The CGMPs require that all sampling plans be scientifically sound and representative of the batch under test (see 211.160(b)). Further, in-process testing of powder blends to demonstrate adequacy of mixing is a CGMP requirement (21 CFR 211.110). Between- and within-location variability in the powder blend is a critical component of finished product quality and therefore should be
evaluated. Drug product manufacturers need to use a science- and risk-based sampling approach to assure (a) adequacy of blend mixing and (b) that sampling of the blend is done at a suitable juncture in the manufacturing process. The sampling and analysis needs to ensure that no differences exist between locations in a blend that could adversely affect finished product quality. Traditional sampling using a powder-thief may have drawbacks and limitations, such as causing disturbance to the powder bed, powder segregation, or other sampling errors. However, powder-thief sampling remains widely used and provides reliable results in many cases. The Agency encourages firms to adopt more innovative approaches to ensuring adequacy of mixing (see, e.g., the PAT guidance). If a manufacturer proposes to use a thief sampling method, the reliability of the method should be evaluated as part of analytical methods development.
