Evaluation of phenotypic tests for detection of Amp C beta-lactamases in clinical isolates from a tertiary care hospital of Rawalpindi, Pakistan.

OBJECTIVE: To evaluate sensitivity and specificity of disc approximation test compared to three-dimensional extract test as a phenotypic gold standard test for detection of AmpC beta-lactamase producing Escherichia coli and Klebsiella pneumoniae.

METHODS: The cross-sectional validation study was conducted from November 2014 to April 2015 at Army Medical College, Rawalpindi, Pakistan. Extended spectrum beta lactamases (ESBLs) were isolated from various clinical specimens. Screening for AmpC beta-lactamases was done by using cefoxitin disc (30µg) showing inhibition zone diameter of <18mm. Screen-positive isolates were subjected to disc approximation test (DAT) and three-dimensional extract test(3-DET).SPSS 20 was used for statistical analysis.

RESULTS: A total of 120 ESBL producing Gram negative rods were included in the study. Out of these 120, 82(68.33%) were found to be AmpC beta-lactamase producing on screening with cefoxitin disc. Amongst these 82 isolates, Escherichia coli were identified in 57(69.51%) and Klebsiella pneumoniae in 25 (30.48%). Phenotypic confirmation by disc approximation test (DAT) identified 43(52.43%). AmpC beta-lactamase producing isolates, whereas gold standard 3-DET showed 38(46.34%) of AmpC beta-lactamase producing isolates. Hence, sensitivity of disc approximation test (DAT) was found to be 88%, specificity was 92%, positive predictive value was 92.68%, negative predictive value was 87.80% and diagnostic accuracy was 90.24%.

CONCLUSIONS: Implementation of disc approximation test in the laboratories can help in identifying AmpC beta-lactamase harbouring organisms.