By: Jeanne Moldenhauer, Excellent Pharma Consulting, Inc.
Introduction
USP has expanded and clarified the use of rapid sterility and rapid microbial test methods primarily through USP monographs and informational chapters: <72>, <73>, <1071>, and <1223, while retaining <71> as the compendial sterility “gold standard.” These incorporations formalize a riskbased framework under which alternative rapid methods may be used for sterile product release, provided they are validated as equivalent or superior to the traditional test. We applaud USP for adding these methods, and hopefully more technologies as well. However, adding these methods to USP alone may not solve all issues with rapidmethods.
When rapid methods were initially introduced, using them product releases were desired, as it was believed that microbiology testing was the rate limiting factor in product release. In fact, a rapid sterility test was identified as the “golden calf” since it would eliminate the time delay for release from 14 days to a shorter time period. This in turn would shorten inventory hold time for work in progress, resulting in cost avoidance.
Unfortunately, many early adopters, who went through heroics to get their technologies validated and implemented, subsequently quit using these methods and went back to traditional methods. This paper discusses some of the technologies used by early adopters, and if applicable, whether they continued using them or went back to traditional methods. When known, these reasons are discussed identifying some other criteria that should be considered in adoption of new methods.
Background
Two new USP general chapters for rapid sterility testing were included in USP <72> and USP <73> which became effective in August 2025.
USP <72> is issued for sterility test methods using RespirationBased Microbiological Methods (identified in this document as System 1). This monograph (which is official) provides a compendial framework for rapid, respirationbased sterility (microbial detection) methods for shortlife products such as certain cell and gene therapies.
USP <73> ATP BioluminescenceBased Microbiological Methods (identified in this document as System 2) This method Describes ATPbioluminescencebased rapid microbiological methods for sterility/contamination detection, especially suited to shortshelflife or advancedtherapy products.
Commentaries further emphasize that USP monograph <72> and USP <73> are new RMM chapters, distinct from the longstanding USP Monograph <71> sterility test and from the information chapter <1071>, and that they are not mandatory until cited in individual monographs, another general chapter, or General Notices.
USP <71> remains the compendial method, requiring incubation of test media for not less than 14 days and serving as the reference standard for demonstrating noninferiority of rapid methods. Alternative sterility tests must be validated against USP Monograph <71> to show that they are equivalent or better in terms of detection capability and reliability for routine use, i.e., not inferior to the compendial method.
USP <1071> is an informational chapter that outlines a riskbased approach to using rapid microbial tests for the release of sterile shortlife products (e.g., CSPs, PET drugs, cell/gene therapies). It emphasizes balancing timetoresult, limit of detection, and sample size in a way that maintains or improves patient safety by providing sterility information before administration.
USP <1223> sets general validation expectations for alternative (including rapid) microbiological methods, covering parameters such as specificity, limit of detection, repeatability, robustness, and equivalence to compendial methods. Sterilityfocused rapid methods are expected to be validated under USP monograph <1223> criteria in addition to any chapterspecific guidance such as <73> or <1071>
Practical Effect of Recent USP Actions
Recognition of Rapid Methods as Acceptable Alternatives
USP’s evolving position explicitly “opens the door” to new rapid sterility and rapid microbial test methods, provided a proper risk assessment and validation demonstrate noninferiority to <71>. Stakeholder guidance (including state and professional bodies applying USP standards) now commonly states that sterility testing may be conducted by USP <71> or a validated alternative rapid method meeting USP <1223> criteria and shown to be noninferior to USP <71>.
Emphasis on ShortLife & Advanced Therapy Products
USP <1071> and USP <73>, as incorporated and updated, specifically target products with short shelflives or timecritical administration (compounded sterile preparations, PET radiopharmaceuticals, and advanced therapies). The chapters support adoption of rapid methods that shorten timetoresult from the traditional 14 days to a few days, enabling samebatch or nearrealtime release decisions without compromising microbial detection performance.
Validation and RiskBased Implementation
USP’s framework requires that firms perform method suitability and productspecific interference assessments to ensure that rapid tests detect relevant microorganisms without inhibition or false results. The riskbased paradigm in USP <1071> instructs users to justify sample size, detection limits, and any tradeoffs in speed versus sensitivity, so that rapid methods enhance, rather than diminish, patient protection compared with the compendial method.
Alignment with Compendial and Regulatory Expectations
Recent implementations show rapid methods being validated “in accordance with both the European and United States Pharmacopoeias,” citing USP <71> and <1223> as the relevant U.S. benchmarks for equivalence. Industry guidance documents and practice standards echo that rapid sterility testing is an “alternative microbiological test method” to USP <71>, not a separate compendial sterility chapter, and must be scientifically justified and documented under the USP validation framework.
Example of Application
For compounded sterile preparations subject to USP <797>, some jurisdictions now instruct that sterility testing may be conducted either via USP <71> or a validated rapid sterility method, so long as the rapid method is demonstrated to be equivalent or superior to the compendial method. i.e., it cannot be inferior, according to USP <1223> and appropriate risk assessment. In practice, this means a pharmacy or manufacturer can adopt an ATPbioluminescence system or other rapid platform for sterility assurance, if they complete method suitability, equivalence studies to USP <71>, and document the approach under the USP informational chapters described previously.
Sterility Testing
Systems 1 and System 2
System 1 and System 2 are both automated, continuous-monitoring blood-culture systems that detect microbial growth via CO₂ production, but they use different sensor chemistries and bottle/media designs that lead to some performance and workflow differences. In Turabian terms you can treat them as two parallel growth-based methods with distinct detection signals, media formulations, and time-to-detection profiles, but the same fundamental principle: organisms must replicate in broth and generate detectable CO₂.
Core detection principle
System 1 uses an internal fluorescent sensor embedded in the base of each culture bottle; as microorganisms grow, they produce CO₂, which dissolves in the broth and alters the pH around the sensor, changing its fluorescence, which the instrument reads at frequent intervals. System 2 instead uses a colorimetric sensor at the bottom of each bottle; CO₂ production and the associated pH change cause a color shift in the indicator, which is detected by optical reflectance or transmittance measurements.
Media and Bottle characteristics
System 1 offers a family of media (e.g., Plus Aerobic/F, Lytic/10 Anaerobic/F, Myco/F) in plastic bottles, optimized for recovery of aerobes, anaerobes, yeasts, fungi, and mycobacteria; many formulations include resin beads that adsorb antimicrobial agents and improve recovery from patients already on antibiotics. System 2 likewise provides a range of plastic culture bottles for blood and sterile body fluids, including FAN (Fastidious Antimicrobial Neutralization) media that incorporate charcoal/resin to neutralize antibiotics and support fastidious organisms.
Performance and time to detection
Comparative studies of System 1 versus System 2 report broadly similar recovery rates overall but somewhat faster average time to detection with System 1 for many organisms; in one platelet-culture study System 1 detected positives about 1.7 hours sooner on average.
A larger comparative clinical study found that System 1 detected more bacteremic episodes than System 2, largely because of improved recovery of Staphylococci, a difference attributed in part to antimicrobial-removing resins in System 1 media not present in the System 2 media tested. More recent work with the System 1 versus System 2 continues to show shorter time to detection for many clinically relevant bacteria and yeasts with System 1, although both systems remain accepted gold standards for bloodstream infection detection.
Workflow and applications
In routine use, both systems incubate bar-coded bottles at 35–37 °C and continuously monitor them; once a positivity threshold in signal is reached the instrument flags the bottle as positive, prompting Gram stain and subculture. System 1 is marketed as a “full-spectrum” solution with modules sized from small bench-top instruments (e.g., FX40) to high-capacity systems and is widely used for blood cultures plus specialized Myco/F Lytic bottles for mycobacteria and fungi. System 2 is likewise modular, supports blood and sterile body-fluid cultures, and in the VIRTUO generation adds automated loading/unloading, fill-volume sensing, and advanced analytics on time to detection and contamination rates.
Adoption
System 1 was the very first rapid sterility test methodology used in a biotech application. System 2 was implemented shortly thereafter. Implementation of these methods, for both in-process and final sterility testing was implemented in some biotechnology application and in compounding pharmacies. To the best of my knowledge, the companies that were early adopters have continued to use these methods.
Analysis
These methods are limited in their application due to the need to use small sample sizes and the fixed size of the media bottles. As such, many pharmaceutical applications would not easily work with these systems. These technologies were widely used in blood banking operations for many years prior to implementation as a product sterility test methodology. These methods were true success stories.
System 3
System3 is a non-growth-based rapid sterility and microbial detection system that combines membrane filtration, universal fluorescent viability labeling, and solid-phase laser cytometry to detect individual viable microorganisms—often within the same shift—rather than after days of incubation. It can detect bacteria, yeasts, and molds (including spores, stressed, and VBNC organisms) at or near the single-cell level in filterable products and process samples, and is positioned as an FDA-accepted rapid microbiological method (RMM) alternative to compendial sterility testing when properly validated.
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Summary comparison (System 1 vs System 2)
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Aspect
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System 1
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System 2
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Detection signal
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Fluorescent CO₂ sensor in bottle base, read noninvasively and continuously.
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Colorimetric pH/CO₂ indicator in bottle base, read optically in real time.
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Growth
requirement
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Requires microbial replication and CO₂ production in broth.
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Same: growth-dependent CO₂ production and pH change.
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Antibiotic
neutralization
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PLUS media with resin to remove/neutralize many antimicrobials.
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FAN/FAN PLUS bottles with charcoal/resin for antimicrobial neutralization.
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Performance
notes
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Often shorter time to detection; higher recovery of some organisms (e.g., staphylococci) in comparative studies.
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Similar overall performance: some studies show slightly slower detection or lower staphylococcal recovery when media lack resins.
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Typical
applications
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Blood cultures, prosthetic joint and other sterile-site samples, platelet screening, mycobacterial/fungal culture.
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Blood cultures, sterile body-fluid cultures, platelet screening; 3D and VIRTUO platforms for different throughputs.
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Principle and core workflow
System 3 operates on a filter–label–scan–confirm sequence: the product is first passed through a sterile single-use membrane device to capture any microorganisms present.⁴ A proprietary, non-fluorescent, membrane-permeant substrate is then added; non-specific intracellular esterases in viable cells cleave this substrate, producing a fluorescent product that is retained within intact cells on the membrane.
The membrane is subsequently analyzed by a solid-phase cytometer that uses a laser to scan the entire filter surface and detect discrete fluorescent events corresponding to individual viable microorganisms. Because detection is based on metabolic activity and fluorescence rather than colony formation, System 3 does not require microbial growth or replication to signal contamination.
Detection characteristics and time to result
System 3 is reported to detect viable microbial cells down to a single organism on the membrane, with sensitivity across several orders of magnitude for bacteria, yeasts, and molds. This sensitivity extends to spores, stressed, fastidious, and viable-but-nonculturable (VBNC) organisms that may not grow—or may grow only slowly—in traditional USP <71> sterility tests.
In compatible products, the full workflow (filtration, labeling, scanning, and confirmation) can be completed in approximately 2–4 hours, enabling actionable sterility or contamination decisions within the same business day and often allowing product release in 1–2 days once validation and any short enrichment steps are defined. This shortens investigation and release timelines by 1–2 weeks compared with the 14-day compendial sterility test, and several white papers and case studies note FDA awareness and acceptance of System 3-based sterility testing in approved applications when validated under USP <1223> and Ph. Eur. rapid method guidance.
Applications and positioning
System 3 is primarily used for filterable sterile drug products (parenterals, biologics), water and WFI, in-process solutions and buffers, and high-value cell-based products, with adaptations for cell-therapy sterility testing and in-process control of media/buffers. Test applications include raw-material testing, in-process product testing, chromatography column “health checks,” and batch release testing aligned with USP <71> sampling schemes but using a rapid, non-growth-based readout.
Because it does not rely on growth, System 3 is often positioned as an “ultra-rapid” RMM that complements or replaces growth-based methods (e.g., System 2-style rapid sterility) for products where filtration and fluorescence are feasible and where accelerated release and early detection of low-level contamination provide significant clinical and economic benefit.
Limitations and suitability considerations
System 3 requires that the product be filterable and compatible with the fluorescent viability chemistry; highly colored or strongly autofluorescent formulations can interfere with detection and may be flagged by the system as incompatible. In such cases, alternative rapid methods or compendial sterility testing remain necessary, underscoring that System 3 is not universally applicable to all sterile products but must be evaluated through product-specific method-suitability studies (filtration, background fluorescence, recovery of challenge organisms) under USP <1223> principles.
Adoption
Adoption of this technology was conducted by the first few adoptions of rapid sterility testing methods on the pharmaceutical drugs side of manufacturing. The reasons for this included the following: shortest test interval (about 4 hours/test), could handle any sample size, could accommodate various rinse dilution sizes, and was applicable to filterable samples.
At least two of the early adopters discontinued use in the first two years of implementation and went back to the traditional sterility test method.
Analysis
This methodology was very attractive to companies that had parametric release for their terminally sterilized products, which if sterility testing was the rate limiting factor for batch release, reduced the hold time from 14 days to less than 1 day. Parametric release is a methodology to tightly control the manufacturing process and have appropriate controls to justify product release without conducting a sterility test. However, the regulatory guidance in effect (to this day) limits the use of the methodology to only terminally sterilized products. For one of the early adopters of parametric release, within a few years they achieved a 2-3 day release time within a few years.
However, manufacturers were looking for the ability to have this shorten release time for aseptically processed products. The Parenteral Drug Association periodically, surveys companies regarding several issues relative to aseptic processing. In their most recent survey, the number of companies aseptically processing products vs. terminally sterilizing products correspond to about 80% of production. Many of these same products, when manufactured as a large volume parenteral, legally must be terminally sterilized. While companies could have changed to terminal sterilization, they chose not to, for a variety of reasons like: didn’t want to buy and qualify sterilizers, did not want to open us “old” regulatory submissions, for fear of having to update all methods used, and so forth.
When rapid sterility tests became available, System 3 seemed to offer the same benefits as parametric release for aseptically filled products.
System 3 uses a quantitative methodology (not a qualitative method) and requires the most validation in the validation guidance. The early adopters did not have significant issues in the validation of the technology (not including that it was time consuming).
The early adopters implemented this technology but then quit using the methodology due to other reasons. Some of these reasons include the following:
- More than one early batch manufactured and tested with this technology failed the test and had to be discarded. Little investigation was conducted to assess whether it was a method suitability issue, or if there was a developmental issue with the method development. In general, manufacturing immediately wanted to go back to the old method when they had to throw out more than one batch, where they couldn’t identify what went wrong.
- Many companies cited big cost avoidances or savings that were projected when they justified the purchases of these technologies. Unfortunately, the accountants could not verify any of these savings. One reason for this was the substantial misconception that microbiology was the rate-limiting factor for product release. There were many factors, e.g., most chemistry testing takes up to three days, if there are deviations or investigations in the batch this can take a significant amount of time to resolve, and so forth.
- Some companies leveraged their testing depth of analysis, based upon testing conducted by other companies where management had friends. While this possibly could be justified, it was important to understand the differences in the product formulations. For example. One company might have had all preserved products while the other didn’t. Another consideration is the ability of the container to generate particles that may interfere with the viability detection method.
Conclusion
While the use of rapid microbiology test methods for sterility testing is highly endorsed, it is important to have company subject matter experts, who need to understand the methodology, and what can interfere with the results obtained, e.g., does the product generate particulates that can fluoresce? Is the product preserved? What product characteristics affect the testing? And so forth.
Another concern is understanding that you may be held accountable to show the cost-savings you claim in your early documentation. Think about how and when full implementation will take place and when cost savings will occur. While likely to show sufficient savings, from personal experience, you may need several years to “fix” all the delays in the process and achieve these avoidances.
Many will see and learn, that microbiology usually is NOT the rate-limiting factor in product release.
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