Pittsburgh children are resistant to common antibiotics at a rate that is double the national average, a finding that has researchers wondering if antibiotic resistance is growing just as much elsewhere but going unnoticed.

Dr. Michael Green and other infectious disease specialists at Children's Hospital found that almost 10 percent of the cultures from pediatric strep throat cases were resistant to macrolides, a class of antibiotics that includes such common drugs as erythromycin, clarithromycin and azithromycin.

Studies conducted elsewhere indicate a national macrolide resistance rate of about 5 percent.

Bacteria can develop resistance to antibiotics over time. Medical experts hope to slow the process and extend the useful lifetimes of the drugs by eliminating the inappropriate use of them for viral illnesses.

Failing to take all of a medicine as prescribed also can help breed resistance.

The higher resistance rates in the Pittsburgh area could be the result of overprescription of antibiotics or a pattern of noncompliance with instructions. But the researchers suspect the reason for the difference has more to do with how data on antibiotic resistance are gathered in other communities.

"I do think Pittsburgh is higher, but we're in the process of trying to get our own accurate [national] data," Green said. "There really is a need to do the kind of surveillance that we're trying to do nationally."

A broader collection of cultures gathered during the entire strep throat season, as was done in the Pittsburgh research, could provide more complete information for other communities, Green said.

Dr. Stanford Shulman, chief of infectious disease at Children's Memorial Hospital in Chicago, is leading a national study that puts the macrolide resistance rate of Group A streptococci at about 4 percent.

"We don't view [the Pittsburgh findings] as alarming, but we believe it warrants continued surveillance," he said.

The Children's study, which is the first to examine antibiotic resistance rates during the course of a entire strep season, appears in this month's issue of the journal Antimicrobial Agents and Chemotherapy.

The macrolide antibiotic azithromycin, which is marketed as Z-pak, is popular, particularly in pediatric practices, because of its convenient once-daily, five-day dosing schedule.

Penicillins, the first-line treatment against strep throat, are typically taken several times a day for 10 days.

To look at local resistance rates, the Children's researchers tested 708 cultures of Group A strep that were obtained from pediatric patients between September 2001 and May 2002.

Overall, 68 or 9.6 percent of the strep samples were resistant to macrolides. Much of that was due to a dramatic increase in resistance seen in the last two months of surveillance, Green said.

Until March, the rate was a bit below 4 percent. But of 135 samples tested in April and May, 47 or 35 percent were resistant. Although several resistant varieties were identified, the big jump was mostly due to just one strep strain.

The reason for the end of season surge could be fallout from increased use of antibiotic during the overlapping cold and flu season, encouraging the survival of resistant strep strains, he said.

A multicenter study headed by Children's researchers is under way in which nine sites each submit about 40 samples per month during strep throat season.

A preliminary review indicates there are ups and downs over time.

The rates in Columbus, Ohio, one of the study sites, were as high or higher than Pittsburgh's, Green said.

Chicago's Shulman said his national study likewise found some areas with equally high rates.

"We have a few sites that get as high as 9 or 10 percent," he noted. "It's fair to say Pittsburgh is not the only area in the United States with rates that are above average."