Haemophilus influenzae blood-stream infection and third-generation cephalosporin susceptibility testing: a...

In this comparative case study, we discuss clinically relevant discrepancies of antimicrobial susceptibility testing (AST) interpretation for ceftriaxone against a non-typable, beta-lactamase negative, ampicillin-resistant (BLNAR) Haemophilus influenzae isolated from a blood culture.

A 74-year-old man presented with a 3 day illness characterized by shortness of breath and dry cough, and was noted to be febrile and hypoxic on admission. A blood culture bottle flagged positive with Gram-negative coccobacilli, later identified as Haemophilus influenzae with the patient commenced on ceftriaxone. The isolate was beta-lactamase negative and antibiotic susceptibility testing (AST) using disc diffusion revealed the isolate resistant to ceftriaxone and ampicillin by EUCAST methodology, with the patient subsequently changed to amoxicillin/clavulanate. Further AST using the CLSI methodology in parallel demonstrated discrepant results between the two susceptibility methods. The patient recovered without complications.

This discrepancy could lead to inconsistent reporting of susceptibilities between laboratories, and consequently antibiotic prescribing, especially for invasive isolates. As more laboratories adopt EUCAST methodologies for AST interpretation in Australia and globally, it is important for clinicians to consider the clinical implications of these methodological discrepancies.