If the culture shows a ceftriaxone minimum inhibitory concentration of 0.25 µg/mL, can we assume the isolate is also susceptible to cephalexin?

Answer

No—do not assume cephalexin is susceptible based on a ceftriaxone MIC of 0.25 µg/mL; susceptibility to first‑generation oral cephalosporins does not reliably track with third‑generation cephalosporins and requires its own testing. 1

• Rationale and algorithm:

  • Third-generation cephalosporin activity (ceftriaxone/cefotaxime) does not predict activity of second/first-generation cephalosporins (e.g., cefuroxime, cephalexin). Guideline statements note that cephalosporin activity cannot be inferred across generations; specifically, “the activity of this or other cephalosporins cannot be predicted by results of in vitro susceptibility tests with cefotaxime or ceftriaxone.” 1
  • Therefore:
    • If you have ceftriaxone MIC ≤0.25 µg/mL but need an oral first‑generation cephalosporin:
      • Do not substitute cephalexin without organism‑specific susceptibility for cephalexin (or its surrogate per lab methods). 1
      • If cephalexin testing is unavailable, choose an oral agent with known susceptibility for the isolate, or continue ceftriaxone if parenteral therapy is acceptable to protect morbidity/mortality/QOL.
  • Pitfalls/caveats:
    • Surrogate relationships exist for some β‑lactams in specific organisms, but they are agent‑ and pathogen‑specific and do not extend from ceftriaxone to cephalexin; avoid cross‑generation inference. 1
    • FDA cephalexin labeling uses cephalothin surrogates for testing and sets its own MIC breakpoints (S ≤8 µg/mL), underscoring the need for direct or validated surrogate testing for cephalexin rather than extrapolating from ceftriaxone. 2

• Practical takeaway:

  • Request cephalexin (or validated surrogate) susceptibility; if not available and an oral step‑down is needed, select an oral agent with confirmed susceptibility rather than assuming cephalexin coverage from a ceftriaxone MIC. 1, 2