Cefdinir Coverage on Proteus mirabilis
Cefdinir has documented in vitro activity against Proteus mirabilis and is FDA-approved for treating infections caused by this organism, though species-specific resistance patterns exist that may limit its clinical utility in certain Proteus infections.
FDA-Approved Coverage
Cefdinir exhibits in vitro MICs of ≤1 mcg/mL against ≥90% of Proteus mirabilis strains, establishing it as an active agent against this pathogen 1.
The FDA label explicitly lists Proteus mirabilis among organisms against which cefdinir demonstrates clinical significance, though notes that "the safety and effectiveness of cefdinir in treating clinical infections due to these microorganisms have not been established in adequate and well-controlled clinical trials" 1.
Clinical Evidence for Urinary Tract Infections
In a large multicenter trial of 661 patients with uncomplicated UTI, cefdinir 100 mg BID for 5 days demonstrated equivalent clinical cure and microbiologic response rates to cefaclor in treating infections where Proteus mirabilis was the second most common pathogen (20 patients) after E. coli 2.
Significantly fewer Proteus mirabilis isolates were resistant to cefdinir (3.7%) compared to cefaclor (6.7%, P<0.003) in admission cultures 2.
Critical Species-Specific Limitation
Proteus penneri (a closely related species) demonstrates natural resistance to cefdinir, with this organism showing intrinsic resistance to several cephalosporins including cefdinir, cefaclor, cefazoline, loracarbef, cefuroxime, and cefotiam 3.
In contrast, Proteus mirabilis is naturally sensitive to all beta-lactams except penicillin G and oxacillin, making species identification clinically important 3.
Comparative Activity
Cefdinir provides good coverage against common respiratory and urinary pathogens, with stability against plasmid-mediated beta-lactamases that commonly affect H. influenzae and M. catarrhalis 4.
For serious Proteus mirabilis infections requiring parenteral therapy, other agents may be preferred based on comparative studies showing superior activity of agents like cefotiam over older cephalosporins 5.
Clinical Pitfalls to Avoid
Do not assume all Proteus species respond equally to cefdinir—Proteus penneri requires alternative therapy due to intrinsic resistance 3.
Cefdinir is primarily renally eliminated and requires dose adjustment when creatinine clearance is <30 mL/min (Cmax increases 2-fold, half-life 5-fold, AUC 6-fold) 1.
For complicated urinary tract infections or pyelonephritis with Proteus mirabilis, consider that cefdinir's clinical trial data primarily supports uncomplicated UTI treatment 2.