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It is the procedure by which all the germs present in the treated material are destroyed. This procedure can be physical, as in the case of moist heat or dry heat, UV and Gamma radiation, or chemical, as in the case of formaldehyde, ethylene oxide, glutaraldehyde and hydrogen peroxide plasma gas.

Sterilization processes are controlled by a combination of physical, chemical and biological techniques designed to evaluate the sterilization conditions and the effectiveness of the process. The physical techniques include control of the cycle time and control of the pressure and temperature indicators inserted in the equipment panel. A correct reading does not ensure the proper functioning of the cycle, but an incorrect reading may be the first evidence of the existence of a problem. Chemical techniques (chemical controls) use substances that are sensitive to one or several of the parameters of the sterilization cycle. They generally indicate the value of one of the parameters reached by means of a change in color. There are chemical controls that list one parameter and others that respond to several in a sterile cycle.

The correct functioning of a sterilization cycle must be checked each time a cycle is run. In the case of steam processes, it is recommended to include at least three biological indicators per cycle, with a minimum frequency of once every three cycles. For ethylene oxide, it is recommended to include at least two biological indicators in each cycle. For dry heat it is recommended to include at least three biological indicators per cycle, with a minimum frequency of once every three cycles. It is important to remember that each time processed biological indicators are incubated, a non-processed indicator should be included to ensure that growth conditions were favorable throughout the incubation period. It is also important to consider that the greater the frequency of use of biological indicators in a process, the greater the security of its proper functioning.

In addition to routine monitoring, you should include biological indicators whenever:

  • A new type of packaging material is used.
  • After training new personnel.
  • After a sterilizer has been repaired.
  • After any change in the sterilizer loading process.

It is always advisable to place sterilization controls in the places that are considered a priori to be most inaccessible to the sterilizing agent.

A key location may be the center of the load, the center of a package, or near the door of the equipment. It is recommended for each particular case to prepare devices of maximum sterilization difficulty, with the materials that are usually used to pack the material to be sterilized.

Whenever this happens, it is recommended that you check the sterilization process. Variables such as time, temperature, pressure, etc. may have to be readjusted. If the problems persist it will be necessary to check the equipment. There may be problems with the impermeability of the packaging material, so it is always advisable to test the packaging material before approving its functionality.

  • Badly washed material, proteins and precipitated salts can cause an encapsulation of the microorganisms, isolating them from direct contact with the sterilizing agent and therefore reducing the process’ effectiveness.
  • Inadequate packaging material.
  • Excessive amounts of packaging material, which can delay the penetration of the sterilizing agent or impede it completely.
  • Overloading, or incorrectly loading the sterilizer.
  • Packages on top of each other without spaces between them where the sterilizing agent can circulate.
  • Too short sterilization times.
  • Insufficient temperature.
  • Too low concentration of sterilizer agent.

Spores are microscopic biological structures that are highly resistant to environmental stresses such as drying, temperature, pressure, osmotic stress, chemical agents and radiation. As they are the most resistant living beings on the planet, they are ideal to be confronted with sterilization processes, since if we make sure that they die, we can affirm the death of any other microorganism present in the sterilized material.

It is the time needed to reduce by one order or 90% a population of bacterial spores subjected to a given sterilization process under a number of conditions established for that process.

Terragene calculates D values at 121°C, 128°C and 134°C for each batch of its BT20, self-contained, BT50 and BT60 steam biological indicators, and spore strips.

Terragene calculates D values at 54°C, 60% RH, 600 mg/l EO for each batch of its biological indicators for EO BT10, self-contained, BT40 and BT60, and spore strips.

Terragene calculates D values at 160°C and 180°C for each batch of its biological indicators for dry heat BT30, semi-self-contained, BT40 and BT60, and spore strips.

Z- Value is a defined parameter for temperature sterilization processes. It is defined as the number of degrees Celsius required to vary the D value in one order. For example, for a given batch, if we have a D value at 121°C of 2.0 minutes and the Z value reported for this batch is 10°C, this means that at 131°C the D value will be 0.2 minutes.

The use of self-contained biological indicators minimizes the occurrence of false positives because it is a closed system that does not allow the exchange of microorganisms but does allow the exchange of gases and steam. Due to their design, it is not necessary to open a paper envelope under sterile conditions and manipulate a paper inoculated with spores, trying to avoid contact with sterile surfaces and/or air currents that could deposit microorganisms on the spore strip.