Understanding Overkill Sterilization: An End to the Confusion - Pharmaceutical Technology

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Understanding Overkill Sterilization: An End to the Confusion
The author clarifies the definition and objectives of overkill sterilization for steam sterilization cycles. Current sterilization practices are reviewed and the validation difficulties associated with the various definitions of overkill sterilization are explored.

Pharmaceutical Technology

Table II: Log reduction and PNSU in selected cycles.
If we were to make a worst-case bioburden assumption, it might be a maximum of 1000 CFU of a resistant sporeformer such as Bacillus subtilis or Bacillus atrophaeus. These mesophilic sporeformers are occasionally found in pharmaceutical plants during environmental monitoring and bioburden sampling. The maximum reported D 121 for these species is <0.5 min. It should be immediately evident from Table II that the 8-min validation (the half cycle) cycle achieves the maximum PNSU that exceeds the 1 in 10–6 expectation for any microorganism with D 121 ≤1 min. For the assumed bioburden of 0.5 min, the log reduction in the half cycle relative to a BI with D 121 = 1 min is greater than 18 logs. The half-cycle process will reproducibly attain the universal expectation of a minimum PNSU on 1 10–6. For bioburden organisms that are not sporeformers, the level of safety is even greater (see Table II).

Overkill sterilization is thus attained for bioburden microorganisms (even those with substantial resistance) in relatively short cycles without the need to double the cycle. The excessive heat input necessary to use a half-cycle approach should not be considered benign. Elastomeric closures, tubing, filters, gasket materials, hoses, and other materials that are commonly sterilized using the overkill method can all suffer adverse effects as a consequence of extended sterilization times. Obviously, the half-cycle approach should never be used for heat-sensitive materials, as it almost always results in unnecessary degradation of the material being processed.

Although the half-cycle method might seem acceptable to some, it strikes this author as an overly conservative approach that belies the customary industry controls on bioburden. There is little, if any, scientific rationale for its continued use in steam sterilization. Requirements for environmental monitoring, cleaning validation, bioburden monitoring, and component preparation all serve to ensure that presterilization bioburden will approach neither the population nor the resistance of the biological indicator. If resistance and count are within reasonable control—that is, being nonthermophilic and meeting the local environmental limits—then any of the alternative approaches can be used.


Process expectations for demonstration of a minimum PNSU of 10–6 are essentially universal. Overkill sterilization in its many variants is only one means of demonstrating that minimum expectation. Its demonstration for routine processing relies on either knowledge or assumptions regarding the bioburden number and resistance and little else. As the challenge microorganism is not present in routine sterilization, only information about the bioburden is relevant to establishment of the desired PNSU.

James Agalloco is president of Agalloco & Associates, PO Box 899, Belle Mead, NJ 08502, tel. 908.874.7558,
He is also a member of Pharmaceutical Technology's Editorial Advisory Board

Keywords: Sterilization


1. PDA, Technical Report 1: Moist Heat Sterilization in Autoclaves, draft 12C, 2002.

2. PDA, Technical Report 22: Process Simulation Testing for Aseptically Filled Products, 1996.

3. "‹1211› Sterilization and Sterility Assurance of Compendial Articles," in USP 29 (US Pharmacopeial Convention, Rockville, MD, 2006).

4. Decision Trees for the Selection of Sterilization Methods (CPMP/QWP/155/96), 1999.

5. Validation protocol, circa 1980.

6. Validation protocol, circa 1990.

7. FDA, 21 CFR 600.11 (b)

8. FDA 21 CFR 212, Proposed June 1, 1976, withdrawn 1991.

9. I. Pflug, Microbiology & Engineering of Sterilization Processes, 10th ed. (Environmental Sterilization Services, St. Paul, MN, 1999).


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