The usual reason to do a composite sample is to save the cost of sampling, testing, and documentation: not a statement FDA wants to hear. The agency's response would simply be, "That is the cost of compliance." From a regulatory point of view, there is a need to justify compositing versus testing all of the containers on some grounds other than just cost. This testing is probably best done using statistical arguments that the composite test is sufficient in the specific situation, that additional testing would not materially improve results, or decrease patient risk.
Compositing risksConsideration of compositing raises several related topics. First, compositing is only done on incoming materials and not on in-process materials or on finished products. Second, compositing should not be done on crucial materials, precursors, APIs, anything that comes in direct contact with the API, in-process materials, or the finished product. The risk is too great. The probability may be low, but the consequence could be lethal to the patient if product quality is not met.
The first justification for compositing is that the risk to the patient is not materially increased by compositing. This justification is supported by the physical situation. Incoming materials generally include liquids, viscous liquids, pastes and emulsions, suspensions, powders, granulations, and solids. The nature of the material affects the sources and structure of the variability.
Take for example a tank car full of a nonviscous liquid. Generally, it is reasonable to assume that the liquid is homogeneous and one sample would be all that is needed. Thirty samples would not give more information about the liquid. The variability in the results would be from the test method itself. Now, suppose the tank car of liquid is contaminated with a heavier liquid that settles in the lowest point of the tank, usually the drain opening. If the single sample is taken from the top part of the tank, we would miss the contamination at the bottom. The sampling scheme must take into account the physical structure of the material and sources of variability.
The most common situation for compositing is the receipt of shipment of drums of a powder. A company wants to open as few of the containers as possible to take samples. The receiving company wants to mix those few samples into a blend and test one sample once. Clearly, all information about variability is lost. Further, if one of the samples that is taken and blended is low (or high) and out of specification, the blend may not be low (or high) enough to signal a difference. Again, the risk to product quality could be high.
Given 100 containers of a powder, many sources of variability can be identified within and between containers. Further, there are issues of deliberate fraud and counterfeit materials that add to possible variability. For example, drums may be filled with a cheap material such as lactose, and then only a small layer of API is put on the top. The common practice of only sampling from the top of the drum misses the lactose.