Instrument sterilization: monitoring

July 1, 2008
Proper instrument sterilization involves a series of six steps. The process involves time, talent, and treasure.

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Proper instrument sterilization involves a series of six steps. The process involves time, talent, and treasure. Offices need to perform quality control activities to measure sterilization effectiveness.

Most sterilization failures involve human error, such as overloading, improper cycle lengths, poor instrument cleaning, or incorrect wrapping. Mechanical failure often is not involved. In fact, many modern sterilizers will not allow processing to continue when a problem occurs.

It is not possible to test each processed item for sterility. Instead, a set of monitoring processes increase the probability that sterilization did occur. Monitoring includes three processes: physical, chemical, and biological.

Physical or mechanical monitoring involves observation of sterilizer gauges, displays, and recordings of unit cycle timers. Mechanical monitoring should occur every load. Correct readings do not guarantee sterilization; however, incorrect readings could be the first indication that problems may be present. Mechanical monitoring, though, does not usually measure problems associated with human errors. Some sterilizers have attached printers, which provide hard-copy records of operational parameters.

Chemical monitoring involves indicators that change color or shape when they are exposed to one or more operational parameters. The CDC recommends placement of an internal chemical indicator into every package. The CDC also asks that an external indicator be used when the internal indicator from outside of the package cannot be seen. Office personnel should receive training concerning the proper interpretation of positive and negative chemical monitoring results.

Basic chemical monitors (Class 1) are tape or indicators present on paper or paper-plastic pouches that change in reaction to a single parameter, such as time or temperature. Class 1 indicators are most useful in distinguishing processed from unprocessed items. Multi-parameter indicators (Class 4) react to two or more critical parameters, such as time and temperature. Class 4 types indicate exposure during a sterilization cycle to the desired parameters. Integrating indicators (Class 5) or "integrators" respond to all critical operational parameters over a defined range of sterilization cycles.

Biological monitoring or spore testing provides the main guarantee of sterilization. The process uses highly resistant, nonpathogenic bacterial endospores (biological indicators or BIs) as microbial challenges. Geobacillus stearothermophilus (née Bacillus stearothermophilus) spores test steam and unsaturated chemical vapor sterilizers, and Bacillus atrophaeus (née Bacillus subtilis) spores test dry heat sterilizers. These are the same spores that sterilizer manufacturers use when testing their units for FDA clearance.

BIs come in three forms: spore strips, spore ampoules, and self-contained spore vials. Spore strips are the most commonly used BI. These are small strips of thick filter paper covered with spores and enclosed in glassine pouches. Offices place pouches into packages and then process them.

Offices usually send the pouches to a sterilization monitoring service for processing. Using an aseptic technique, service personnel open the pouches, place the strips into tubes of growth media, and culture them. If sterilization has killed the test spores, then there will be no growth in the tube (clear). This is a negative result. If growth occurs (a cloudy tube), the result indicates sterilization failure and a positive test. Culturing for G. stearothermophilus is at 56°C, and at 37°C for B. atrophaeus.

Ampoules are sealed glass, which contain spores suspended in a culture broth solution. Ampoules test steam sterilizers. After processing, the ampoules undergo culturing at 56°C, and if viable spores are present, they will grow.

The components of self-contained spore vials include a spore strip separated from a sealed glass ampoule that contains growth medium and a pH indicator system. There are caps with holes that allow steam penetration and a hydrophobic filter, which serves as a bacterial barrier.

After sterilization, crushing of the vial joins the growth media with the processed spore strip. Vials undergo incubation and then are examined for bacterial growth. The pH indicator in the growth media changes from purple to yellow with the generation of acid. Some spore vials can go into a special incubator/reader, which delivers rapid results (in one hour). The readers even display a "red light" for positive results or a "green light" for negative results.

Dr. Charles John Palenik is the director of Infection Control Research and Services at the Indiana University School of Dentistry. In 2003, he was chairman of the Executive Board of OSAP, dentistry's resource for infection control and safety. Direct infection control questions to [email protected].

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