Understanding SAL

Sterility is defined as being free from all living organisms.1 Since it’s not practical to test every device for the absence of microorganisms, the concept of sterility is assumed to be a statistical probability. The sterility assurance level (SAL) defines the probability of a non-sterile unit for a validated sterilization process. For example, when the SAL is 10-6, it means that there is one chance in a million that a device is not sterile.

Biological indicators are used to develop and validate sterilization processes, and therefore estimate SAL.

  • Load monitoring with biological indicators
    The Importance of Proper Resistance

    BIs contain a known number of live microorganisms (spores) to assess the adequacy of a sterilization cycle. Because BIs measure the lethality and effectiveness of sterilizers, it’s important that the spores within a BI are of the right resistance. Offer of BI Comparison for vaporized hydrogen peroxide

  • Sterilization monitoring
    Guidelines Summary

    The AAMI ST79 Comprehensive guide to steam sterilization and sterility assurance in health care facilities is a go-to resource for all healthcare facilities that process medical instruments with steam sterilization. Get access to a convenient 3M summary of the quality assurance recommendations in the guideline.

  • All fields are required unless indicated optional

  • 3M takes your privacy seriously. 3M and its authorized third parties will use the information you provided in accordance with our Privacy Policy to send you communications which may include promotions, product information and service offers. Please be aware that this information may be stored on a server located in the U.S. If you do not consent to this use of your personal information, please do not use this system.

  • Submit

Our apologies...

An error has occurred while submitting. Please try again later...

Thank you!

Your form was submitted successfully

Biological Indicators on white background

Biological Indicators (BIs)

The education and science of BI technology for sterilization.


Biological indicators (BIs) are critical to sterilization assurance.

  • Comprehensive sterilization monitoring is more than checking cycle temperatures and times. It’s verifying that conditions within a load were adequate to kill a population of resistant microorganisms. A biological indicator (BI) is the only way to directly measure the lethality of a sterilization cycle.

    BIs contain bacterial spores that are even harder to kill than any potential remaining microorganisms that may have survived the surgical equipment decontamination process. When the sterilization cycle is sufficient to kill the spores in the BI, one can assume the microorganisms on the instruments have also been killed.

    3M has been developing and improving technology for sterilization assurance for decades. In fact, we invented rapid steam BI technology and launched our first rapid readout BI in 1991.

How a BI works

  • 3M Attest Rapid Readout BI


  • Clinician using a BI


  • Gloved hand holding Biological Indicator crusher


  • 3M™ Attest™ Auto-reader 490/490H System


  • The theory behind the BI is that if your process is effective enough to kill a large population of highly resistant spores, it will also kill a lower number of less resistant organisms on the medical devices. Because it detects the killing of microbial spores, a BI is capable of yielding information that is more valuable than any other sterilization monitor.

    After exposure to the sterilization process, self-contained BIs are activated to provide the spores with optimized growth media, and then must be incubated at the appropriate temperature to determine if any spores survived. For this reason, results are not immediately available. However, advances in BI technologies have led to rapid readout BIs, which provide the end user with actionable results in less than 30 minutes.

What is sterility assurance level (SAL)?

  • Sterile is the state of being free of all living microorganisms and is usually expressed as a probability.1 A sterility assurance level (SAL) is a number that tells you the probability of a viable microorganism being present on an item after sterilization.1 This number expresses the likelihood that a microorganism survived the sterilization. An SAL of 10-6 (or 10 to the negative sixth power) indicates there is one chance in a million that a device is not sterile and is generally accepted as appropriate for an item intended to contact compromised tissue.1

    The sterilizer manufacturer is responsible for providing a sterilizer and process that can achieve the desired SAL and uses BIs as a tool to estimate SAL and develop and validate their sterilization processes. The user is responsible for monitoring the performance of the sterilizer (e.g., with a BI) to ensure it is operating as intended.1

Chemical indicators vs. biological indicators

  • Photo of 3M Attest steam chemical integrators

    Chemical indicators (CIs) verify that specific conditions for sterilization like time, temperature level and sterilant (steam, vaporized hydrogen peroxide or ethylene oxide) were reached inside the sterilizer or individual package. CIs will reach their end point (e.g., change color) to show that the pack or load was exposed to the specified conditions for sterilization (e.g., time, temperature level and sterilant). They are an important part of the sterilization assurance process, showing you immediately that one or more sterilization parameters may have failed.

    CIs do not directly measure lethality with viable spores, but CIs are still an important indicator to any sterilization program — they can provide immediate results to demonstrate they were exposed to the specified sterilization conditions. However, CIs do not tell you if the potentially infection-causing microbes present in that pack or load were exposed to a lethal dose of sterilant. Only a BI is a direct measure of the lethality of a sterilization cycle. BIs are used to monitor the effectiveness of sterilizers and individual sterilization cycles.

BIs are key to complying with standards and guidelines — and protecting patient safety.

  • Illustration of sterilization cart

    AAMI standards specify that BIs are "the only sterilization process monitoring device that provides a direct measure of the lethality of the process."2 Releasing every load based on a BI result (every load monitoring) is a best practice for sterile processing departments. A negative BI result from every single load processed confirms the lethality of each cycle.

    CDC, AORN, and AAMI guidelines all state that implants should be quarantined until a BI result is known, except in emergency situations.

gloved hand holding dental instruments
Dental sterilization monitoring results in 24 minutes.

Learn how to help protect your patients and practice.


Which BI do you need?

  • Photo of 3M™ Attest™ Super Rapid Readout Biological Indicator 1492V

    Need help figuring out which 3M™ Attest™ Biological Indicator or BI process challenge device (PCD) solution to use at your facility? Use the 3M™ Attest™ Bl Selector Tool. After you answer a few questions, we can suggest an option or two.

Key Resources

Contact a 3M Sterilization Representative

Learn how 3M Sterilization Monitoring Solutions can help you be confident that every sterilization cycle is monitored prior to instrument use, so every patient can receive a high level of care.

Have a technical question?
Call us at:

Have a technical question?
Call us at:


1. CDC Guideline for Disinfection and Sterilization in Healthcare Facilities, 2008; https://www.cdc.gov/infectioncontrol/pdf/guidelines/disinfection-guidelines-H.pdf [accessed June 27, 2023].
2. ANSI/AAMI ST79:2017 Comprehensive guide to steam sterilization and sterility assurance in health care facilities.