There are currently EUCAST clinical breakpoints for nitrofurantoin and mecillinam, however these breakpoints can only be used to interpret tests performed on E. coli (nitrofurantoin) or E. coli, Citrobacter species, Klebsiella species, Raoultella species, Enterobacter species and P. mirabilis (mecillinam) from uncomplicated UTI.
Some UK laboratories extrapolate the nitrofurantoin and mecillinam clinical breakpoints to interpret all Enterobacterales, however, this is not recommended.
EUCAST have recently reviewed these breakpoints and have expanded the mecillinam breakpoints to include Citrobacter spp., Raoultella spp., and Enterobacter spp.. In previous years, the clinical breakpoint was for interpretation of E. coli, Klebsiella spp. and P. mirabilis only. The change followed a BSAC/EUCAST collaborative study which investigated whether the current breakpoints could distinguish between isolates with and without resistance mechanisms in species other than those listed. After reviewing current clinical evidence EUCAST decided that the breakpoints could also be used for Citrobacter spp., Enterobacter spp. and Raoultella spp.
After a similar review, the nitrofurantoin clinical breakpoints could not be extended to include any other Enterobacterales species.
Nitrofurantoin susceptibility in Enterobacterales varies, with E. coli particularly susceptible and Proteeae (e.g. Proteus, Morganella and Providencia spp.), some Klebsiella species and Pseudomonas species carrying intrinsic resistance. Infections with Enterobacterales other than E. coli or staphylococci other than S. saprophyticus are commonly associated with upper urinary tract or complicated infections. EUCAST clinical breakpoints are based upon pharmacokinetic data, microbiological data and clinical experience. Most nitrofurantoin and mecillinam clinical data are derived from acute UTI studies, mostly comprising of E. coli (Hüttner et al., 2015). For these reasons, EUCAST considers Enterobacterales other than E. coli, P. aeruginosa, Acinetobacter species, staphylococci other than S. saprophyticus, streptococci other than group B, H. influenzae, Moraxella catarrhalis, Neisseria species and anaerobes as poor targets for nitrofurantoin or inappropriate for nitrofurantoin therapy. For mecillinam, P. aeruginosa, Acinetobacter species, Staphylococcus species, Enterococcus species, streptococci, H. influenzae, M. catarrhalis, Neisseria species and anaerobes were considered poor targets or inappropriate for mecillinam therapy.
BSAC CURRENTLY RECOMMENDS: DO NOT REPORT nitrofurantoin or mecillinam results for those species without breakpoints.