In the execution of its legislated responsibilities, the United States Food and Drug Administration commonly refers to standard test methods detailed in the United States Pharmacopeia (USP). Microbiological test methods (contained in general chapters) are listed in chapters <51> to <80> with details regarded as enforceable where referenced as a test method. USP <61> \"Microbiological Examination of Nonsterile Products: Microbial Enumeration Tests\" is a globally harmonized chapter that has been successfully employed for the enumeration of microorganisms recoverable from nonsterile finished drug products. The content of USP <61> is not always scientifically principled nor emphatically understood by all pharmaceutical microbiologists. Consequently, misunderstanding and misapplication of USP <61> may result in analyses and assessments of microbiological quality that are flawed or erroneous. In this article, clarification is provided to assist the pharmaceutical microbiologist in the appropriate and intended use of USP <61>, including provision of details not always commonly known or understood.

Microbiologists dread investigating results that are outside of the specifications. However, the investigation must be performed. A decision to reject the batch does not remove the requirement to investigate the failure. Unfortunately, microbiological assay samples are often consumed during the test or the results of the data are obtained several days after the sample is tested. This delay from testing to results often renders the original sample dilution not valid for further testing. Thus, this delay can hinder finding a root cause. Knowing the root cause can aid with microbial control, defining corrective actions, and defining preventative actions that are required to minimize or eliminate the potential for reoccurrence. The root cause is an essential component in understanding the patient impact, status of the product, and regulatory reporting requirements. This article discusses out of specification results obtained from the microbial examination of nonsterile product assays and the subsequent root cause investigation. The focus of this article is on aberrant results obtained from a validated assay with established acceptance criteria.