Practice Tips #107: Failing A Sterilizer Spore Test

Current CDC guidelines and standard best management practices (BMPs) in infection control (as well as many local regulations) recommend performing a spore test on your sterilizer weekly. A test of your sterilizer using a biological indicator (“BI”) is necessary to confirm that it is effective at rendering loads sterile. But what happens when your sterilizer fails the test? What can and should you do?

Image of two Crosstex Confirm 10 Biological Indicator Vials

PHOTO: Biological Indicator Vials

Failed a Spore Test?

First, remove the sterilizer from service. You can’t use a sterilizer unless you know it is properly functioning. Next, repeat the spore test. According to studies, the most common cause of failure of a spore test is improper handling of the test media resulting in post-sterilization contamination. A simple re-test while exercising caution in handling may result in a passed test and you can resume use of your sterilizer.

There are a number of other operator-related issues that could result in a failed spore test, including:

  • Overloading (i.e. sterilizer too full)
  • Improper or excessive packaging (wraps or pouches)
  • Inadequate space between packages within the load
Image of dental autoclaved overloaded with instruments and cassettes

PHOTO: An Overloaded Autoclave

Review proper sterilization procedures and techniques with staff before repeating the test. Then perform a second test, making sure certain operator-related issues have been addressed and/or assurances taken to avoid them. If the second test also results in failure, it’s time to start looking more in-depth at your sterilizer.

Sterilizer Errors

Most automatic sterilizers display error codes if there is a failure during a sterilization cycle. We’ve decoded the errors for some sterilizers (such as the Statim) in Practice Tips #20 and #21.  Check these out if you are getting any error messages.

If your sterilizer is not displaying any error messages (or if you have a manual sterilizer), you’ll need to rely more on your powers of observation.

In order for a sterilizer to render a load sterile, it requires the presence of a particular temperature and pressure for a minimum amount of time. These three things are related. The greater the temperature and pressure, the lower the time required. Most autoclaves run at about 135° C (275° F) and 30 psi (pounds per square inch), with a cycle time at that temperature and pressure of 3-7 minutes. This is all that is required to achieve overkill, so sterility can be assured (see steam sterilization principles).image of temperature and pressure gauges embedded into a wall like a clock

Run a cycle and observe your sterilizer throughout the cycle. Watch for any obvious signs of problems, such as steam leaks. Make certain the sterilizer achieves full sterilization temperature and pressure (using the gauges on the sterilizer). Have a stop watch handy and time how long the sterilizer is at full sterilization temperature and pressure and compare this time to the specifications in your owner’s manual (minimum should be 3 minutes, of course).

Sterilizer Repairs

If steam leaks are evident, replace the leaking component(s) or correct other mechanical issues that may lead to a leak (e.g. lubricate the door hinges on a Midmark Ultraclave). If it is a door gasket issue, see Practice Tip #93 to see how to replace and install them properly.

Image of PM kit for Midmark M11

PHOTO: Midmark M11 Preventive Maintenance Kit, #MR-72

Once you address any mechanical problems and you have a successful spore test, you may return your sterilizer to service. Log the successful spore test in accordance with local regulations. Be sure to keep the log for as long as prudent for legal reasons.

A number of possible causes for a failed spore test can be the result of failed components, that should be replaced as part of your routine sterilizer maintenance. Most sterilizers require replacement of various components annually, although many have more frequent requirements. Contact us for specifications for you sterilizer or check your owner’s manual.

In order to be assured your sterilizer is functioning properly and your staff is operating your sterilizer consistently and correctly, it’s important to perform regular biological testing of your sterilizer. As many failures can be the result of operator error, it’s also a good idea to periodically review the operation of your sterilizer with staff.

You can print and post our routine sterilizer maintenance schedule in the sterilization area for your staff to reference as well. It may be convenient to bookmark or print and keep handy our issues of Practice Tips #27 and #28 devoted to trouble-shooting sterilizers or tech tips #20 and #21 regarding decoding Statim error messages.


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