Most Probable Count (MPC) in Environmental Monitoring

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In pharmaceutical cleanrooms and controlled environments, active air sampling is a critical component of environmental monitoring. It ensures that airborne microbial contamination remains within defined limits, protecting product sterility and patient safety.

During active air sampling, microbial particles impact a nutrient agar plate through a grid of holes in samplers such as slit-to-agar samplers or RCS (Reuter Centrifugal Sampler). However, the raw colony count on the agar plate often underestimates the actual number of microorganisms in the sampled air because multiple particles can impact the same hole. This is where the Most Probable Count (MPC) and Feller correction come into play.

🌿 What Is Most Probable Count (MPC)?

The Most Probable Count (MPC) is the statistically corrected number of microbial colonies in a given air volume during active air sampling. It accounts for the possibility of multiple particles hitting the same hole or impact site on the agar surface. MPC provides a more accurate estimate of the airborne microbial load compared to the raw colony count.

MPC is essential for:

  • Comparing results with ISO 14644-1 cleanroom classification limits or GMP alert/action levels.
  • Trend analysis and contamination control in sterile areas.
  • Regulatory compliance and reporting.

🔬 How MPC Is Calculated Using Feller Correction

The calculation is based on the principle that some holes may receive more than one particle. Feller (1950) derived a statistical correction to estimate the most probable number of microorganisms:

Formula:

MPC = -N × ln((N - r)/N)

Where:

  • N = Total number of holes or impact sites on the sampler plate
  • r = Number of holes that received at least one colony (i.e., occupied holes)
  • ln = Natural logarithm

Example:

Suppose you perform an air sample using a sampler with 400 holes. After incubation, 150 holes show microbial growth.

  1. Plug into the formula:
    MPC = -400 × ln((400 - 150)/400)
  2. Calculate:
    MPC = -400 × ln(250/400) = -400 × ln(0.625)
  3. ln(0.625) ≈ -0.4700 → MPC = -400 × (-0.4700) ≈ 188 CFU

The Most Probable Count (MPC) for the air sample is therefore approximately 188 CFU, which is the corrected estimate of airborne microorganisms.

📌 Key Points About MPC in Active Air Sampling

  • MPC corrects for underestimation in raw colony counts caused by multiple particle impactions.
  • Essential in ISO Class 5 (Grade A) and Grade B areas where accurate counts are critical.
  • Most modern samplers provide Feller tables or software to calculate MPC automatically.
  • Results are used to compare with alert and action limits for cleanroom compliance.
  • Trend analysis using MPC ensures early detection of contamination sources.

💡 Best Practices for Accurate MPC in Environmental Monitoring

  • Ensure the sampler is calibrated and validated before use.
  • Use proper aseptic technique during air sampling.
  • Sample during both operational (dynamic) and at-rest (static) conditions.
  • Use sufficient sample volume to detect low-level contamination.
  • Record raw colony counts and calculate MPC for each sample for consistency.
  • Investigate any excursion beyond alert or action limits promptly.

📜 Regulatory References

  • EU GMP Annex 1: Manufacture of Sterile Medicinal Products
  • ISO 14644-1: Classification of Air Cleanliness in Cleanrooms
  • FDA Guidance for Sterile Products: Emphasizes reliable environmental monitoring and corrected counts

🌟 Conclusion

The Most Probable Count (MPC) is a critical tool for accurate environmental monitoring in pharmaceutical cleanrooms. By applying Feller correction to raw active air sampling results, microbiologists can obtain a true estimate of airborne microorganisms, ensure compliance with regulatory limits, and maintain sterile conditions. Proper understanding and calculation of MPC are essential for reliable contamination control and product safety.

💬 About the Author

Siva Sankar is a Pharmaceutical Microbiology Consultant and Auditor with extensive experience in sterility testing, validation, and GMP compliance. He provides consultancy, training, and documentation services for pharmaceutical microbiology and cleanroom practices.

📧 Contact: siva17092@gmail.com
📱 Mobile: 09505626106

Disclaimer: This article is for educational purposes and does not replace your laboratory’s SOPs or regulatory guidance. Always follow validated methods and manufacturer instructions.

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