A rapid review indicated higher recruitment rates in treatment trials than in prevention trials.

Objectives: To test the hypothesis that the percentage of patients screened that randomize differs between prevention and therapy trials.

Study Design And Setting: Rapid review of randomized controlled trials (RCTs) identified through published systematic reviews in August 2013. Individually randomized, parallel group controlled RCTs were eligible if they evaluated metformin monotherapy or exercise for the prevention or treatment of type 2 diabetes. Numbers of patients screened and randomized were extracted by a single reviewer. Percentages were calculated for each study for those randomized: as a function of those approached, screened, and eligible. Percentages (95% confidence intervals) from each individual study were weighted according to the denominator and pooled rates calculated. Statistical heterogeneity was assessed using I(2).

Results: The percentage of those screened who subsequently randomized was 6.2% (6.0%, 6.4%; 3 studies, I(2) = 100.0%) for metformin prevention trials; 50.7% (49.9%, 51.4%; 21 studies, I(2) = 99.6%) for metformin treatment trials; 4.8% (4.7%, 4.8%; 14 studies, I(2) = 99.9%) for exercise prevention trials; and 43.3% (42.6%, 43.9%; 28 studies, I(2) = 99.8%) for exercise treatment trials.

Conclusion: This study provides qualified support for the hypothesis that prevention trials recruit a smaller proportion of those screened than treatment trials. Statistical heterogeneity associated with pooled estimates and other study limitations is discussed.