Sterility Testing is a critical microbiological quality control process used to detect viable microbial contamination in pharmaceutical sterile products, and other products that require sterility assurance. The appropriate testing method depends on the product, its characteristics, and the applicable compendial standards and quality requirements.
Understanding Sterility Testing and Why It Matters
Sterility testing helps determine whether viable microorganisms can be detected in a representative sample of a pharmaceutical product under defined test conditions. It is an important part of a holistic sterility assurance strategy that also includes validated sterilization processes, aseptic processing, environmental monitoring controls, personnel practices, and contamination prevention.
USP <71> Sterility Tests describes the traditional compendial approach to sterility testing, while USP <72> and USP <73> describe microbiological methods for detecting contamination in short-life products. USP <72> uses respiration-based detection, and USP <73> uses ATP bioluminescence to detect microbial contamination.
What Is Sterility Testing?
Sterility testing is a microbiological test used to detect viable microorganisms in a pharmaceutical product or other article that is required to meet sterility requirements.
Under USP <71>, the test is designed to detect microbial contamination using defined procedures and culture conditions. It applies to products such as injectable preparations, ophthalmic preparations, sterile bulk substances, and other products or medical devices where sterility must be demonstrated.
USP Sterility Testing Methods
There are three USP chapters particularly relevant to understanding traditional and alternative approaches to sterility testing:
- USP <71> Sterility Tests
- USP <72> Respiration-Based Microbiological Methods for the Detection of Contamination in Short-Life Products
- USP <73> ATP Bioluminescence-Based Microbiological Methods for the Detection of Contamination in Short-Life Products
The three chapters use different detection principles and may be appropriate for different products and testing objectives.
The appropriate method should be selected based on the product, method suitability, applicable requirements, and the scientific needs of the testing program.
USP Sterility Tests
What Is USP 71?
USP <71> Sterility Tests is a chapter of the United States Pharmacopeia (USP) that describes standardized methods for detecting viable microbial contamination in pharmaceutical products and other products that are required to be sterile.
USP <71> is widely recognized and is also referenced by FDA as the principal source for sterility testing methods, including test procedures and media.
USP 72 Respiration-Based Microbiological Methods
What is USP 72 Sterility Test?
USP <72> describes alternative rapid microbiological methods (RMMs) that detect microbial contamination by measuring respiratory or metabolic activity rather than visible growth. Instead of waiting for colonies to develop (as in USP <71>), these methods detect viable microorganisms through indicators such as oxygen consumption or carbon dioxide production.
USP <72> is intended for sterility assurance testing of short shelf-life products and advanced therapies, including cell and gene therapy products, and does not rely on operator visual confirmation of turbidity.
How Does USP 72 Work?
The product is introduced into an appropriate culture system and incubated under controlled conditions.
As viable microorganisms grow, they produce metabolic changes. The automated detection system monitors these changes and can identify microbial activity without relying solely on visual turbidity.
At Eagle Analytical, the source material indicates that USP <72> testing uses the BacT/ALERT® system, an automated microbial detection platform that monitors microbial metabolic activity.
USP 73 ATP Bioluminescence-Based Microbiological Methods
What Is USP 73?
USP <73> describes ATP bioluminescence-based microbiological methods for detecting microbial contamination in short-life products.
The method detects ATP, an energy molecule found in living cells. When microbial ATP reacts through a bioluminescence reaction, light is produced. The instrument measures this light and converts the signal into an objective analytical result.
USP describes <73> as a risk-based method for detecting microbial contamination in short-life products and products that need to be administered as soon as possible. It may also be used as an in-process control for certain product intermediates, cell media, or process solutions.
USP 71 vs. USP 72 vs. USP 73
Choosing between sterility testing methods requires more than comparing turnaround times.
The product formulation, , sample characteristics, microbial recovery requirements, method suitability, and applicable regulatory expectations all need to be considered.
Feature | USP <71> | USP <72> | USP <73> |
Primary Approach | Traditional growth-based sterility test | Respiration-based detection | ATP bioluminescence |
Typical timeline in Eagle materials | 14 days | 7 days | 7 days |
Visual turbidity interpretation | Yes | No | No |
Automation | Limited compared with RMM platforms | Automated | Instrument-based |
Short-life products | Less suited when 14 days is not feasible | Designed for short-life products | Designed for short-life products |
Method Suitability | Required | Required | Required |
Formulation-specific | Yes | Yes | Yes |
Microbial identification support | Yes | Yes | Yes |
The most important difference is the detection principle. USP <71> relies on traditional culture and observation of microbial growth, while USP <72> detects respiratory or metabolic activity and USP <73> uses ATP bioluminescence to detect microbial activity.
The Role of Sterility Testing in a Holistic Sterility Assurance Program
A sterility test provides information about the units tested, but it cannot replace contamination prevention and process control.
FDA guidance emphasizes that sterility testing has limitations related to sample size and that samples should represent the batch and processing conditions.
An effective sterility assurance strategy may include:
- Validated sterilization processes
- Aseptic processing controls
- Environmental monitoring
- Personnel training and qualification
- Facility and equipment controls
- Container closure integrity where applicable
- Bioburden control
- Appropriate microbiological testing
- Documented investigations and corrective actions
Together, these controls create a stronger framework for protecting product quality and patient safety.
How Eagle Analytical Supports Sterility Testing
Eagle Analytical is an FDA and DEA registered, A2LA ISO 17025 accredited testing laboratory serving pharmaceutical, manufacturing, and compounding facilities. The company’s testing capabilities include traditional and rapid sterility testing approaches described in USP <71>, USP <72>, and USP <73>.
Our microbiology laboratory supports clients with scientifically appropriate testing methods, method suitability studies, controlled incubation, microbial detection, and investigation support.
- USP <71> Sterility Testing: Eagle performs both membrane filtration and direct inoculation, depending on the product and its characteristics.
- USP <72> Respiration-Based Sterility Testing: Eagle uses the BacT/ALERT® automated microbial detection system to monitor microbial metabolic activity.
- USP <73> ATP Bioluminescence Testing: Eagle uses the Celsis® system to detect microbial contamination through ATP bioluminescence.
- Solid phase cytometry-based rapid microbiological method for the detection of contamination in clear aqueous solutions. Eagle offers ScanRDI – alternative rapid sterility testing.
Selecting the appropriate sterility testing method requires an understanding of the product, formulation, and testing objectives. Eagle’s experienced laboratory team can help clients determine the appropriate testing approach and establish method of suitability before routine testing begins.
Conclusion
Sterility testing is an important part of pharmaceutical microbiological quality control, but it should never be considered a standalone guarantee of product sterility.
USP <71> provides the traditional compendial approach using membrane filtration or direct inoculation and microbial growth detection. USP <72> and USP <73> provide alternative approaches designed for detecting microbial contamination in short-life products, using respiration-based detection and ATP bioluminescence, respectively.
The right method depends on the product, formulation, , testing requirements, and demonstrated method suitability.
For pharmaceutical manufacturers and compounding facilities, choosing an experienced testing laboratory can help ensure that the selected method is scientifically appropriate and that the resulting data supports product quality, regulatory expectations, and patient safety.
Need sterility testing for your product? Contact Eagle Analytical to discuss USP <71>, <72>, or <73> testing and method suitability requirements.
Frequently Asked Questions About Sterility Testing
What is the purpose of sterility testing?
Sterility testing is used to detect viable microbial contamination in a representative sample of a pharmaceutical product under defined test conditions. It is one component of a broader sterility assurance program.
What is the difference between USP , USP , and USP ?
USP <71> is the traditional compendial Sterility Test based on microbial growth and observation. USP <72> uses respiration or metabolic activity to detect microbial contamination, while USP <73> uses ATP bioluminescence.
