Diagnostic accuracy of colistin agar test medium for the detection of colistin resistant gram-negative rods directly from positive blood cultures

Background: Colistin (polymyxin E) is an important treatment choice against infections with multidrug-resistant (MDR) Gram-negative rods (GNRs). The mechanism of its activity is its bactericidal action, which involves disrupting the integrity of the bacterial outer membrane. Resistance is usually caused by changes in the lipid A portion of lipopolysaccharides, which reduce the affinity of colistin for binding, leading to a loss of antimicrobial activity. Material and Methods: Positive blood culture bottles containing Gram-negative rods after Gram staining were directly inoculated onto CAT medium supplemented with 4 µg/mL colistin. Growth after 18–24 hours of incubation at 37 °C indicated colistin resistance. The outcomes were compared with conventional culture and susceptibility testing (Vitek-2, CLSI 2024). Species that are intrinsically resistant were excluded. Known colistin-resistant and -susceptible bacterial strains were used for quality control. Results: The CAT medium was 85.13% sensitive, 83.81% specific, and 84.66% diagnostically accurate in identifying colistin-resistant Gram-negative rods. With an ROC AUC of 0.845 (95% CI: 0.795–0.895), it demonstrated good overall diagnostic performance, indicating its reliability, precision, and high discriminatory power compared with conventional methods. Conclusion: The CAT medium has good diagnostic performance for the direct detection of colistin-resistant Gram-negative rods in positive blood cultures. Keywords: Colistin, Gram-negative bacteria, Gram-negative rods.