Evaluation of colistin susceptibility among Klebsiella pneumoniae isolates by broth microdilution and colistin agar test.

BACKGROUND

Colistin is a last-resort antibiotic against multidrug-resistant (MDR) Klebsiella pneumoniae. Reliable susceptibility testing is critical but challenging due to colistin's complex pharmacodynamics and testing limitations. This study evaluates the performance of the Colistin Agar Test (CAT) against the gold standard Broth Microdilution (BMD) method.

METHODS

Study was conducted after confirming stability of colistin in agar plate. A total of 150 non-duplicate clinical isolates of K. pneumoniae (103 colistin intermediate, 47 resistant) were tested using both BMD and CAT. Antimicrobial susceptibility testing (AST) of these isolates was performed using VITEK-2. Quality control was ensured using CLSI-recommended strains. Statistical parameters including Categorical Agreement (CA), Essential Agreement (EA), Major Error (ME), Very Major Error (VME), sensitivity, specificity, predictive values, and Cohen's Kappa were calculated using BMD as reference.

RESULTS

MICs ranged from 0.125-16 µg/mL (BMD) and 0.125-8 µg/mL (CAT). CA and EA for CAT were 96 % and 87.3 % respectively. ME and VME were 4.8 % and 2.1 %. Sensitivity, specificity, PPV, and NPV of CAT were 97.9 %, 95.1 %, 90.1 %, and 98.9 % respectively. Cohen's Kappa showed almost perfect agreement (0.91).

CONCLUSION

CAT demonstrated high agreement with BMD and showed promise as a screening tool for colistin susceptibility in K. pneumoniae. While BMD remains the gold standard, CAT offers a feasible alternative for routine use, particularly in resource-limited settings. However, resistant results by CAT should be confirmed with BMD to avoid false resistance reporting and potential therapeutic exclusion of colistin.