Clinical trials dropout rates

To complicate matters further, there is a high rate of dropout among investigators. Many conduct one or two trials and choose to never conduct another one, leading to a dilemma in which 50% of US principal investigators have opted out of the clinical trials business. The reasons cited are that clinical research interferes too much with other responsibilities such as private practice medicine or academic obligations, or they lack the infrastructure to handle today s rigorous trials. 

A systematic review of clinical trials indicated that St. John s wort has been well tolerated in trials, with no serious adverse effects reported. Patient dropout rates and adverse effects reports were similar to those of placebo (Knuppel and Linde 2004). 

A systematic review of adverse events reported in clinical trials of St. John s wort indicated that data from 35 doubleblind randomized trials showed that dropout and adverse event rates in patients receiving St. John s wort extracts were similar to placebo, lower than with older antidepressants, and somewhat lower than with SSRI antidepressants. Dropout rates due to adverse events ranged from 0 to 5.7% in 17 observational studies that included 35,562 patients. No serious adverse events were reported in any of the studies (Knuppel and Linde 2004). 

A review of clinical trials on cranberry juice indicated a high dropout rate for many of the trials, although no significant adverse events were reported in the trials reviewed (Jepson et al. 2007). 

One hundred twenty-five events would provide 90% power, and 95 events would provide 80% power to illustrate the 95% Cl is < 1.8 based on a Cox proportional hazard model. Therefore, in a clinical trial with an expected event rate of 1 event per 100 patient-years and a 5% annual dropout rate, 90% power would be achieved by enrolling 200 patients per month for 24 months and following all enrolled patients for an additional 24 months. The total trial duration would be 48 months. If, however, the true event rate was actually 0.75%, the power would decrease to 80%—a doubling of the type 2 error. Furthermore, if the event rate were actually 0.5%, then the power would drop to 65%, a type 2 error 3.5 times as high. Because it is hard to predict CV event rates in noncardiac populations, such overestimates are not uncommon. Similarly, if the actual event rate... 

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