Stability of hybridization activity of Coccidioides immitis in live and heat-killed frozen cultures tested by AccuProbe Coccidioides immitis culture identification test.

Frozen hyphal suspensions of Coccidioides immitis were evaluated for suitability as positive control cultures in the AccuProbe C. immitis culture identification test. The genetic probe hybridization activity of heat-killed and viable frozen cultures, stored at -20 and -70 degrees C and tested over a 10-month period, was compared to that of a freshly grown culture, and the results were evaluated based upon the manufacturer's established positive and negative photometric light unit (PLU) cutoff values. All C. immitis suspensions produced positive hybridization values well above the positive and negative cutoff values, and no significant decrease in hybridization activity was observed with the frozen cultures after 10 months of storage. The frozen, heat-killed suspensions produced PLU values with less variability (coefficient of variation, 8% over 10 months than the fresh or frozen viable cultures and were deemed the most stable and sale form of positive control material to use.