Ceftriaxone Susceptibility Testing 

Ceftriaxone is a third-generation cephalosporin with broad-spectrum activity against Gram-negative and Gram-positive bacteria, including Enterobacterales, Streptococcus pneumoniae, and Haemophilus influenzae. It is widely used in the treatment of community-acquired pneumonia (CAP), meningitis, urinary tract infections (UTIs), and sepsis.

Testing Methodologies

The susceptibility of ceftriaxone is determined using Clinical and Laboratory Standards Institute (CLSI) and European Committee on Antimicrobial Susceptibility Testing (EUCAST) guidelines, employing the following methodologies:

• Broth Microdilution (BMD) Method:
  • Considered the gold standard for minimum inhibitory concentration (MIC) determination.
  • Conducted in cation-adjusted Mueller-Hinton broth (CAMHB).
  • CLSI-defined MIC breakpoints for Enterobacterales:
    • Susceptible (S): ≤1 μg/mL
    • Intermediate (I): 2 μg/mL
    • Resistant (R): ≥4 μg/mL
• Automated Susceptibility Testing Systems:
  • Platforms such as VITEK 2, BD Phoenix, and Microscan determine ceftriaxone MIC values.
  • May require confirmatory BMD testing for borderline-resistant isolates.

Application and Clinical Relevance

• Treatment of Gram-Negative and Gram-Positive Infections:
  • Ceftriaxone is a key antibiotic for treating severe bacterial infections, including meningitis, pneumonia, gonorrhea, and bloodstream infections.
  • Frequently used in combination with azithromycin for gonococcal infections.
• Detection and Monitoring of Resistance Mechanisms:
  • Resistance mechanisms include ESBL production (CTX-M, TEM, SHV), AmpC β-lactamases, and carbapenemases (KPC, NDM, OXA-48, VIM, IMP).
  • Confirmatory ESBL phenotypic testing (e.g., double-disk synergy test or combination disk tests) is essential.
• Surveillance and Resistance Trends:
  • Continuous susceptibility testing is crucial for monitoring emerging resistance patterns.
  • Helps optimize antimicrobial stewardship programs by preventing empirical overuse.

Ceftriaxone susceptibility testing plays a critical role in diagnostic microbiology and infection control, particularly in community-acquired and MDR infections. Broth microdilution and disk diffusion methods remain primary approaches, while automated systems offer rapid assessment. Due to the increasing prevalence of β-lactamase-producing pathogens, ongoing surveillance and refinement of MIC breakpoints are essential for optimal therapeutic decision-making.