The combination disk diffusion extended-spectrum β-lactamase (ESBL) confirmation test (CDT) is used for the confirmation of ESBL production in Enterobacteriaceae and usually takes 16-20 h to results. In this study, we searched for the shortest possible incubation time without a reduction in reliability. A total of 125 ESBL screening-positive isolates were subjected to CDT and were molecularly characterised by microarray. Inhibition zones were read every hour over 6-18 h of incubation. Concordance between earlier and 18-h readings was calculated for each hour. Results were validated on 224 isolates during routine clinical practice. For the initial 125 isolates, concordance (Cohen's κ) between the 6-h and 18-h readings was 0.88 [95% confidence interval (CI) 0.78-0.96; P <. 0.001]. The earliest time point for full concordance with the 18-h reading was 10 h. Validation of the 10-h reading for 224 clinical isolates resulted in a concordance of 0.99 (95% CI 0.98-1.0) between the 10-h and 18-h readings. Overall concordance on all 349 isolates was 0.99 (95% CI 0.97-1.0). Reading after 10 h of incubation has an excellent correlation with results after 18 h of incubation. This can significantly reduce the turnaround time for ESBL detection in laboratories with long opening hours or providing a 24/7 service. Consequently, there is a potential for implementing infection control measures up to 8 h earlier.

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Keywords Combination disk diffusion test, ESBL, Rapid reading, Turnaround time
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Journal International Journal of Antimicrobial Agents
van den Bijllaardt, W, Voermans, P.C.M. (Patricia C.M.), Buiting, A.G.M, Mouton, J.W, & Muller, A.F. (2017). Shortening the incubation time for the combination disk diffusion extended-spectrum β-lactamase (ESBL) confirmation test: How far can we go?. International Journal of Antimicrobial Agents. doi:10.1016/j.ijantimicag.2017.03.032