwent prostate biopsy within 12 months from a raised prostate specific antigen in the screening group and 28% in the control group.

CONCLUSIONS: Similar proportions of men were prostate specific antigen-tested in both the screening group and control group, yet only a minority of contamination prostate specific antigens led to biopsy. Also, men in the screening group started screening at a younger age. These could both be explanations for our result that organized screening is more effective in reducing prostate cancer mortality than non-organized testing. When carried out properly and compared to an unscreened population, the effects of organized screening are likely even greater than previously shown in the Göteborg screening trial.