Comment on 'statistical consideration and challenges in bridging study of personalized medicine': a modified variance for sensitivity analysis.

A pivotal clinical trial is often necessary to assess drug efficacy in the intended to use (IU) population. Ideally, patients should be enrolled based on a positive test result from a well-characterized companion diagnostic (CDx). However, the central challenge is that patients are instead recruited on the basis of a clinical trial assay (CTA) result. This challenge arises because, CTA is available at all local labs; the time delay to enable enrollment based on CDx could result in a significant proportion of patients being unable to participate, adversely affecting precision and/or bias. The difficulty is therefore that patients are recruited on the basis that their CTA result is positive (CTA+) but the goal is to assess the drug efficacy in patients positive by the companion diagnostic (CDx+). In this commentary, we will examine an apparent weakness of a variance formula that is proposed in the context of a sensitivity analysis. We will develop an alternative formula, and argue that this should be used instead. [ABSTRACT FROM AUTHOR]