Biological indicators (BIs) are used globally to monitor the efficacy of sterilization processes for medical products (1–5).
BIs contain high numbers—generally 104 to 106—of bacterial endo-spores that are highly resistant to the sterilization process for which they are designed. The spores are
placed on or in a carrier material such as paper and packaged to protect the BI before as well as after sterilization and
before culturing. BIs are useful for validation and routine monitoring of ethylene oxide, steam, dry heat, low temperature
steam formaldehyde, vapor hydrogen peroxide, and related plasma sterilization systems.
In the United States, BIs sold for use in healthcare facilities are considered medical devices and are regulated by the US
Food and Drug Administration good manufacturing practices (GMPs) as promulgated in the Quality System Regulation of 1996 (6).
BIs sold to manufacturers of sterile medical products are considered manufacturing components. Regardless of the intended
use, BIs must be manufactured under carefully controlled conditions to assure quality, reproducibility, and predictable performance
Reducing biological indicator incubation time and the existing FDA protocol
How much incubation time is required to provide sufficient information to allow the user to make a correct decision regarding
the acceptability of the sterilization process? In the mid-1980s, the FDA Center for Device and Radiological Health (CDRH)
issued a protocol (hereafter termed the RIT [reduced incubation time] protocol) for BI manufacturers and users to follow to
reduce the incubation time for BIs from the normal seven days to a shorter time based on the results of testing defined in
the protocol (8). The RIT protocol requires manufacturers to expose 100 BIs from each of three lots to separate sterilization
processes; the results of the exposure to the sterilization processes must result in 30 to 80 nonsterile BIs from each lot
after incubation at the appropriate conditions for seven days. Achieving this outcome requires carefully-controlled reduced
lethality (fractional) sterilization runs for which highly specialized sterilization equipment is often used.
To determine a reduced incubation time for a BI/sterilization process according to the RIT protocol, the number of nonsterile
BIs for each of the three lots is recorded after periodic reviews up to seven days of incubation; the number of nonsterile
BIs at seven days is considered "100%." The number of nonsterile BIs at incubation times less than seven days is compared
with the respective seven-day value, and a percentage is calculated; the qualified reduced incubation time is the longest
time, for one or more of the three lots, to first exhibit a number of nonsterile BIs that is ≥ 97.0% of that observed after
seven days of incubation. A typical example appears in Table I with the BIs being scored daily.
Table I : Example of data necessary to meet the FDA CDRH criteria of determining a reduced incubation time of less than 7
As required, for each of the three lots tested, the number of nonsterile BIs after seven days of incubation was greater or
equal to 30 and less than or equal to 80. For lots 2 and 3, the 97.0% criterion was met after one day of incubation; for lot
1, however, the 97.0% criterion was met after two days of incubation. Based upon these results, a reduction of incubation
time from seven to two days is acceptable.