Eight Quick Tips on Inferring Antibiotic Susceptibility Results

Antibiotic susceptibility testing is commonly used to select appropriate antimicrobial therapy or deescalate to a narrower spectrum agent. Because not every antibiotic is tested, clinicians must infer certain types of antibiotic susceptibility based on bacterial patterns of resistance and sensitivities of other antibiotics.
The following are 8 quick tips regarding inferred antibiotic susceptibility, which are based on the EUCAST clinical breakpoints document.
1.Amoxicillin can be inferred from ampicillin. Amoxicillin is generally not tested, and ampicillin is less commonly used as an oral formulation. In other words, if a bacterial isolate is susceptible to ampicillin, it is also susceptible to amoxicillin.
2.Cefoxitin testing is used for most cephalosporin and carbapenem Staphylococcus susceptibilities. Cefoxitin is a surrogate for cephalosporin susceptibilites EXCEPT for the cephalosporins that have no Staph coverage (ceftazidime, cefixime, and ceftibuten). Similarly, all carbapenem susceptibility for Staph is inferred from cefoxitin.
3.Susceptibility to erythromycin infers macrolide susceptibility. If erythromycin is susceptible (typically for Strep and Staph), clarithromycin and azithromycin are also susceptible.
4.Enterococcus faecium penicillin resistance infers resistance to all B-lactams. E. faecium penicillin resistance primarily occurs via an alteration in the B-lactam binding site (PBP – penicillin binding protein) such that no B-lactams are active, including cephalosporins and carbapenems. While the mechanism is similar to methicillin-resistant Staph aureus, the genetic mutation in the PBP is different.
5.Enterococcus susceptibility to ampicillin infers other penicillin-derivative susceptibility. Due to the mechanism of B-lactam resistance with Enterococcus, susceptibility of ampicillin infers susceptibility of broader-spectrum penicillins (such as piperacillin).
6.All B-lactam susceptibility of B-hemolytic Streptococcus (groups A, B, C, G) is inferred from penicillin. Streptococci that are not S. pneumoniae have very poor resistance patterns — over time, they have not been genetically gifted in developing resistance mechanisms. Most are susceptible to penicillin, and penicillin susceptibility infers susceptibility to all B-lactams (including cephalosporins and carbapenems).
7.For Staphylococcus, norfloxacin infers susceptibility to other quinolones. Norfloxacin sensitivity infers that ciprofloxacin, levofloxacin, moxifloxacin, and ofloxacin will also be sensitive. The opposite is not true, however, in that other quinolones may be sensitive despite norfloxacin resistance.
8.For Staphylococcus, tetracycline infers susceptibility to other tetracyclines. Tetracycline sensitivity infers that doxycycline and minocycline will also be sensitive. The opposite is not true, however, in that these tetracyclines (the drug class) may be sensitive despite tetracycline (the drug) resistance.