Background: Direct antimicrobial susceptibility testing (dAST) remains controversial despite widespread use. This study examined the veracity of dAST from urine samples containing clinically significant Enterobacterales. It investigated the use of the AI capability of the APAS® Independence (Clever Culture Systems, Switzerland) in providing a solution in lieu of manual plate-in-hand reading, and investigated the DxM 6100 Autoplak (Beckman Coulter) as a means of automated inoculation.
Method: 192 urines with suspected uncomplicated UTI were enrolled in the AI capability study. Samples were selected on the basis of having a leucocyte count ≥ 50 x10^6/L. Mueller Hinton plates (Edwards Group, 04091) were inoculated using a sterile cotton swab dipped into the urine sample and a RETRO C80 inoculator. A panel of 12 antimicrobial discs were applied, and the plates incubated according to CLSI guidelines. Following incubation, the plates were read by APAS (dAST-APAS), and plate-in-hand methods using electronic calipers (d-AST manual). Significant isolates were tested in the Vitek 2 (Biomerieux), providing an MIC and S/R interpretation, and by disc diffusion using the same disc panel used for dAST. Discs were read by both APAS Independence (iAST-APAS) and plate-in-hand methods (iAST-manual). The Vitek 2 result was used as the reference result for comparison against the test methods. Results were documented in a blinded form and categorical agreement (CA), as well as major error (ME) and very major error (VME), were determined.
A further series of urine samples were enrolled to compare manual inoculations and automated inoculation using the DxM 6100 Autoplak. Zone sizes from both methods were read in the APAS and compared.
Results: There was a high degree of categorical agreement between dAST-APAS and dAST-manual (95.4%, 908/948), and between the iAST-APAS and iAST-manual (97.2% 867/892). When comparing dAST-manual with Vitek, the CA was 93.1% (824/885). When comparing dAST-APAS with Vitek the CA was 91.3% (843/922). dAST inoculated using the Autoplak versus manual methods demonstrated no significant difference in results when measured by the APAS.
Conclusions: The DxM 1600 Autoplak and the APAS® Independence offer a combined modular solution to dAST of urines, allowing earlier provision of results, comparable to those obtained by isolate testing