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Sterilizer Test Failures

By Charles A. Hughes

What does a facility do when faced with a “positive” spore test after monitoring their sterilizer? According to the CDC (Center for Disease Control and Prevention) guidelines…

“If spores are not killed in routine spore tests, the sterilizer should immediately be checked for proper use and function and the spore test repeated. If the spore tests remain positive, use of the sterilizer should be discontinued until it is serviced”.

Now that private-offices and clinics are spore testing sterilizers as recommended and/or required by local, state and federal agencies; more and more questions are being asked regarding the proper use and function of sterilization equipment. The spore test itself is sometimes even questioned along with the person or Lab culturing the test to see if it grows (Positive = Non Sterile) or does not grow (Negative = Sterile). Let’s review each of these questions as they are related to possible sterilizer failure.

Sterilizer Function

Depending on the type of sterilizer, it will function differently from other types used to process instruments and materials. For example, a Steam sterilizer functions differently than a Chemical vapor, which functions differently from a Dry heat sterilizer. What type(s) of sterilizers do your facilities use? All users must know the sterilizer type, as operation, packaging materials, loading instructions and even the spore test are all specific to the type of sterilizer being used.

Steam sterilizers commonly operate at 250F/121C for 30 minutes or 270-273F/132-134C for 10 minutes. These are exposure times only and do not include temperature come up, come down or dry times. Special Note: Proper drying of wrapped loads is critical in order to maintain sterility until point of use.

At 250F, pressure should read between 15-18 psi and 30-32 psi at 273F. If your sterilizer times, temperatures or pressure readings vary from these, be sure to check the Operator/Service Manual to verify you are running an approved cycle. Users who do not have an Operator/Service Manual should contact the sterilizer manufacturer for a replacement. Steam sterilizers require distilled water and all manufacturers warn against using tap water as it can damage the sterilizer and reduce the quality of the steam, which can cause sterilizer failure. In addition, door gaskets should be cleaned daily and repaired or replaced as needed. A poor door seal, clogged air trap jet, blocked safety valve and/or inadequate water fill levels can all cause a steam sterilizer to fail. Steam sterilizers should be tested at least weekly with Bacillus stearothermophilus spores that are commercially available in paper strip, self-contained vials or sealed glass ampoule formats.

Chemical vapor sterilizers operate at 270F/132C for 20 minutes with no dry time. Newer units also have a 7-minute FLASH cycle, but is limited to “single, unwrapped instruments” only. This type of sterilizer does not use distilled water, but rather a proprietary solution (alcohol/formaldehyde) available from the manufacturer (Barnstead/Thermolyne, formerly MDT Corp.). It is important that the sterilizer dispense the proper amount of solution during each cycle, because 270F/132C for 20 minutes with heat only, is not enough to kill spores. Again, door seals must be secure, traps must be cleaned and the solution must be viable (shelf-life = 24 months). Chemical vapor type sterilizers should be tested at least weekly with Bacillus stearothermophilus spores, but only in the paper strip format.

Dry heat sterilizers are available in convection and rapid heat transfer designs with drastically different cycle parameters. For example, traditional convection type sterilizers should be operated at 320F for 1-2 hours depending on load size. The Cox Rapid Heat Transfer type sterilizer available from Alfa Medical operates at 375F with preset cycles of 6, 8 and 12 minutes. Dry heat sterilizers only require electricity to operate and depending on the manufacturer, some units run on 110V and some require 220V. Convection and Rapid Heat Transfer dry heat sterilizers should be tested at least weekly with Bacillus subtilis spores, but only in the paper strip format.

Operator Error

Selecting the correct cycle conditions, packaging materials and spore tests are the responsibility of the Operator. For example, running an “unwrapped” cycle with a “wrapped” load will cause sterilization failure, as will using the wrong packaging materials with the correct cycle. Be sure your facilities know how to select cycle conditions and proper packaging materials. Overloading is another reason why sterilizers fail, as the recommended cycle time is not enough to compensate for the load size. New sterilizers are now required to submit maximum load testing to the FDA when applying for approval, showing they have compensated for large loads; however, older sterilizers usually have not. Users should limit the size of their loads or add additional time to compensate for large loads per the manufacturers instructions. Chemical vapor type sterilizers and convection type dry heat sterilizers have been found to be especially load sensitive.

Operating a sterilizer from a COLD START can also cause sterilizers to fail and therefore, as many manufacturers now recommend running a warm up cycle first, in order to preheat the sterilizer; while other manufactures tell you to add a specific amount of exposure time to their standard cycle when running from a COLD START. Once again, be sure to check the Operator/Service Manual from the manufacturer.

Spore Tests and Testing Labs

Since most chemical indicators turn color by heat only, spore testing is considered the best way to document all parameters for sterilization (time, temperature & sterilant contact) have been achieved.44 Distributors who sell spore tests are not required to be FDA registered as they typically do not re-package or re-label them. Testing labs, however, usually do and therefore, they must be FDA registered and meet strict QSR (Quality Systems Regulations) guidelines. Lab technicians who are not careful can contaminate the spore test during transfer and culturing, especially with Bacillus subtilis spores, which are incubated at 35-37C. When selection a Testing Lab for 3rd party verification, be sure they are experienced (i.e. gram stain all positives to verify spore growth and not Lab error prior to reporting that your test has failed), are FDA registered and can document that their spore test is approved for your specific type of sterilizer!


  • CDC Guideline for Handwashing & Hospital Environmental Control, Biological Monitoring of Sterilizers, 1985.
  • OSHA Technical Notes, #23, March 1, 1990.
  • Hughes, C., Surgical Services Management: Sterility is Event Related, Not Time Related. Vol. 2, No. 10, October 1996.
  • JADA, Biological Indicators for Verifying Sterilization. Vol 117, October 1988.

About the author…

Chuck is the VP for Infection Prevention at Crosstex/SPSmedical. Crosstex/SPSmedical is one of the largest sterilizer testing laboratories in N. America and the company develops, manufactures and markets a full line of sterility assurance products. Certified as a Health Education teacher, Chuck has worked for over 25 years in the manufacturing industry in areas of R&D, Regulatory Affairs, Microbiology, Marketing and Sterilization Training. A member of AAMI, IHSCMM, ASHSCP and OSAP, Chuck has extensive lecturing experience, including presentations to sterilizer and instrument manufacturers, OR departments, Dental organizations, and CS organizations throughout the U.S. and Canada.