Evaluation of VITEK 2 and three protocols for direct susceptibility testing on Gram-negative bacilli from positive...

Bloodstream infections are associated with high mortality, while early and appropriate antibiotic therapy can significantly reduce fatal outcomes. Direct antibiotic susceptibility testing (AST) from positive blood cultures has the potential to enable earlier administration of antimicrobials compared with routine AST. The aim of the study was to evaluate the performance of three direct AST protocols for Gram-negative bacilli using positive blood culture fluids.

AST performed on subcultured isolates with VITEK 2 was defined as protocol 1 and used as the reference method. From positive blood culture bottles signaling Gram-negative bacilli, VITEK 2 (protocol 2), modified disk diffusion (protocol 3) and rapid disk diffusion (protocol 4) recommended by EUCAST. The results of protocols 2, 3 and 4 were compared with protocol 1.

The categorical agreements (CA) of protocols 2, 3, and 4 compared with protocol 1 for all strains were 95.9%, 80.7%, and 80.6%, respectively. Very major discrepancy (VMD) rates were 0.5%, 9%, and 1%, major discrepancy (MD) rates were 2.5%, 2.9%, and 6.9%, while minor discrepancy rates were 2.4%, 3.7%, and 3.5%, respectively.

VITEK 2 AST performed directly from blood culture bottles demonstrated the highest categorical agreement and the lowest discrepancy rates across all isolates and antibiotics. This approach is practical in laboratories equipped with mass spectrometry for direct identification and offers a cost-effective option for widespread use in clinical microbiology settings.