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. method: 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 threedimensional 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%. conclusion: implementation of disc approximation test in the laboratories can help in identifying ampc betalactamase harbouring organisms. © 2016,pakistan medical association. all rights reserved.