Sterility Indicators: Strict controls must be carried out on products to be labeled ‘sterile’. Such controls must then ensure, the absence of viable microorganisms from these products. There are two types of controls:
- Controls on the process of sterilization i.e. sterilization monitors or sterilization indicators.
- Sterility testing of the products.
Monitoring of the sterilization process can be achieved by the use of physical, chemical, or biological indicators of the sterilization performance.
1. Physical indicators
(i) Moist heat: A master process record (MPR) is prepared as part of the validation procedure for a particular autoclave and for each specified product and load configuration. This may then be used as a reference for the process record obtained from a single thermocouple placed in a strategic part of each load (batch process record, BPR). The MPR should be checked at annual intervals and whenever significant changes occur in the BPR when compared with the MPR. Microprocessor–controlled sterilization cycles are now a part of modern autoclaves. Pressure is measured by pressure gauges or through pressure transducers.
(ii) Dry heat: In dry heat sterilization processes, a temperature record chart is made of each sterilization cycle and is compared against a master temperature record.
(iii) Radio sterilization: A plastic dosimeter gives an accurate measure of the radiation dose absorbed and is considered to be the best technique currently available for the radio sterilization process.
(iv) Gaseous methods: For gaseous sterilization procedures, elevated temperatures are monitored for each sterilization cycle by temperature probes and routine leak tests are performed to ensure gas-tight seals. The gas concentration is measured independently of pressure rise, often by reference to the weight of the gas used. Pressure and humidity measurements are recorded.
(v) Filtration: Bubble point pressure test is a technique employed for determining the pore size of filters and may also be used to check the integrity of certain types of filter devices immediately after use. The principle of the test is that the filter is soaked in an appropriate fluid and pressure is applied to the filter. The pressure difference when the first bubble of air breaks away from the filter is equivalent to the maximum pore size. When the air pressure is further increased slowly, there is a general eruption of bubbles over the entire surface. The pressure difference is equivalent to the mean pore size.
2. Chemical indicators
Chemical monitoring of a sterilization process is based on the ability of heat, steam, sterilant gases, and ionizing radiation to alter the chemical or physical characteristics of a variety of chemical substances.
(i) Browne’s tubes: The most commonly used chemical indicators for heat processes are Browne’s tubes. These are small sealed tubes containing a reaction mixture and an indicator. Exposure to high temperature completes the reaction producing a change in the color of the indicator (Table.1). All four types change from red through yellow-brown to green, the latter color only being achieved after a specified time at the given temperature.
Table.1: Types of Browne’s tubes
(ii) Witness tubes: Witness tubes consist of single crystalline substances of known melting point contained in glass tubes e.g. sulfur (115 oC), succinic anhydride (120 oC), benzoic acid (121 oC), etc. A dye may be included to show more clearly that the crystals have melted. Such a device only indicates that a certain temperature has been reached. Exposure time can be calculated by putting the crystals in one end of an ‘hour-glass’ tube, the volume of the crystals and the diameter of the constriction of the tube is adjusted so that the time for transfer of the melt is the same as that required for the sterilization at the required temperature.
(iii) Heat-sensitive tape: Heat-sensitive tape is used quantitatively in the Bowie-Dick test. This is a test to determine that all air has been removed from dressings and that subsequent steam penetration has been even and rapid. The tape is placed suitably wrapped at the center of a test pack. All the bars on the tape should change color to demonstrate full penetration of the steam.
(iv) Royce sachet: The Royce sachet is a chemical indicator for ethylene oxide sterilization. This consists of a polythene sachet containing magnesium chloride, HCl, and a bromophenol blue indicator. A given concentration-time exposure to ethylene oxide results in the formation of ethylene chlorohydrin and a color change from yellow to purple.
(v) Chemical dosimeters: Chemical dosimeters give an accurate measure of the radiation dose absorbed and are considered to be the best technique currently available for controlling radiation sterilization. Qualitative indicators made of radiosensitive chemicals impregnated in plastic are also available. The indicator changes from yellow to red during irradiation.
3. Biological indicators
Biological indicators consist of a suitable organism deposited on a carrier and are distributed throughout the sterilizer load. At the end of the sterilization process, the units are recovered and cultured to determine the presence or absence of survivors. The biological indicator measures sterilization processes directly and can integrate all sterilization parameters. The selected organism should possess high and reproducible resistance to the sterilizing agent, and should be genetically stable, readily characterizable, and non-pathogenic. The viability of the organisms, the storage conditions before use, and the incubation and culture conditions after sterilization must be standardized for the results. The organisms used as biological indicators are usually resistant bacterial spores (Table.2).
Table.2: Biological indicators for monitoring sterilization processes
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