Comparative evaluation of Vitek®2 and broth microdilution method for colistin susceptibility testing of Gram-negative isolates from intensive care unit in a tertiary care hospital.

INTRODUCTION: Colistin is the last resort treatment against resistant Gram-negative bacteria, necessitating reliable and rapid means for sensitivity testing of colistin. Automated systems like VITEK®2 are adopted to determine the minimum inhibitory concentration (MIC) due to easy usage. Broth microdilution (BMD) for colistin MIC was suggested by EUCAST and CLSI.

OBJECTIVE: To compare and evaluate colistin MIC by BMD and VITEK®2 against Gram-negative organisms from the ICU in a tertiary care hospital.

METHOD: Clinically significant organisms isolated from ICU patients were included. MIC was determined using BMD and VITEK®2. Very major error (VME), major error (ME), essential agreement (EA), categorical agreement (CA), positive predictive value (PPV), negative predictive value (NPV), sensitivity, and specificity were analysed.

RESULT: 533 isolates were obtained from blood (435,81.60%), respiratory samples (57,10.70%), pus and exudates (20,3.80%), urine (18,3.40%), and CSF (3,0.60%). The Enterobacterales were K. pneumoniae (185,34.70%) E. coli (73,13.70%) and E. cloacae (26,4.90%) while non-fermenters were A. baumannii (209,39.20%) and P. aeruginosa (40,7.50%). The VITEK®2 sensitivity was >99%; specificity ranged from 14.28-52.94%. PPV was 93.81% while NPV was 93.75%. VME ranged from 47-100% between isolates. ME was up to 20%. The highest VME was obtained in E. coli (100%). The total EA and CA observed were 68.5% and 99.79% respectively.

CONCLUSION: Automated system VITEK®2 failed to detect the resistance in 32 (60%) isolates. The obtained VME and ME values were >3%, which is unacceptable as per the standard guidelines. EA of ≥90% wasn't obtained. Sensitivity for VITEK®2 was >99%, but had low specificity (14.28%). Hence, VITEK®2 is not reliable for colistin susceptibility testing.