When Sterilization Indicators Disagree: A Structured Investigation Framework

Sterilization monitoring is strongest when physical, chemical, and biological evidence is interpreted together. Each method examines a different aspect of the process: physical records document programmed conditions, chemical indicators respond to specified process conditions, and biological indicators use resistant microorganisms to challenge the process [1]. Because these monitors do not measure exactly the same thing, their results may occasionally disagree.

A discordant result should not be dismissed as a harmless anomaly or interpreted in isolation. It should trigger a documented review of the load, sterilizer, monitoring materials, placement, and test procedure. Monitoring results provide process evidence; they do not independently establish that every item in a load is sterile [2].

What discordant monitoring results can look like

Examples include:

• A cycle record appears acceptable, but an internal chemical indicator does not reach its specified endpoint.
• Physical parameters and chemical indicators appear acceptable, but a biological indicator is positive.
• A biological indicator is negative while the cycle record shows that a required condition was not achieved.
• An external process indicator changes as expected, but an internal indicator suggests inadequate penetration.
• One monitoring device gives an unexpected result while other devices from the same load appear normal.

These situations are not interchangeable. A negative biological indicator does not prove that every item in the load received adequate sterilization conditions [2]. An acceptable chemical-indicator response also does not prove that the load is sterile. Chemical indicators respond to defined process conditions and may help identify problems involving packaging, loading, or sterilizer operation [2].

First response: control the affected load

Until the discrepancy is understood, place the affected load on hold in accordance with the applicable device and cycle instructions, validated process, and facility policy. Do not release packages solely because one monitor passed. If an internal or external chemical indicator has not reached its specified endpoint, follow the indicator instructions and the facility’s established disposition procedure before use. An external process indicator can help distinguish processed from unprocessed items; it does not replace an internal indicator when penetration into the package or device must be assessed [1].

If the physical record shows that a required cycle condition was not achieved, the load should be managed according to the applicable validated process and facility policy rather than released on the basis of a different monitor. As a conservative facility decision principle, one negative result should not be used to erase an unexplained failure from another monitoring method.

Preserve evidence before repeating the process. Retain the cycle printout or electronic record, indicator results, load contents, packaging information, operator identification, sterilizer identification, and relevant photographs. Retain the original monitoring devices when practical, particularly when a biological indicator or unusual chemical response is involved.

Use a question-based investigation

1. Was the correct monitor used?

Confirm that the indicator was intended for the sterilization method and cycle type. Review the product instructions for placement, incubation or readout, interpretation, and controls. For biological indicators, verify that the appropriate microorganism and preparation were used for the process. CDC identifies Geobacillus stearothermophilus as relevant to biological-indicator monitoring of steam sterilization and several low-temperature processes, while Bacillus atrophaeus is associated with ethylene oxide and dry heat monitoring [1].

Check the expiration date, lot number, storage conditions, packaging integrity, and possible exposure to unintended heat, moisture, sterilant, or sunlight. A faulty, degraded, or incorrectly handled monitor can produce a misleading result, but a suspected monitor problem should be evaluated alongside the process evidence rather than assumed in advance [1].

2. Was the monitor placed correctly?

Review whether the indicator was positioned in the location or challenge device specified by the sterilizer, test-pack, or monitoring-device instructions. Placement should represent the intended challenge, such as a difficult-to-penetrate location or a selected item within the load. A test placed incorrectly may not provide interpretable evidence.

For chemical indicators, determine whether the device was placed inside the package when internal monitoring was required and whether the package was assembled and loaded correctly. External indicators provide limited process-status information; they do not replace internal monitoring when penetration must be evaluated [1].

3. What do the physical records show?

Review the complete cycle record rather than only the final cycle status. Examine exposure time, temperature, pressure, vacuum phases, alarms, aborted phases, and deviations from the validated or manufacturer-specified cycle. For steam sterilization, CDC describes routine mechanical monitoring as including review of time and temperature records and pressure information [1].

An incomplete, unavailable, or inconsistent physical record is a significant investigation finding. The facility should apply its approved acceptance criteria and disposition procedure; a negative biological-indicator result should not be treated as sufficient justification to release a load when the documented cycle does not meet the applicable requirements.

4. Could loading, packaging, cleaning, or sterilant delivery explain the result?

Inspect the load configuration, package dimensions, density, orientation, and contact between packages. Consider whether packaging materials were compatible with the sterilization method and whether instruments were adequately cleaned, opened, disassembled, and arranged to permit sterilant contact. CDC identifies improper cleaning, incorrect loading, overloading, improper packaging, and unsuitable packaging materials among possible causes of sterilization-monitoring failures [3].

For steam sterilization, also consider air removal, steam delivery, condensate, and other utilities. Entrapped air can prevent steam from reaching a chemical indicator in an air-removal test [4]. An unusual result may therefore indicate a problem involving equipment operation or the sterilization environment rather than the indicator alone.

Responding to a positive biological indicator

A positive biological indicator requires prompt action under the facility’s approved procedure. For a suspected positive result, take the sterilizer out of service and notify the appropriate supervisor and infection-control or other qualified personnel. Place suspect loads on hold and assess whether items already distributed or used require recall, reprocessing, or other action. This assessment is particularly important for implantable devices and should follow facility policy, applicable requirements, manufacturer instructions, and the organization’s risk-management process.

Before interpreting the result, verify the control biological indicator, indicator identity and readout, cycle settings, sterilizer records, monitor storage, and operator technique. When mechanical and chemical results suggest that a steam sterilizer operated properly, CDC notes that a single positive spore test may not by itself prove sterilizer malfunction; testing should be repeated promptly [1]. This does not eliminate the need to control suspect loads or evaluate possible recall and reprocessing actions.

CDC’s suggested protocol for a positive biological indicator in a steam sterilizer includes repeating biological-indicator testing in three consecutive cycles. If repeat tests remain positive, the sterilizer should remain out of service while equipment and steam supply are evaluated, and affected items should be considered for recall and reprocessing as appropriate [5]. This three-consecutive-cycle protocol is specific CDC guidance for this steam-sterilizer scenario; it is not a universal rule for every sterilization method or every facility situation. Non-steam processes should be managed according to their applicable instructions, validated procedures, and facility policy.

Making a release decision

Load release should be based on predefined acceptance criteria and the total available evidence, not on the most reassuring result. A useful decision record should state:

• Which monitoring result was discordant.
• Whether the load was quarantined and when.
• What evidence was reviewed.
• Whether the monitor, cycle, load, or equipment was identified as a likely contributor.
• Whether the load was released, reprocessed, recalled, or escalated.
• What corrective and preventive actions were assigned.

When the evidence cannot establish that the applicable process conditions were achieved, withhold the load and obtain qualified technical review. Document the rationale for the final disposition rather than relying on an isolated negative result.

Practical prevention measures

CSSD teams can reduce recurring discordance by standardizing indicator placement, documenting each load clearly, training staff to recognize abnormal results, reviewing monitor storage conditions, and trending failures by sterilizer, cycle, load type, operator, and indicator lot. Reviewing these data may reveal repeated loading problems, packaging defects, equipment drift, or training gaps.

Limitations

Monitoring indicators provide process evidence; they do not independently prove that every device in a load is sterile. Interpretation depends on the sterilization method, device and packaging configuration, monitor instructions, cycle records, validated processes, and facility procedures. This framework is educational and should be adapted to applicable sterilizer manufacturer instructions, local requirements, and institutional policies.

Educational disclaimer: This article is for professional education and does not replace facility procedures, applicable requirements, manufacturer instructions, or evaluation by qualified sterilization and infection-prevention personnel.

References

1. Sterilizing Practices. Centers for Disease Control and Prevention. 2024-06-24.
2. Glossary: Biologic Indicator and Chemical Indicator. Centers for Disease Control and Prevention. 2024-06-24.
3. Best Practices for Sterilization Monitoring in Dental Settings. Centers for Disease Control and Prevention. 2024-06-24.
4. Steam Sterilization. Centers for Disease Control and Prevention. 2024-06-24.
5. Table 12: Suggested Protocol for Management of Positive Biological Indicator in a Steam Sterilizer. Centers for Disease Control and Prevention. 2024-04-25.

Educational overview. Follow the applicable product instructions, validated procedures, local regulations and your facility’s approved policies.