Evaluation of the Accelerate Pheno System for Identification of Acinetobacter Clinical Isolates and Minocycline Susceptibility Testing

Identification of A. baumannii complex.Among the 101 isolates evaluated, 30 were initially identified as non-baumannii species by the clinical microbiology laboratory and were examined further by rpoB sequencing. These were identified as A. pittii (n = 5 isolates), Acinetobacter ursingii (n = 5), A. calcoaceticus (n = 4), A. nosocomialis (n = 3), Acinetobacter radioresistens (n = 3), A. baumannii (n = 3), Acinetobacter lwoffii (n = 2), Acinetobacter johnsonii (n = 2), Acinetobacter baylyi (n = 1), and Acinetobacter bereziniae (n = 1). The species of one isolate, initially identified as Acinetobacter haemolyticus, could not be identified definitively by rpoB sequencing. In summary, there were a total of 74 A. baumannii isolates and 27 non-baumannii Acinetobacter isolates.

Accelerate pheno system-based species identification.Two isolates, one A. baumannii and one A. ursingii, could not be identified due to an insufficient number of cells upon testing; this result was repeated when the isolates were tested a second time. For these two isolates, AXDX yielded an error code, signaling the laboratory would need to test by an alternative method. Of the remaining 73 A. baumannii isolates, all were identified as A. baumannii by AXDX. Among the 11 non-baumannii A. baumannii complex isolates, 5 of 5 A. pittii, 3 of 3 A. nosocomialis, and 2 of 4 A. calcoaceticus isolates resulted in a positive identification by the A. baumannii probe in the Accelerate Pheno system. The 2 A. calcoaceticus isolates that were not identified by AXDX yielded nonidentification upon repeat testing. Among the 15 non-A. baumannii complex isolates, 2 (1 A. radioresistens and 1 A. baylyi) resulted in a positive identification by the A. baumannii probe in the Accelerate Pheno system, while 13 resulted in nonidentification by the A. baumannii target probe, with 11/13 producing an “off panel” result via signal from the universal bacterial fluorescence in situ hybridization (FISH) probe and 2/13 producing no result due to too few cells.

Table 1 summarizes the results. Overall, the sensitivity of identifying A. baumannii complex strains was 97.6% (82 of 84), and the specificity was 86.6% (13 of 15).

Few studies have assessed the performance of FDA-cleared rapid ID systems for testing blood cultures for their ability to correctly identify Acinetobacter spp. In the multicenter trial to evaluate AXDX, 69 of 70 isolates of A. baumannii complex were correctly identified (98.6% sensitive), and 3 specimens negative for A. baumannii among a total of 1,857 tested yielded a false-positive A. baumannii complex result (99.8% specificity) (8). The BioFire BCID panel (bioMérieux, Durham, NC), which was designed to identify A. baumannii alone, was shown in a multicenter evaluation to falsely identify 5 non-baumannii isolates among 23 (21.7%) tested as A. baumannii. These included all 4 A. pittii isolates and the 1 A. junii isolate evaluated in the study. In this study, all 51 A. baumannii isolates were correctly identified (12). The Verigene BC-GN test kit (Luminex, Austin, TX), in contrast, identified 62 isolates of Acinetobacter spp. in clinical trials, of which 2 were false positives (not Acinetobater spp.). Additionally, during this study, one isolate of A. baumannii was not detected by the Verigene BC-GN test kit. Unfortunately, in this study, the species of Acinetobacter were not elucidated (13). As such, laboratorians and physicians should be aware that a result of A. baumannii in blood from these rapid identification systems may in fact be A. baumannii, A. baumannii complex, or in some cases, non-baumannii complex species. Importantly, for A. baumannii complex species which are considered to be of clinical significance, AXDX positively identified all strains of A. baumannii, A. pittii, and A. nosocomialis and two of four isolates of A. calcoaceticus.

Accelerate Pheno system-based AST of minocycline.Minocycline is active against the majority of A. baumannii strains and considered one of the few treatment options available for infections caused by extensively drug-resistant A. baumannii (1). The susceptibility breakpoint currently approved by the CLSI is an MIC of ≤4 μg/ml, with an MIC of 8 μg/ml considered intermediate and ≥16 μg/ml resistant. By the standard BMD, MICs were reliably determined for 100 of 101 tested isolates. One isolate did not grow sufficiently in the BMD panels to accurately determine the MIC. Of the 100 isolates, 85 were susceptible, 14 intermediate, and 1 resistant by BMD. By AXDX, MICs were determined for 98 isolates, where 69 were susceptible, 24 were intermediate, and 5 were resistant (Fig. 1). The isolate that did not grow in BMD panels also did not yield an MIC on AXDX.

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FIG 1

Minocycline MIC (μg/ml) distributions of Acinetobacter isolates by AXDX. S, susceptible; I, intermediate; R, resistant.

The essential agreement (±1 log 2 MIC agreement) of AXDX-based MICs with BMD was 98.0% (96/98). The categorical agreement was 75.5% (74/98); all errors were minor errors, i.e., cases in which one AST method yielded an intermediate interpretation and the other yielded either a susceptible or resistant interpretation. There were no very major errors (i.e., susceptible by AXDX, resistant by BMD) or major errors (resistant by AXDX, susceptible by BMD). The majority of minor errors (20/24) involved AXDX reporting an MIC one dilution higher than the modal BMD MIC (16 μg/ml versus 8 μg/ml in 5 isolates, 8 μg/ml versus 4 μg/ml in 15 isolates). As seen in Fig. 1, the relatively high rate of minor errors was caused by the MIC distribution for minocycline, which bisected the susceptibility breakpoint. Nonetheless, the bimodal distribution of the MICs and the excellent essential agreement suggests that AXDX will rapidly identify the majority of Acinetobacter isolates with low MICs, for which minocycline may be considered a treatment option. A previous study demonstrated similar performance for minocycline MIC determination for Acinetobacter spp. by AXDX. Among 224 isolates evaluated, essential agreement was 97.4% and categorical agreement was 92.1%, and 17 minor errors and 1 major error were observed (8). Previous studies have demonstrated that other commercial AST instruments have similarly yielded a high rate of minor errors when testing A. baumannii against minocycline. In one study, which evaluated the performance of Spanish laboratories to accurately determine A. baumannii AST results, minor errors of 88.6% were observed across all systems evaluated (14). A second study demonstrated minor errors of 15% to 40% across different disk diffusion, Sensititre, and Etest methods (15).