Understanding the Tip:
What is bracketing and why it matters:
Bracketing is a reduced stability testing design wherein only the extremes (highest and lowest) of product variables—such as strength or package size—are tested. The assumption is that stability characteristics of intermediate configurations will fall within the tested range. This strategy significantly reduces the number of stability samples and tests while still maintaining scientific robustness, especially in scenarios involving multiple strengths or pack sizes that share the same formulation and packaging material.
Scenarios where bracketing is beneficial:
Bracketing can be used:
- Across different strengths with a linear formulation scale-up
- For multiple pack sizes using identical primary packaging
- When variations in fill volume do not affect product stability
This approach allows faster product development and submission with fewer resources and no compromise on data integrity.
Regulatory and Technical Context:
Guidelines supporting bracketing design:
ICH Q1D provides guidance on bracketing and matrixing strategies in stability testing. WHO TRS 1010 also endorses bracketing when scientifically justified. CTD Module 3.2.P.8.1 and 3.2.P.8.3 should clearly describe the rationale and data supporting the bracketing approach. Agencies like FDA and EMA accept bracketing, provided the design rationale is sound, and the stability of the untested configurations can be reasonably inferred from the tested extremes.
Common audit concerns related to bracketing:
Inspectors
- The scientific justification for omitting intermediate strengths/sizes
- Evidence that all configurations are compositionally and materially equivalent
- Statistical or historical support validating similarity of degradation behavior
Insufficient justification may result in a demand for additional stability data or rejection of shelf-life claims.
Best Practices and Implementation:
Establish strong scientific justification for bracketing:
Demonstrate:
- Formulation linearity and proportionality across strengths
- Consistency in manufacturing process and primary packaging
- Similar exposure profiles (e.g., oxygen, moisture ingress) across pack sizes
Use prior stability or development data to support the assumption of similar degradation trends.
Document the bracketing strategy within your stability protocol:
Clearly define:
- Tested configurations (e.g., lowest and highest strength)
- Omitted configurations and justification for omission
- Shelf-life assignment strategy based on bracketing data
QA and regulatory review must endorse the bracketing design prior to execution.
Track results closely and reassess if variability is observed:
Monitor:
- Real-time stability results for the bracketed samples
- Out-of-trend behavior that may necessitate additional testing
- Any deviations in storage conditions that could differentially impact omitted configurations
If required, add intermediate strengths or configurations to the testing program to confirm assumptions.
Bracketing in stability testing is a powerful efficiency tool when scientifically justified. It reduces workload, expedites product timelines, and optimizes resource use—provided that the integrity of shelf-life assignment and regulatory expectations are fully upheld.