6.3 Sterilization Monitoring

Three Layers of Assurance

No single monitor proves a load is sterile. ANSI/AAMI ST79 builds confidence from three independent layers that must all agree before a load is released:

• Physical / mechanical monitors — The sterilizer's gauges, digital displays, and printouts that record time, temperature, and pressure for each cycle. They confirm the machine reached and held its parameters but say nothing about steam reaching the load interior.
• Chemical indicators (CIs) — Heat- or chemical-reactive inks that change to show a device or pack was exposed to the sterilizing agent. They detect exposure and certain process failures but do not prove sterility.
• Biological indicators (BIs) — Calibrated populations of highly resistant bacterial spores; if they are killed, the process is proven lethal to spores. BIs are the only direct measure of sterilization efficacy.

Think of it as defense in depth: mechanical proves the machine ran, chemical proves the agent reached the pack, and biological proves spores died. The CIS exam will frame a scenario where the printout looks perfect but an internal CI failed — the lesson is that one passing layer does not release the load.

Chemical Indicator Types 1–6

Chemical indicators are classified by their response, per ANSI/AAMI/ISO 11140-1. Higher numbers respond to more critical variables, not necessarily 'better':

Type 1 CIs go on the outside of packs; an internal CI (commonly Type 4, 5, or 6) is placed inside each pack to confirm the agent penetrated to that location. A frequent exam trap treats a passing Type 5 integrator as a substitute for a BI — it is not. A Type 5 correlates to BI performance and supports early release decisions, but it does not replace routine and implant biological monitoring.

Biological Indicators and Process Challenge Devices

A biological indicator (BI) contains a known population of bacterial spores chosen for their high resistance to the specific process. The organism is matched to the method:

• Geobacillus stearothermophilus — used for steam and for vaporized hydrogen peroxide.
• Bacillus atrophaeus — used for ethylene oxide (and historically dry heat).

After exposure, the BI is incubated; no growth confirms the spores were killed and the process was lethal, while growth indicates failure. Self-contained and rapid-readout BIs shorten incubation from days to as little as 20 minutes to a few hours, enabling timely release.

BIs are usually placed inside a process challenge device (PCD) — a representative test pack or a manufactured challenge that mimics the most difficult-to-sterilize item in a load. The PCD is positioned in the area of the chamber least favorable to sterilization (often the front bottom near the drain in a steam sterilizer).

ANSI/AAMI ST79 directs that the sterilizer be BI-monitored at least weekly, preferably every day it is used, and in every load containing an implant (with the implant ideally quarantined until the BI result is read). A control BI from the same lot is incubated unexposed each day to confirm the spores are viable and the incubator is working.

The Bowie-Dick Test

The Bowie-Dick test is a daily check of air removal and steam penetration in a dynamic-air-removal (prevacuum) sterilizer. It is not a sterility test — it confirms the vacuum system removes air so steam can penetrate the load.

How it is run:

1. Performed each day the sterilizer is used, before the first processed load.
2. Run in an otherwise empty chamber so the only challenge is the test pack itself.
3. Uses a Bowie-Dick test pack (a stacked-towel pack or a commercial disposable pack containing a Type 2 indicator sheet).
4. A uniform color change across the indicator passes; a non-uniform or center-pale result signals trapped air or a vacuum/leak problem, and the sterilizer is taken out of service until the cause is corrected.

Gravity-displacement sterilizers do not require a Bowie-Dick test because they do not rely on a vacuum system for air removal. Two exam traps to avoid: the Bowie-Dick pack runs alone in an empty chamber (a loaded chamber invalidates it), and a Bowie-Dick pass proves air removal, not that any specific load was sterilized — that is still the job of CIs and BIs.

Releasing Loads and Responding to a Positive BI

Releasing a load requires reviewing all monitoring before items leave the sterile storage area:

• Confirm the mechanical printout shows correct time, temperature, and pressure.
• Confirm the external (Type 1) and internal CIs passed.
• For implant loads, do NOT release until the BI is read; use a rapid-readout BI and a Type 5 integrating indicator, and document any emergency release.

What to Do on a Positive Biological Indicator

If the test BI shows growth (positive) while the control behaved correctly, treat the load(s) as non-sterile and act immediately:

1. Take the sterilizer out of service and do not run further loads in it.
2. Recall and quarantine every load processed since the last negative (passing) BI; retrieve items that have not yet been used.
3. Notify the appropriate personnel — the supervisor, infection prevention, and affected clinical areas; if an item was already used on a patient, follow facility infection-prevention escalation.
4. Investigate the cause — review the printout, loading, operator technique, and recent maintenance; have the sterilizer serviced if a malfunction is suspected.
5. Do not return the sterilizer to service until the cause is corrected and it passes monitoring (typically three consecutive negative BI cycles in an empty chamber, per facility policy and AAMI guidance).

A positive control with a negative test BI, by contrast, only confirms the spores were viable and the incubator worked — it is the test BI growing that indicates a sterilization failure.