Evaluating the formulae for integrated lethality in ethylene oxide sterilization using six different endospore forming strains of bacteria, and comparisons of integrated lethality for ethylene oxide and steam systems.

Bacterial endospores from six different species of bacteria were exposed to a spectrum of ethylene oxide (EtO) sterilizing conditions. Temperature was varied from 40 to 60 degrees C and the ethylene oxide concentration was varied from 300 to 750 mg/L. Relative humidity was maintained at 60+/-10% RH. The fraction negative procedure was used to determine the D value for each of the test conditions. Bacterial species tested included Bacillus atrophaeus ATCC # 9372, Bacillus smithii ATCC # 51232, Bacillus subtilis "5230" ATCC # 35021, Bacillus subtilis, DSM # 4181, Bacillus pumilus ATCC # 27142, and Geobacillus stearothermophilus ATCC # 7953. All spore preparations were inoculated on filter paper strips packaged in blue, sterilizable glassine pouches. G. stearothermophilus was the least resistant organism tested. The most resistant organisms tested were B. atrophaeus and B. subtilis "5230". The B. subtilis "5230" strain was slightly more resistant than B. atrophaeus at conditions of 54C and EtO concentrations of 400, 600, and 750 mg/L, as well as at 60C/750mg/L EtO. The other species were between these extremes. This empirical data allowed the application of the recently published formula for converting D values from one set of conditions to another and evaluations of accuracy. The measured D values also allowed the determination of Z values based on temperature variations. These formulae, when applied to process temperatures independent of gas concentration, result in a Z value of approximately 32 degrees C that appears to be similar for all species tested. These data support the application of the previously published formulae 1-6 and allow the same approach to integrated lethality for ethylene oxide processes as is commonly applied to steam sterilization. A review of steam sterilization and related principles was conducted for comparison of integrated lethality for these two methods of sterilization. Errors associated with D values, Z values, extrapolation, and integrated lethality for both methods of sterilization are discussed.

Factors affecting tailing in ethylene oxide sterilization part 1: When tailing is an artifact... and scientific deficiencies in ISO 11135 and EN 550.

Current approaches for validation of EtO sterilization cycles identified in ISO 11135 and EN 550 are scientifically deficient and result in full cycle times that are too short. This review article identifies the scientific flaws promulgated in the standards that lead to overestimation of process lethality. The reasons for these deficiencies are design-of-experiment errors that either fail to mathematically account for lag factors or that treat dissimilar samples as a single statistical sample set. Industry's movement toward parametric release, based on erroneous assumptions where exposure time rather than equivalent time is used, may create serious problems. The authors discuss these errors in detail, as well as reasonable methods for correcting them based on formulae for integrated lethality. Application of these methods will allow more accurate calculations of process lethality that are supported by biological data and appropriate mathematical analyses.