Biomedical subjects
Publications and source records attributed to A Tallentire.
Neither of the standards for the validation and routine control of sterilization of medical devices by ionizing radiation provides detailed guidance on how to substantiate a 25 kGy sterilizing dose. This article describes the basis of a new method for substantiating this dose and how the resulting procedure has been applied in practice.
This article discusses the limitations of the methods offered in the radiation standards for selecting radiation dose. It highlights the difficulty of using these methods for product-specific dose-setting and, in particular, their inability to determine this for products with low bioburdens. Issues relating to the appropriateness of 25 kGy are also discussed.
A limiting case model, developed earlier to provide a conservative estimate of the proportion of non-sterile items (P) in an irradiated population of items contaminated with micro-organisms of a single type, has been extended to include items having organisms of different types. Estimates of P are compared with exact values derived on purely theoretical grounds and with values obtained by computer simulation of microbiological inactivation on items.
The primary function of a package enclosing a sterile medical or surgical device (e.g. syringe, needle, scalpel, etc) or a tray wrap enveloping a set of surgical items is to be an absolute barrier towards micro-organisms. Generally, the barrier function operates from the time that the item is exposed to the sterlization treatment until it is used. It follows that the material(s) constituting the package or wrap must permit the sterilization treatment to perform effectively, yet present a barrier to potential microbiological contaminants during post sterlization storage, handling and transport.
A given integrated radiation dose delivered from a LINAC as a train of pulses (50/s), characteristically of 0.1 to 5 microseconds pulse length with dose rates within the pulse between 0.38 and 38 krads/microsecond, inactivates bacterial spores in water suspension more effectively than the same dose given as Co60 gamma rays. This enhancement of radiation damage occurs both in the presence and in the absence of oxygen and is not explained by either pulse dose rate or pulse length alone, but is monotonically related to the product of these pulse parameters, pulse dose. The enhancement appears to result from the interaction, within individual spores, of free radical species of average lifetime of about 2-5 microseconds. The time scale over which these species operate suggests that they are freely diffusable. Prevention, in part, of their damaging effect by the presence of selective scavenging agents is evidence that OH radicals are involved. Measurements of H2O2 yield for irradiation conditions that show a gradation of enhancement of damage correlate strongly with the extent of damage observed.
A method (designated the net positive approach) is described for scoring results of sterility tests done with more than one recovery condition on product items treated with inactivating agents at levels less than those used in sterilization processes proper (subprocess treatments). Three tests of sterility, one per recovery condition, constitute a single sample. A growth in one or more recovery conditions is scored as a net positive for that sample. The net positive approach reduces the occurrence of false-negatives and assigns a value to the proportion of items reported as nonsterile (proportions positive) that is conservative.
The numbers and types of microorganisms contaminating a commercially available laboratory animal diet were examined by plate-count methods at all stages of production. During the cooking and pelleting stages of manufacture there was a marked reduction in numbers of heat-labile vegetative contaminants introduced via the raw materials, and at subsequent stages plate counts at 37 and 25 degrees C corresponded closely to counts of viable aerobic bacterial endospores. It would seem that the count on the pelleted diet was determined principally by the numbers of bacterial spores being introduced with the ingredients. The response to gamma-irradiation of the innate microflora contaminating the pelleted diet was characteristic of that generally seen with populations of aerobic bacterial endospores.
Using a stopped-flow mixing and pulsed irradiation apparatus, a study has been made of the decay, to a harmless form, of radiation-induced species that would otherwise be lethal to spores on contact with oxygen. Aqueous suspensions of Bacillus megaterium spores were irradiated with electrons for approximately 1 s; at various times after irradiation oxygen in solution was added. As the interval between anoxic irradiation and introduction of oxygen increased, the fraction of spores surviving increased. This change in survival reflects the decay of potentially lethal species. The presence of electron-affinic radiosensitizers during irradiation enhanced the decay rate of this damage, the greatest enhancement being seen with sensitizers of the highest electron affinity. In contrast, the nitroxyl-free radical sensitizer TAN fixed the radiation-induced damage so that no increase in survival, and hence no decay, was seen.
Using a stopped-flow mixing and pulsed irradiation apparatus, a study has been made of the decay, to a harmless form, of radiation-induced species which would otherwise be lethal to spores on contact with oxygen. Aqueous suspensions of Bacillus megaterium spores were irradiated with 600 krad of electrons given over approximately 1 s; at various times after irradiation oxygen in solution was added. As the interval between irradiation and introduction of oxygen increased, the fraction of spores surviving increased. For spores irradiated in a deoxygenated condition the decay of the potentially lethal species, reflected by this change in survival, proceeded as if two parallel first-order reactions with half-lives of 9 and 120 s operate. In contrast, for spores equilibrated with oxygen and then irradiated, the decay is described by a single first-order expression with an associated half-life similar to that of the faster of the two reactions operating in anoxia.
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