Background: Factors associated to increased antimicrobial resistance among bacterial pathogens have been widely discussed and need to be evaluated. In Finland resistance to erythromycin in group A streptococci has become an important problem among outpatients. The aim of this study was to investigate whether occurrence of erythromycin resistance among group A streptococci isolated from noninvasive infections correlates with the patients' age and sex.
Methods: Group A streptococci isolated from 10 162 patients were tested for erythromycin resistance in 21 regional microbiologic laboratories located throughout Finland. The age of every patient and the sex of 8121 (80%) patients were known. The statistical significance of the association between the patients' age or sex and the occurrence of erythromycin resistance in group A streptococci, isolated from throat swab samples (8568 isolates) or pus samples (1594 isolates), was measured by logistic regression analysis.
Results: When erythromycin resistance of the isolates was regressed with the patients' age and sex, the age of the patient was a clearly significant predictor for the throat isolates (beta coefficient = -0.0114, SD 0.0029, observed value of t test statistic = -3.89, P = 0.0001) but not for the pus isolates. The odds ratio for age was 0.99 with a 95% confidence interval of 0.98 to 0.99. Thus the expected risk of erythromycin resistance on a group A streptococcal throat isolate decreased with increasing age by 1% per year. No significant association between the patients' sex and the occurrence of erythromycin resistance was found.
Conclusions: Significant differences may exist between different age groups in the frequency of antibiotic-resistant isolates among outpatients, perhaps caused by differences in antibiotic prescribing. Thus overall resistance levels do not necessarily represent all age groups, especially children.