Growth Promotion Testing (GPT) is a routine and essential part of microbiological quality control for sterile and non-sterile pharmaceutical laboratories. When media qualification practices are aligned with day-to-day testing practices, they support consistent performance, clear scientific rationale, and confident audit discussions.

This overview highlights commonly applied GPT practices grounded in USP guidance and established microbiological principles, with practical context to help laboratories qualify media in a way that is both effective in routine use and easy to defend during inspection.

How GPT Is Commonly Applied in Practice

Across the industry, laboratories design GPT programs to confirm that media performs as intended under routine testing conditions. While procedures vary, effective GPT programs tend to share common characteristics:

• Practical and repeatable execution
• Alignment with routine laboratory workflows
• Emphasis on consistency and comparability over time

GPT is intended to demonstrate suitability for use, not exact numerical precision. Because GPT involves living organisms, some variability is expected. The goal is not to make every result look identical, but to show that media consistently supports acceptable growth when tested under defined conditions. For that reason, GPT programs are strongest when they are repeatable, clearly documented, and evaluated in context over time.

USP Framework for Growth Promotion Testing

Several key pharmacopeias provide GPT guidelines. The relevant USP chapters are harmonized with the JP and EP. The harmonized framework includes:

• Media
• Incubation conditions
• Testing period
• Acceptance criteria

These GPT guidelines allow laboratories flexibility of implementation within their quality systems.

Solid and Liquid Media: Acceptance in Context

Solid Media

USP <61>, acceptance criteria for GPT for Microbial Enumeration Tests, states that “growth obtained on solid media must not differ by a factor greater than two from the calculated value for a standardized inoculum.” As of the publishing date for this blog, the USP does not contain a definition of a “Standardized Inoculum.” Although one does not exist in the USP, Dr. Radhakrishna Tirumalai defines a “Standardized Inoculum” in the PDA Publication Industrial Pharmaceutical Microbiology Quality Control as one that is “prepared using the Standard culture collection microorganisms as indicated in the chapter and should meet the “five passage rule” and have an accurately determined microbial count commercially available, or one that has been standardized in-house to provide and accurate number of CFUs.”

There are many ways to complete the factor of two comparison; however, the most scientifically sound way is to compare the new batch of media to a previously approved batch. For GPT of media used in USP <62>, Tests for Specified Organisms, it is recommended to also run a control with ≤100 CFU on non-selective agar to support interpretability.

In practice, many laboratories focus on overall comparability and performance trends across batches, recognizing that small numerical differences are expected in biological systems.

Liquid Media

Liquid media are considered suitable when clearly visible growth occurs that is comparable to previously approved media. Parallel testing and use of non-selective agar provides proof the liquid media was inoculated with a viable culture of NMT 100 CFU.

Internal Verification and Audit Confidence

Internal verification of media and controls plays an important role in supporting confident audit discussions by helping laboratories demonstrate suitability for routine use. In addition to supplier documentation, laboratories should evaluate performance under their own test conditions, including organism recovery, growth promotion, and method-specific performance characteristics. USP <1117> emphasizes the importance of verifying that media and controls are appropriate for their intended use and maintaining records that demonstrate consistent performance over time.

Trending historical recovery data can provide useful context during investigations and inspections, particularly because microbiological recovery inherently exhibits biological variability. Published microbiological studies, including work by Tidswell et al., support the use of trend-based interpretation rather than overreliance on isolated recovery results.

To meet the USP <1117> requirements for incoming inspection of media and controls, a laboratory must:

• Verify control performance such as identity and manufacture warranty
• Confirm performance within their own methods and environment
• Maintain historical recovery data to demonstrate consistent performance over time

During inspections, this approach allows laboratories to clearly explain why a control or medium is considered suitable based on observed performance, rather than relying solely on documentation or labels.

Frequently cited microbiological literature, including work by Tidswell et al., supports evaluating microbial recovery under routine laboratory conditions, reinforcing the importance of interpreting recovery data in context, recognizing inherent biological variability and the value of trend-based interpretation. For GPT programs, this means laboratories must confirm that media consistently supports acceptable growth under the laboratory’s defined test condition, while recognizing expected biological variability.

Why This Approach Holds Up in an Audit

Auditors generally look for:

• Clear rationale for testing practices
• Consistent execution over time
• Evidence that materials are fit for their intended use

When internal verification and trending are part of routine GPT practices, laboratories are well positioned to demonstrate control over their processes without the need for reactive justifications during inspection.

Key Takeaways

Growth Promotion Testing helps confirm that microbiological media are suitable for their intended use. GPT programs that work well in practice typically:

• Align with routine laboratory workflows
• Emphasize comparability and consistency over time
• Use internal verification to support confident audit discussions

When GPT practices are clearly defined and consistently applied, they support both reliable testing and smooth inspections.

Microorganism controls for GPT

Looking for GPT control strains? Microbiologics brings confidence without complexity to your GPT program. For over 40 common reference strains, EZ-Accu Shot simplifies GPT with refrigerator storage and a quick dissolve format that delivers 10 to 100 CFU per inoculum. When you pair EZ-Accu Shot with our custom environmental isolate controls, your laboratory has a comprehensive and consistent GPT program. Find your GPT strains at Microbiologics.com.

Sources

Tirumalai, Radhakrishna, PhD and Edward Tisdel, PhD. (2024) Industrial Pharmaceutical Microbiology Quality Control. PDA.