Physical/Mechanical indicators monitoring the sterilization process


  • Sterilization is an essential stage which is used in the processing of any product.
  • These products are destined for parenteral administration, or for contact with broken skin, mucosal surfaces, or internal organs where the threat of infection exists.
  • The sterilization of microbiological materials, soiled dressings and other contaminated items is necessary to minimize the various health hazards associated with these articles.
  • The sterilization processes involve the use of a biocidal agent or physical microbial removal process to a product or preparation.
  • These processes target the killing or removing of all microorganisms.
  • There are various physical indicators that are used in the monitoring of the sterilization process.
  • They are as follows:

1. Heat sterilization processes

  • A temperature record chart is made of each sterilization cycle with both dry and moist heat (i.e. autoclave) sterilizers in this process.
  • This chart forms part of the batch documentation and is compared against a master temperature record (MTR).
  • The temperature should be taken at the coolest part of the sterilizer which is recommended.
  • The thermocouples are used that are placed at selected sites in the chamber which might be inserted directly into test packs or bottles.
  • This also provides further information on heat distribution and penetration within sterilizer.

2. Gaseous sterilization

  • Temperature probes are used to monitor the elevated temperature for each sterilization cycle.
  • Routine leak tests are also performed to ensure gas tight seals.
  • Records of pressure and humidity measurements are kept.
  • Gas concentration is measured independently of pressure rise, often by reference to weight of gas used.

3. Radiation sterilization

  • The use of a plastic (often Perspex) dosimeter is recommended which gradually darkens in proportion to the radiation absorbed.
  • This gives an accurate measure of the radiation dose and is considered to be one of the best techniques currently available in radio-sterilization process.


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4. Sterilizing filters

  • These filters are subjected to a bubble point pressure test which helps in determining the pore size of filters.
  • These are also used to check the integrity of certain types of filter device (membrane and sintered glass) immediately after use.
  • The principle of this test is that the wetted filter is subjected to an increasing air or nitrogen gas pressure differential in its assembled unit.
  • The pressure difference which is recorded when the first bubble of gas breaks away from the filter is the maximum pore size of the filter used.
  • When the gas pressure is increased further slowly, there is a general eruption of bubbles over the entire surface.
  • The pressure difference recorded now is related to the mean size of the pore.
  • A pressure differential which is below the expected value would signify a damaged or faulty filter.
  • Measuring the diffusion of gas through a wetted filter at pressures below the bubble point pressure is also a modified test for membrane filters.
  • A faster diffusion rate than expected would again indicate a loss of filter integrity.
  • A filter is considered ineffective when an unusually rapid rate of filtration occurs.




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4. HEPA filters

  • These filters are used to supply sterile air to aseptic workplaces.
  • Its efficiency is tested by the generation of upstream of dioctylphthalate (DOP) or sodium chloride particles of known dimension followed by detection in downstream filtered air.
  • Retention efficiency is recorded as the percentage of particles removed under defined test conditions.
  • Microbiological tests are not normally performed.




Physical/Mechanical indicators monitoring the sterilization process