Sterility testing is designed to determine whether a material contains viable (living) microorganisms that are capable of growth. Rather than detecting a specific molecular marker, sterility checks rely on a biological principle: if living microbes are present and placed under supportive conditions, they will multiply and become detectable over time. A “defined incubation period” refers to a pre-established timeframe during which a sample is maintained under conditions that support the growth of a broad range of microorganisms. This period is necessary because microbial contamination may initially be present at very low levels and requires time to proliferate to detectable levels. Sterility testing is intended to detect: Bacteria (both Gram-positive and Gram-negative) Yeasts Molds The test outcome is qualitative rather than quantitative—samples are typically reported as showing evidence of microbial growth or no observable growth at the end of the incubation period. What sterility testing does—and does not—tell you Sterility checks specifically assess viable contamination, meaning organisms that are alive and capable of replication. As a result: A sample that passes sterility testing is considered free of detectable living microorganisms Sterility testing does not detect: Endotoxins or other microbial byproducts Nonviable (dead) microorganisms Viruses or fastidious organisms that cannot grow under the test conditions Because of this, sterility testing is often complemented by other assays (such as endotoxin testing) to provide a more complete contamination profile. Why incubation time matters Microorganisms vary widely in how quickly they grow. Some bacteria replicate rapidly, while others—and most fungi—grow more slowly. The defined incubation period is chosen to: Allow sufficient time for slow-growing organisms to proliferate Reduce false-negative results caused by delayed growth Provide consistent and comparable results across samples Observations are typically made throughout the incubation window to monitor for visible signs of growth, such as turbidity or colony formation.