Mandated by regulatory requirements such as USP <797> Pharmaceutical Compounding – Sterile Preparations and Current Good Manufacturing Practices (CGMP), sterility testing is a quality control measure for aseptically produced pharmaceutical products. However, the compendial testing method outlined in USP <71> Sterility Tests presents significant statistical, microbiological, and time limitations. The implementation of a robust quality assurance program and the adoption of rapid sterility testing methods are essential to overcome such limitations.
Limitations of the compendial method
The USP sterility test method, first introduced in 1936 and subsequently revised, has been criticised for its limitations. Studies such as those by Frances Bowman (1969) emphasised the need for multiple culture media due to the inability of any single medium to support the growth of all microorganisms. Bryce (1956) highlighted that sterility tests only assess organisms capable of growing under test conditions and that the sample size is so restricted that it provides only a gross estimate of the sterility of a product lot.
Traditional sterility testing, as outlined in USP <71>, relies on methods involving direct inoculation or membrane filtration followed by a 14-day incubation period in growth-promoting media. This approach presents several key limitations:
- Statistical limitations – sterility testing only evaluates a small fraction of a given batch. As a result, it is possible for contaminated units to go undetected, leading to the release of nonsterile product.
- Microbiological limitations – the growth media used in the compendial methods do not support the proliferation of all microbial species. Fastidious organisms or viable but non-culturable (VBNC) microbes may not be detected, creating potential sterility assurance gaps.
- Time limitations – the 14-day incubation period significantly delays product release, increasing holding costs and impeding timely patient access and use time.
Quality by design: building sterility into every system
Since sterility testing alone cannot provide absolute assurance of product sterility, a comprehensive quality assurance program integrating the principles of quality by design (QbD) must be developed, implemented, and practiced. Sterility assurance must be an intrinsic feature of the entire production system, from the onset, rather than the outcome of a sterility test. By integrating QbD principles into all aspects of aseptic processing, encompassing facility and engineering controls, environmental monitoring, personnel training, and process validation, compounders are less reliant on a single quality control test, which is only representative of the microbial quality of the samples tested, to disposition a compounded sterile preparation
USP <797> and alternative sterility testing methods
Recent revisions to USP <797> now permit the use of alternative methods, provided they meet specific validation criteria, offering opportunities to improve testing efficiency and reliability. USP General Notices state that alternative testing methods may be used if they can be shown to provide equivalent or better results than compendial methods. Further, according to USP <6.30> Alternative and Harmonized Methods and Procedures, alternative methods must be fully validated per USP <1225> Validation of Compendial Procedures and must demonstrate comparable results within allowable limits. Alternative methods aim to simplify sample preparation, improve precision and accuracy, reduce run time, and allow for automation.
Evolution of alternative methods for sterility testing
USP <1071> Rapid Microbial Tests for Release of Sterile Short-Life Products: A Risk-Based Approach describes various technologies for rapid microbial tests and their operating parameters relative to user requirement specifications. Among these requirements is the expectation for a low limit of detection and a faster time to result.
To address the limitations of culture-based detection, direct microscopic examination methods emerged in 1977 (Kronvall and Myhre, 1977). These methods utilised a cell stain that only reacted in cells with intact chromosomes to make them fluoresce under ultraviolet light exposure and direct epifluorescent microscopic examination. This cell staining concept underwent decades of improvement. Over time, advancements in microbial detection led to the development of solid-phase laser scanning cytometry, which allows detection and enumeration of microorganisms within hours rather than days or weeks. This concept formed the technical basis for the development and validation of the ScanRDI® sterility test.
Benefits of ScanRDI® for rapid sterility testing
Scan Rapid Detection Imaging (ScanRDI®), a fluorescence-based technology that detects viable microorganisms within hours is an FDA-accepted alternative documented in Drug Master File (DMF) #14621 submitted to the FDA in 1999. ScanRDI® rapidly detects viable microbial cells down to one microorganism without relying on microbial growth or multiplication, eliminating the need for specialised culture conditions and extended incubation periods.
Like the compendial method, the ScanRDI® testing protocol utilises the same sampling protocols outlined in USP <71>, detects all the standard USP organisms, and requires method suitability testing. Furthermore, it has been shown to provide consistent and reliable results that have been prospectively validated as published by Smith et al, (2010).
From 2007 to 2018, Eagle utilised ScanRDI® testing to determine the sterility of compounded sterile preparations. During this period, nearly 40,000 tests were performed across 1,500 drug compounding categories for 500 pharmacy locations; believed to be the most complex data set accumulated. A retrospective validation of the products tested indicated a sterility failure rate of 0.96% (383), with 96% resulting sterile, and 3.4% resulting incompatible (no test). In parallel, a retrospective validation of over 45,000 USP <71> sterility tests across 1,500 drug compounding categories for approximately 1,000 pharmacy locations was performed, resulting in a sterility positive rate of 0.63% (288). With comparable sterility failure rates, the ScanRDI® sterility test method protocol has proven to be an effective, reliable, and efficient alternative method to the compendial USP <71> sterility test protocol. At Eagle, method suitability is testing is performed on every unique client formulation across all sterility testing platforms.
Key benefits of ScanRDI® include:
- Rapid turnaround – detects viable organisms in as little as two hours, expediting batch release.
- Increased sensitivity – capable of detecting single viable cells, including VBNC organisms that traditional sterility tests may miss.
- No growth requirement – unlike culture-based methods, ScanRDI® does not rely on microbial replication, reducing the risk of false negatives
Regulatory compliance, patient safety, and business risk
Despite its limitations, sterility testing, in conjunction with a robust quality assurance program, remains a regulatory requirement under USP <797> and CGMP for aseptically produced products. The most efficient strategy for compliance and operations is the implementation of rapid sterility testing methods such as ScanRDI®. At Eagle, we understand that achieving sterility assurance requires more than just passing a sterility test, as it alone cannot assure product sterility. In addition to laboratory testing, our team provides end-to-end support for aseptic processing at all stages of sterility assurance, helping clients design and implement facility controls, qualify equipment, processes and personnel, develop and validate processes and programs, provide on-site aseptic training, and perform compliance audits.
Ultimately, compliance with sterility requirements is not just about meeting regulatory mandates—it is about patient safety, access, and satisfaction. Rapid sterility testing ensures faster product release, preventing testing time from cutting into patients’ in-use time. Additionally, reduced inventory hold times translate to improved operational efficiency and cost savings. By integrating a robust quality assurance program with rapid sterility testing, compounders can confidently disposition products while mitigating business risks and protecting public health. With Eagle as your aseptic processing partner, you gain a team of experts dedicated to ensuring compliance, efficiency, and patient safety at every step of the process.
References
Bowman, F.W. 1969. The Sterility Testing of Pharmaceuticals. Journal of Pharmaceutical Sciences. 58 (11): 1301 – 1308.
Nagarkar, P.P., S.D. Ravetkar and M.G. Watve. 2001. Oligophilic Bacteria as Tools to Monitor Aseptic Pharmaceutical Production Units. Applied and Environmental Microbiology. 67 (3): 1371 – 1374.
Kronvall, G., and E.Myhre. 1977. Differential staining of bacteria in clinical specimens using acridine orange buffered at low pH. Acta Pathol Microbiol Scand B. 85 (4):249-254.
Jones, D.L., M.A. Brailsford, and J-L. Drocourt. 1999. Solid-Phase, Laser-Scanning Cytometry: A New Two-Hour Method for the Enumeration of Microorganisms in Pharmaceutical Water. Pharamcopeial Forum 25(1): 7627 – 7645.
Bryce, D.M. 1956. Tests for the Sterility of Pharmaceutical Preparations: The Design and Interpretation of Sterility Tests. Journal of Pharmacy and Pharmacology. 8 (2): 561 – 572.
Smith, R., Von Tress, M., Tubb, C., & Vanhaecke, E. (2010). Evaluation of the ScanRDI as a rapid alternative to the pharmacopoeial sterility test method: Comparison of limits of detection. PDA Journal of Pharmaceutical Science and Technology, 64(4), 356-363.
