In CLSI M100 2010, new revised interpretive criteria for cephalosporins (cefazolin, cefotaxime, ceftazidime, and ceftriaxone) and aztreonam susceptibility were established.  Following evaluation of pharmacokinetics-pharmacodynamics (PK-PD) properties and limited clinical data, and on the basis of dosage regimens the breakpoints are revised.

Cephalosporin	CLSI  M100 S-19 (2009) Sensitive	CLSI M100 S-20 (2010) Sensitive
Cefotaxime	≥23	≥26
Ceftriaxone	≥21	≥23
Ceftazidime	≥18	≥21
CSeftizoxime	≥20	≥21
Aztreonam	≥22	≥21

 When using the new interpretive criteria, routine ESBL testing is no longer necessary before reporting results (e.g. it is no longer necessary to edit results for cephalosporin, aztreonam, or penicillin from susceptible to resistant. However until laboratories implement the new interpretive criteria, ESBL testing should be routinely performed as described previously. ESBL testing may still be useful for epidemiological and infection control purposes.