Chemical, Enzyme and Biological Indicators for VHP: What Each One Tells You

Three questions, three indicators

When a VHP cycle is developed or checked, three different questions come up at each location in the space:

• Did the vapour get here? — answered by a chemical indicator.
• How much lethal exposure did this point get compared with others? — answered quickly by an enzyme indicator.
• Were resistant spores killed here? — answered only by a biological indicator.

Confusing these questions is a common source of trouble. A chemical indicator that has changed colour is often treated as a pass, when it only shows that some hydrogen peroxide was present.

Chemical indicators

A chemical indicator (CI) is a printed strip or label whose ink changes colour when it reacts with hydrogen peroxide. Indicators for VHP are designed for the vapour process and are classified under ISO 11140-1; most used in rooms and isolators are process or exposure indicators.

• What they are good for: cheap, instant, and easy to place in large numbers. They show quickly whether vapour reached hidden areas, behind equipment, inside cabinets and at the far end of a room.
• What they cannot show: whether the concentration and time were enough to kill spores. Many respond to a fraction of the dose needed for a 6-log reduction. Some also respond to condensation differently from the vapour.
• Typical use: distribution checks during cycle development, a quick check after changes to the room or load, and as a routine visible sign that a load passed through a pass box cycle.

Enzyme indicators

An enzyme indicator (EI) carries a dried layer of a heat-stable enzyme on a carrier. Hydrogen peroxide inactivates the enzyme; afterwards the remaining activity is measured, usually with a bioluminescence reader, and compared with an unexposed control. The result is a number — a log reduction in enzyme activity — available within minutes of the cycle ending.

• What they are good for: a graded result rather than pass or fail, so you can rank locations from best to worst exposure and see the effect of moving a fan or changing the injection rate the same day.
• What they cannot show: spore kill. The enzyme’s response has to be correlated with biological indicators for the process; it is a fast surrogate, not a replacement.
• Typical use: cycle development and optimisation, locating worst-case positions before biological indicator runs, and investigations after a failure.

Biological indicators

A biological indicator (BI) for VHP normally carries about 10⁶ spores of Geobacillus stearothermophilus on a stainless steel carrier in a vapour-permeable pouch, made to ISO 11138-1. After the cycle the carriers are transferred to growth medium and incubated at 55–60 °C. No growth means the spore population was reduced by at least six logs at that location.

• What they are good for: they are the only indicator that measures microbial lethality directly, which is why qualification and periodic requalification rely on them.
• Limitations: results take days, the result is growth or no growth, and individual BIs vary in resistance. Occasional positives from a single unusually resistant carrier are a known issue; how to handle them is part of the validation approach.
• Typical use: performance qualification at worst-case locations, requalification, and verification after significant changes.

The science behind the 6-log criterion and D-values is covered in biological indicators and the 6-log reduction; what to do when a BI grows is covered in VHP cycle failure troubleshooting.

Side by side

	Chemical indicator	Enzyme indicator	Biological indicator
Measures	Presence of H₂O₂	Loss of enzyme activity	Kill of resistant spores
Result	Colour change	Number (log reduction of activity)	Growth / no growth
Time to result	Immediately	Minutes	Days of incubation
Equipment needed	None	Reader	Incubator, growth medium, microbiology lab
Proves sterilisation?	No	No — must be correlated with BIs	Yes, for the carrier and location tested
Main use	Distribution mapping, routine exposure check	Development, worst-case finding, investigations	Qualification and requalification

Using them together

1. Map with chemical indicators. Place many CIs across the room or isolator, including corners, under equipment, inside cabinets and at the far end from the generator. Locations that change slowly or weakly are candidates for worst case.
2. Tune with enzyme indicators. Put EIs at the weak locations and a few reference points. Adjust injection rate, hold time, fan positions or load arrangement and compare results the same day.
3. Qualify with biological indicators. Place BIs at the confirmed worst-case locations, plus positions required by your protocol, and run the qualification cycles.
4. Monitor routinely. Depending on risk, use CIs on every load, and BIs at a defined frequency or after changes.

Placement and handling

• Place indicators so the vapour reaches them the way it reaches the surface they represent — not shielded by tape, clips or the bag they came in.
• Record each location on a drawing with an ID, and use the same positions in repeat runs.
• Avoid placing indicators where condensation drips or collects unless that is the condition you are trying to test.
• Keep unexposed controls from the same lot for every enzyme and biological indicator run.
• Store indicators as the manufacturer specifies; absorbed moisture and heat can change their response.