Issues on the selection of non-inferiority margin in clinical trials

The determination of non-inferiority margin is an important and confusing issue which directly influences the acceptability of a new medication. We reviewed the published literature, International Conference on Harmonization of Technical Requirements for Registration of Pharmaceuticals for Human Use (ICH) Guidelines and Committee for Proprietary Medicinal Products (CPMP) papers on the selection of non-inferiority margin and the corresponding statistical tests in clinical trials, in order to provide practical recommendations and suggestions for establishing reference criteria for the non-inferiority margin in China. Data sources The literature on the selection of a non-inferiority margin and statistical tests was mainly extracted from relevant English articles on non-inferior clinical trials published from 1990 to 2007. The starting point (1990) was chosen due to lack of such papers published prior to 1990. This literature was searched via PubMed, Medline and Chinese Knowledge Information (CNKI). ICH guidelines and CPMP papers were downloaded from their official websites. The keywords "clinical trial", "non-inferiority" and "non-inferiority margin" were used. Study selection Forty-three original articles and critical reviews, ICH E10 guideline and CPMP papers were selected.

The non-inferiority testing with treatment difference and ratio are commonly used, where the non-inferiority margin is determined with and without historical data. Traditionally, this margin is treated as a fixed value, while developed methods take the variation into account in the determination of this margin, on which the test depends is more convincing. The mixed margin consisting of a margin based on treatment difference and a margin based on treatment ratio can exactly control the type I error at the desirable level and obtain a better power. In this review, we also provide some recommendations and suggestions for the selection of the non-inferiority margin in the western countries and China.

Besides the magnitude and variability of the standard control and placebo effects, the determination of the margin depends on other factors, such as, the safety profiles of the two comparative therapies, method of administration and product cost. Hence the margin should be made specifically for a certain case. Reference criteria for different medications should be given to avoid the arbitrary conclusion.