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Biological Indicator Resistance Testing

By Courtney Lang, Study Director, EO Department, BS, RM(NRCM)

Most medical device manufacturers have defined acceptance criteria when it comes to the resistance of their biological indicators (BIs). The resistance of a BI can be determined by verification of the D-value or by verification of the survival/kill time response characteristics.

The D-Value is the time, in minutes, required to kill one log or 90 percent of the population of indicator organisms under stated exposure conditions.

The D-value can be calculated using either the survivor curve method or the fraction negative method. The survivor-curve method establishes a D-value based on the number of surviving organisms after each exposure.  This method requires a minimum of five different exposure times; one exposure at time zero, where the samples are not subjected to the sterilant, one exposure that reduces the population to 0.01% of the original population, and a minimum of three exposure times covering the intervals between those mentioned above.

The fraction negative method indirectly calculates a D-value based on the fractional growth of the BIs when immersed in growth medium. There are two procedures used in this method: the Holcomb-Spearman-Karber (HSK) procedure and the Stumbo-Murphy-Cochran (SMC) procedure. The HSK procedure requires a minimum of five different exposure times which must result in one set of samples demonstrating all growth, two sets of samples demonstrating fractional growth, and two sets of samples demonstrating no growth. The SMC procedure requires a minimum of three different exposure times, each yielding fractional growth of the organism.

The survival/kill time testing is performed using the survival and kill times provided on the Certificate of Analysis. Following the survival time exposures, all BIs are expected to demonstrate growth during BI sterility testing. Following the kill time exposures, all BIs are expected to demonstrate no growth during BI sterility testing. Each test is performed in duplicate to demonstrate reproducibility of the results.

The testing conducted at Nelson Laboratories follows either ISO 11138 Part 1 or USP <55>.