What indicates that instruments have been sterilized during biological indicator tests?

In the context of biological indicator tests for sterilization, a strip of paper that changes color is a critical indicator of whether instruments have been effectively sterilized. These strips are typically impregnated with spores of bacteria that are highly resistant to the sterilization process, such as Bacillus stearothermophilus for steam sterilization. When the sterilization process has successfully killed these spores, the color change in the paper strip provides visual confirmation that the conditions necessary for sterilization were met, such as time, temperature, and moisture levels.

This method is essential because it directly tests the efficacy of the sterilization process rather than relying on external factors like the sterilization solution's color, temperature readings, or electronic outputs. While these other factors can be important aspects of monitoring the sterilizer itself, they do not offer definitive proof that the sterilization was successful. The color change in the biological indicator strip provides a clear, reliable indicator of the effectiveness of the sterilization process.