Disk diffusion has been the mainstay for antimicrobial susceptibility testing (AST) in most clinical microbiological laboratories since Bauer, Kirby et al. first described this technique in the 1960s . During the past decade automated AST microdilution systems based on determination or extrapolation of minimal inhibitory concentrations have been introduced in the diagnostic market, e.g. systems like the Vitek 2 (BioMérieux), Phoenix (Becton-Dickinson), or Microscan (Siemens Healthcare Diagnostics).
The main advantages of commercial microdilution systems including automated reading and rapidity are compromised by the still lower sensitivities in the detection of important resistance mechanisms compared with the disk diffusion method, e.g. inducible macrolide-lincosamide-streptogramin resistance (MLSB-Type), extended spectrum beta-lactamases (ESBL), and AmpC beta-lactamases [2–5]. In addition, some combinations of resistance mechanisms are not reliably detected by automated microdilution systems e.g. ESBL in Enterobacteriaceae isolates co-producing chromosomally- or plasmid-encoded AmpC beta-lactamases . The sensitivity for detection of resistance mechanisms largely depends on the composition of the antibiotic drug panel used in the automated microdilution systems, which cannot be changed or modified by the user [2, 7].
The disk diffusion method readily permits detection of inducible phenotypes and most combinations of resistance mechanisms including ESBL and AmpC co-production. The antibiotic panel composition is flexible and enables the clinical laboratory to readily adjust the composition of panels to its needs [8, 9]. Disadvantages of the disk diffusion method are its labour cost due to manual measurements and manual data documentation, and the investigator dependence and variation of results .
During the past decade several systems have been developed to automate disk diffusion readings. Systems like Sirscan (i2a, Montpellier, France), OSIRIS and ADAGIO (both BIO-RAD, Marne La Coquotte, France), Oxoid Aura (Oxoid Ltd., Basingstoke, UK), or BIOMIC (Giles Scientific Inc., Santa Barbara, California, USA) are able to automatically read inhibition zone diameters and incorporate expert systems for AST interpretation. These systems allow fully automated (Sirscan) or semi-automated reading (ADAGIO, Aura, BIOMIC), documentation and data interpretation using expert systems. The few studies available investigating the performance of automated zone reading systems indicate a high agreement with standard manual calliper (correlation coefficients ranging from 0.91 to 0.96) resulting in only few susceptibility categorisation errors [10–15]. However, some systems are no longer available (OSIRIS, Oxoid Aura), or have reported practical problems for routine use (BIOMIC) .
No studies are available investigating, if and to which extent fully automated zone reading is able to facilitate standardisation of inhibition zone diameter measurements. High reproducibility and low variation of results become even more important in the light of the new CLSI and EUCAST AST guidelines that contain smaller intermediate susceptibility categories or, in case of EUCAST, have even partially abandoned the use of the intermediate category. Directly adjacent susceptible and resistant categories lead to a higher frequency of major and very major errors (i.e. susceptible to resistant, resistant to susceptible) simply due to technical reasons, i.e. variation of individual measurements [17–19].
This study aimed at comparing the fully automated Sirscan with standard calliper measurements assessing: i) The agreement of inhibition zone diameter results (comparability), ii) The frequency of discrepancies in susceptibility categorisation (accuracy), and iii) Variation of repeat diameter measurements (reproducibility and precision).