Most autoclave failures that are blamed on the machine are caused by how the chamber was loaded. Packs touching the walls, pouches stacked paper-to-paper, mixed metal and porous loads, and containers without vents all change how steam moves, and a biological indicator placed in the wrong spot records the result. Loading is the one variable staff control several times a day, which makes it the first place to look when results become inconsistent.
Good loading starts with the packaging decision. A Class 4 dual-indicator sterilization pouch works only if the film side faces the same way as its neighbours and the pouch is not compressed by the load above it.
What are the loading mistakes that cause the most failures?
| Mistake | What happens | Fix |
|---|---|---|
| Packs touching chamber walls | Condensation and uneven heating; wet packs | Leave a gap at the walls and door |
| Stacking pouches paper to film | Steam cannot reach the paper face | Stand pouches on edge, film facing the same way |
| Overloading the chamber | Air pockets and failed indicators in dense loads | Stay within the manufacturer's load limit; leave space between trays |
| Mixing metal, porous and liquid loads | Different heating rates; wet packs and slow cycles | Run separate cycles by load type |
| Blocking the chamber drain or filter | Poor steam circulation and wet loads | Keep the drain clear; check the filter weekly |
| Unvented containers or closed lids | Trapped air; failed sterilization inside | Use vented containers or open lids per the manufacturer |
| Indicator placed on the chamber floor | A result that does not represent the load | Place it in a process challenge device inside the load |
How should pouches be arranged in the chamber?
Stand pouches on edge with the paper side facing the same direction, and leave a finger-width gap between them. Laying them flat in a stack is the single most common cause of a pouch that passes an external indicator and fails inside. If you must use a tray, keep the pouches upright in a rack rather than layered.
For wrapped sets, place heavier items on the lower shelf and lighter packs above, without compressing either. Do not stack trays so that the upper tray rests on the packs below; use the shelf positions the manufacturer provides.
Why does mixing load types cause wet packs?
Metal heats quickly and holds heat, porous loads absorb steam, and liquid loads cool the chamber as they boil. In a mixed load, the chamber reaches its set point, the metal is already hot, the liquid is still catching up, and the cycle ends with condensation on the wrapped packs. The indicator may pass and the pack may still be wet, which is a recontamination risk once the pack cools on the shelf.
Separate the three families. A clinic that runs one metal cycle, one porous cycle and one liquid cycle will repeat fewer loads than one that mixes everything to save a cycle.
Where should the biological indicator go in the load?
Inside a process challenge device, placed in the position that is hardest to sterilize, typically the lower centre of a dense load or inside a representative pack. Not on the chamber floor, not near the door, and not loose on a shelf. The indicator is a test of the worst case; putting it in the easy case produces a passing record that proves nothing.
If you run several loads a day, use one indicator per load rather than one per day. In Ontario, the dental standard is a biological indicator for each sterilizer on every day it is used, and a single test cannot cover loads of different composition. For a clinic running wrapped and hollow loads, a test for each cycle type is the safer reading of the requirement.
What should staff check before closing the door?
Four quick checks take under a minute: no pack touching a wall, no stack of flat pouches, no blocked drain, and the indicator in place inside the load. Add a fifth at the end of the cycle: inspect for wet packs before moving anything to the shelf. A wet pack is not a sterile pack, whatever the indicator says.
Related reading
- Can sterilization pouches be re-autoclaved?
- Small steam sterilizer classes N, S and B for dental clinics
- Why does autoclave tape stay pale tan after a cycle?
Download the printable sterilization log, or build one with the log generator. Ask a Compliance Specialist at our consultation desk. Ordering for a clinic, lab or care home? Wholesale and multi-site ordering covers account setup for institutional buyers. Start with a 24-hour biological indicator 25-pack to test a loading change against a real result. Reselling? become a distributor.
Frequently Asked Questions
Can you overload an autoclave and still pass the indicator?
Sometimes, which is why overloading is dangerous. A dense load can pass in one position and fail in another. Stay within the manufacturer's load limit and place the indicator in the hardest position.
Should pouches be placed flat or on edge?
On edge, with the paper faces aligned and gaps between pouches. Flat stacks trap air and prevent steam from reaching the paper side.
Why are my packs wet after the cycle?
Wet packs usually come from mixed load types, an overloaded chamber, a blocked drain or steam that is too wet. Separate load types and check the drain and steam supply.
How many biological indicators for multiple loads in one day?
Use one per load, and one per cycle type where loads differ in composition. A single daily test does not represent a wrapped load and a hollow load at the same time.
CliniEco Medical is a licensed medical device establishment (MDEL #35334).
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