Comparison of Rapid Resa-Imipenem NP Test with Conventional Disc Diffusion Method for Detection of Carbapenem Susceptibility in Acinetobacter Baumannii.

Objective: To compare rapid Resa-Imipenem NP test with conventional disc-diffusion method for carbapenem susceptibility testing in Acinetobacter baumannii.

Study design: Cross-sectional analytical study. Place and Duration of the Study: Department of Microbiology, The Armed Forces Institute of Pathology / National University of Medical Sciences, Rawalpindi, Pakistan, from December 2022 to November 2023.

Methodology: A rapid resazurin-based test to assess imipenem susceptibility was performed on the growth of Acinetobacter baumannii in 202 clinical samples from different clinical settings, and the results were compared with the conventional disc-diffusion testing method. Bacterial suspensions of the isolates were added to imipenem and resazurin (a viability colourant) containing Mueller-Hinton broth on a 96-well polystyrene microtitre plate and incubated for 2.5 hours. A colour change from blue to pink indicated resistance. Sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and diagnostic accuracy were calculated. Cohen's kappa assessed agreement between tests.

Results: Out of 202 isolates, 157 were imipenem resistant and 45 were imipenem sensitive. The sensitivity, specificity, PPV, and NPV were 99.4%, 100%, 100%, and 97.8%, respectively. Diagnostic accuracy was 99.5% with one very major error (VME, 0.5%). Cohen's Kappa showed excellent agreement between the two tests (<0.001).

Conclusion: The rapid Resa-Imipenem NP test is simple, reliable, and significantly faster, providing accurate results within 2.5 hours compared to 18-24 hours by conventional disc-diffusion.

Key words: Acinetobacter baumannii, Imipenem, Rapid resazurin test, Microbial sensitivity tests, Disk diffusion method, Carba-penems.