Biological and Chemical Tests for Sterilization Control

How the chemical and biological indicators that verify the actual effectiveness of every sterilization cycle work, and how often they should be performed according to ISO 11140 and ISO 11138.

An autoclave that works correctly from a mechanical standpoint doesn't automatically guarantee that every single cycle achieved actual sterility conditions on the treated instruments: for this reason the protocol includes a multi-level system of indicators, each designed to verify a different aspect of the process. Chemical indicators, compliant with ISO 11140-1, are strips or labels that change color when exposed to certain physical cycle parameters — temperature, exposure time, presence of steam — and come in several types (from Type 1, which simply indicates exposure to the cycle occurred, up to Types 5 and 6, migrating indicators that respond to multiple parameters simultaneously, offering a more rigorous check).

The Bowie-Dick test, also a specific chemical indicator (ISO 11140-4), verifies the vacuum pump's ability to completely remove air from the chamber during the pre-vacuum phase: a standardized test pack is subjected to a cycle at 134°C for 3.5 minutes, and uniform color change, typically from blue to pink, confirms that steam correctly penetrated even porous loads. The Helix test serves an analogous function but specifically for hollow bodies — turbines, handpieces, contra-angles — simulating the geometry of these instruments to verify that steam reaches even the narrowest, hardest-to-penetrate channels.

Biological indicators represent the most rigorous level of verification, because they don't just measure the cycle's physical parameters but the process's actual ability to kill microorganisms. They contain spores of Geobacillus stearothermophilus, a heat-resistant bacterium chosen precisely for its high heat resistance, making it a particularly reliable "worst case" indicator. After the sterilization cycle, the vial is incubated at a temperature between 55 and 62°C: if the culture medium remains its original color (typically purple), it means no spore survived and sterilization was successful; a color change to yellow, a sign of bacterial growth, indicates process failure, requiring immediate removal of the autoclave from service until checked.

The recommended frequency of these checks follows a cascading logic: the vacuum test is performed daily before the day's first cycle, multi-parameter chemical indicators are placed in every single sterilization cycle, Bowie-Dick and Helix tests are typically performed every two weeks, while the biological spore test should be performed at least once a month, and always after any extraordinary maintenance work on the autoclave, before returning it to clinical service. In case of a positive biological test result, international guidelines recommend repeating the test only after taking the necessary corrective measures, keeping the sterilizer out of service until satisfactory results are confirmed.

On Oralzon you'll find multi-parameter chemical indicators, Bowie-Dick and Helix test kits, and biological indicators with dedicated incubator to complete your sterilization quality control protocol.