Investigating Sterilizer Test Failures

Investigating Sterilizer Test Failures

What does your facility do when faced with a "positive" spore test after monitoring your sterilizer? According to CDC (Center for Disease Control) 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 & 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 = Not Sterile) or does not grow (negative = Sterile). Let's review each of these questions as they relate to possible sterilizer failure.


Depending on the type of sterilizer your facility has, it will function differently from other types used to process instruments and materials. For example, a Steam sterilizer functions differently than a Chemical vapor sterilizer, which functions differently than a Dry heat sterilizer. What type of sterilizer do you have? 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 your facility has.

Most Steam sterilizers operate at 250ºF (121ºC) for 30 minutes and 273ºF (134ºC) for 10 minutes, plus dry time. At 250ºF, pressure should read between 15-18 psi and 30-32 psi at 273ºF. If you are operating at temperatures and times that differ from these, or you are not achieving these pressure readings; be sure to check the Operator/Service Manual to see you are in compliance. If you do not have a Manual, contact the manufacturer for a replacement. They are required to provide you one; however, they may charge a fee for it. 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 are tested with B. stearothermophilus spores which are commercially available in paper strip, self-contained vial or sealed glass ampule formats.

Chemical vapor sterilizers operate at 270ºF for 20 minutes with no dry time. Newer units also have a 7 minute FLASH setting, but is for "single, unwrapped instruments" only. This type of sterilizer does not use distilled water, but rather a proprietary solution (alcohol/formaldehyde) available only from the manufacturer (Barnstead/Thermolyne, formerly MDT Corp.). It is important that the sterilizer dispense the proper amount of solution during each cycle, because 270ºF 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 mths). Chemical vapor type sterilizers (Chemiclave) are tested with B. stearothermophilus spores, but only in the paper strip format.

Conventional, convection type Dry heat sterilizers operate at 320ºF down to 280ºF for 1-2 hours; however, new "Rapid Heat Transfer" sterilizers are now available. These dry heat units operate at 375ºF for as little as 6 minutes (Cox sterilizer by Alfa Medical) to a high of 52 minutes (Guardian sterilizer by Dentronix). These types of sterilizers only require electricity to operate. Depending on the manufacturer, some units run on 110V and some require 220V. Convection and Rapid heat transfer Dry heat sterilizers are tested with B. subtilis spores, but only in the paper strip format.


Selecting the correct cycle conditions, packaging materials and spore test are all the responsibility of the Operator. For example, running an "unwrapped" cycle with a "wrapped load" will cause the sterilizer to fail, as will using the wrong packaging materials with the correct cycle. Be sure all Operators know how to select cycle conditions and proper packaging materials.

Overloading is another reason why many sterilizers fail, as the recommended cycle time is not enough to compensate for the load size. New sterilizer manufacturers now include maximum load testing with their submissions to the FDA to show they have compensated for "large loads"; however, older sterilizers may require significantly increased cycle times depending on the size of the loads. Chemical vapor and convection Dry heat type sterilizers have been found to be especially load sensitive.

Operating a sterilizer from a COLD START can also cause the sterilizer to fail and therefore, many manufacturers now recommend that you run an empty cycle first in order to pre-heat the sterilizer. While other manufacturers tell you to add a specific amount of time to their standard cycles when running from a COLD start.

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. Distributors who sell spore tests are not required to be FDA registered as they typically do not re-package or re-label them. However, testing labs usually do and therefore, they must be FDA
registered and meet strict GMP guidelines. Lab technicians who are not careful can contaminate the spore test during aseptic transfer and culturing, especially with B. subtilis which is incubated at 37ºC. When selecting a testing lab, be sure they are experienced (i.e. gram stain all "positives" to document 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!

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