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From January, 1992, to January, 1994, penicillin-resistant (minimal inhibition concentration (MIC) > 0.06 μg/ml) Streptococcus pneumoniae (PRSP) isolates accounted for 48 (17%) of 283 isolates from acute otitis media (AOM) or recurrent AOM in 246 ambulatory patients in rural Kentucky. By broth microdilution, relatively penicillin-resistant (MIC ≥ 0.06 to 1.0 μg/ml) and highly penicillin-resistant (MIC ≥ 2.0 μg/ml) strains were detected in 25 (16%) and 23 (15%), respectively, of 157 pneumococcal middle ear isolates. Using 1994 National Committee for Clinical Laboratory Standards breakpoints for pneumococci (unavailable for oral cephalosporins except cefuroxime), highly PRSP strains were almost uniformly susceptible to clindamycin and vancomycin. In contrast highly PRSP strains were resistant to most oral antimicrobials customarily used for AOM with one-third of strains highly resistant (MIC ≥ 2.0 μg/ml) to ceftriaxone. Serotypes 6B, 19F and 23F accounted for 95% of highly PRSP strains and serotype 9V for 48% of relatively PRSP strains. By multivariate analysis, otitis-prone condition (P = 0.0008) and number of antibiotic courses before day of culture (P < 0.0001) were independently predictive of PRSP. Highly PRSP isolates were more commonly isolated from patients recently treated within 3 days (30%) vs. those who completed therapy more than 3 days earlier (2%) (P < 0.0001). Rates of successful treatment outcome for patients with PRSP strains were: 76% (13 of 17) for amoxicillin/clavulanate, 63% (5 of 8) for amoxicillin (60 to 80 mg/kg/day) and 70% (7 of 10) for oral clindamycin for 10 to 14 days of therapy; and 78% (14 of 18) for parenteral ceftriaxone (25 or 50 mg/kg/day) once daily for 3 to 8 days. This is the first large series of prospectively identified AOM isolates in the United States to document the emergence of PRSP as a frequent pathogen of this common pediatric disease.