Sentinel surveillance for Streptococcus pneumoniae antibiotic nonsusceptibility.

Most outpatient illnesses caused by S pneumoniae tend to be treated empirically, without identification of the organism. Accurate, community-specific drug-resistant S pneumoniae surveillance is critical in helping determine initial therapy. Consequently, clinicians in areas with a low percentage of high-level penicillin resistance might gain confidence in treating presumptive outpatient infections with an appropriate antibiotic, thereby reducing the risk for multidrug resistance. Measuring the prevalence of pneumococcal resistance to penicillin and other antibiotics enables epidemiologists and healthcare providers to monitor trends and develop guidelines for optimal empiric therapy. The ideal method for accurate tracking of antimicrobial-resistance patterns in a community is a population-based active surveillance system that collects strains for susceptibility testing in a reference laboratory, such as the Centers for Disease Control and Prevention’s (CDC) ABC study. However, this method is costly, time-consuming, resource intensive, and therefore can only be implemented in limited areas of the country. Sentinel surveillance is a system in which data is collected from a limited sample of hospitals that are representative of a given area of the country. It differs from passive surveillance studies such as the Alexander Network or Protekt, in that a sample is allocated beforehand to a series of hospitals according to their catching areas and the number of isolates reported. South Carolina’s sentinel study published in this issue of the Journal, although not the first one, is an attempt to accurately establish resistance shifts in a relatively small area. Sentinel surveillance has been found to accurately predict penicillin nonsusceptibility in states that had low between-laboratory variation, although it has been shown inadequate when the prevalence of resistance is rather low. S pneumoniae antimicrobial resistance has been found inconsistent across the United States. Nonsusceptibility to penicillin among invasive pneumococcal isolates has been shown to range from 15 to 35% among populations in the CDC’s national surveillance system. Among the passive surveillance studies that include noninvasive isolates, resistance prevalence for the Southeast in 2001 was 18.8% for intermediate and 17.8% for fully resistant (Alexander Network), while an average of years between 2000 to 2003 was 15.7% for intermediate and 28.5% for resistant (Protekt study). The findings from the South Carolina sentinel surveillance study both confirm the fact that resistance prevalence is high in the southeastern part of the country, and that penicillin resistance is not declining in that area, as might had been suggested because of the impact of the pneumococcal vaccine. The “shift” found from high to intermediate penicillin resistance may be a function of the source of the isolates or the population structure. In South Carolina about half of the isolates were from nonsterile sites, and the paucity of otitis media or children’s isolates may be the reason for the apparent shift. Sentinel studies need to pay extra attention at the isolate source and age structure of their sample. Unfortunately the only gold standard available comes from the CDC population study, and the distribution of the source (within invasive) and the age distribution need to be carefully matched. It would be recommended that sentinel studies reweight their sampling scheme accordingly. Sentinel surveillance studies help document emerging pneumococcal resistance and have an important role in trying to accurately understand and predict the future impact of increasing resistance to macrolides and fluoroquinolones.