The primary platform used for pyrazinamide (PZA) susceptibility testing of Mycobacterium tuberculosis is the MGIT culture system (Becton Dickinson). Since false-resistant results have been associated with the use of this system, we conducted a multicenter evaluation to determine the effect of using a reduced cell density inoculum on the rate of false resistance. Two reduced inoculum densities were compared with that prescribed by the manufacturer (designated as “BD” method). The reduced inoculum methods (designated as “A” and “C”) were identical to the manufacturer's protocol in all aspects with the exception of the cell density of the inoculum. Twenty genetically and phenotypically characterized M. tuberculosis isolates were tested in duplicate by ten independent laboratories using the three inoculum methods. False-resistant results declined from 21.1% using the standard “BD” method to 5.7% using the intermediate (“A”) inoculum and further declined to 2.8% using the most dilute (“C”) inoculum method. The percentages of the resistant results that were false-resistant declined from 55.2% for the “BD” test to 28.8% and 16.0% for the “A” and “C” tests, respectively. These results represent compelling evidence that the occurrence of false-resistant MGIT PZA susceptibility test results can be mitigated through the use of reduced inoculum densities.

Background Many U.S. clinical laboratories lack capacity to definitively identify fungi or perform antifungal susceptibility testing (AFST). To expand testing access, CDC’s Antibiotic Resistance Laboratory Network (AR Lab Network) provides Candida species identification and AFST to U.S. facilities for clinical and public health purposes. We describe the first three years of Candida AR Lab Network resistance data. Methods Isolates from any body site with species identification and AFST performed July 2016–June 2019 are included. Submissions were based on clinical and public health need. Patients may have multiple isolates. The 7 AR Lab Network regional laboratories used matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) or DNA sequencing for species identification. AFST was performed using broth microdilution for azoles and echinocandins (anidulafungin and micafungin) and Etest for amphotericin B. This analysis focuses on non-albicans Candida species with Clinical and Laboratory Standards Institute M60 minimum inhibitory concentration breakpoints and C. auris, which has CDC-proposed tentative breakpoints. Results Participation increased from healthcare facilities from 2 states submitting in 2016 to 35 states in 2019. Species identification was performed on 5,234 non-albicans isolates. AFST was performed on 4,222 (81%) isolates, including 2,395 C. glabrata, 815 C. auris, 267 C. parapsilosis, 125 C. tropicalis, 35 C. guilliermondii, and 32 C. krusei. Of isolates with AFST and body site indicated, 22% (900/4,102) were from blood. We found 85% of C. auris, 8% of C. glabrata, and 5% of C. parapsilosis isolates were resistant to azoles; 33% of C. auris isolates were resistant to amphotericin B; and 2% of C. glabrata, 1% of C. auris, and 1% of C. parapsilosis isolates were resistant to echinocandins. Although intrinsically resistant to fluconazole, C. krusei isolates were not resistant to voriconazole. Multidrug resistance was present in 32% of C. auris and 1% of C. glabrata isolates. Conclusion AR Lab Network has expanded access to rapid Candida testing, including AFST, and provides real-time surveillance. Results can be used to detect emerging species and resistance and guide public health action and healthcare practices. Disclosures All Authors: No reported disclosures