A comparison of Etest, M.I.C.Evaluator strips and CLSI broth microdilution for determining B-lactam antimicrobial susceptibility in Haemophilus influenzae

β-Lactamase-positive ampicillin-resistant (BLPAR) strains of Haemophilus influenzae generally have relatively high ampicillin MICs, but remain susceptible to cephalosporins and co-amoxiclav. β-Lactamase-negative ampicillin-resistant (BLNAR) strains, on the other hand, have relatively low ampicillin MICs and low levels of resistance or decreased susceptibility to cephalosporins and co-amoxiclav due to altered PBP3. Some strains have both β-lactamase and altered PBP3: these β-lactamase-positive co-amoxiclav-resistant (BLPACR) strains have relatively high ampicillin MICs due to the β-lactamase and low MICs of other β-lactam antibiotics associated with altered PBP3.