Primary and secondary outcomes
In total, 46 of 83 RCTs (55%) did not achieve their targeted recruitment within the pre-planned study period with a maximal extension period of 6 months (Table 2). Recruitment was not achieved within the pre-planned study period with a maximal extension period of 12 months in 41 RCTs (49%). Of these 41 RCTs, 29 studies had a total recruitment period of up to five years, and 12 RCTs finished their recruitment within five to ten years.
Nineteen RCTs (23%) stopped prematurely due to recruitment issues. Of these 19 RCTs, four studies reached 0 to 10% of their recruitment target, six studies 10 to 20%, two studies 20 to 30%, five studies 30 to 60% and two studies reached 70 to 80% of their planned recruitment target.
The mean recruitment period was 50 months (range 12-96 months) for RCTs with recruitment failure versus 31 months (range 12-91 months) for RCTs without recruitment failure. Twenty-two RCTs had a recruitment period of over 48 months. The actual absolute recruitment rate was 4.5 inclusions per month in RCTs with recruitment failure compared to 18.5 inclusions per month in RCTs without recruitment failure (p<0.001).