CHICAGO (Reuters) – Health clinics in drug and grocery stores tend to attract patients without a primary care doctor who might otherwise fall through the cracks, U.S. researchers said on Wednesday.

The clinics, which offer convenient hours for flu shots, treatment of sore throats and other basic services, have expanded rapidly at retail chains like Walgreen Co, CVS Caremark and Wal-Mart Stores.

The study, which appears in the journal Health Affairs, is among the first to look at who visits these clinics, which are expected to grow from the current 1,000 outlets in the United States to 6,000 by 2011.

Only 39 percent of patients who used a retail clinic said they had a primary care doctor, the study showed. That compared with 80 percent of people in a national sample who said they had a regular doctor.

“One of the primary concerns that has been raised is that retail clinics will disrupt primary care relationships,” said Dr. Ateev Mehrotra of University of Pittsburgh School of Medicine and a researcher at the nonprofit RAND Corp.

“What we found is for the majority of people who are going there, there is no primary care relationship to disrupt,” Mehrotra said in a telephone interview. “These are people who are not being served right now.”

Mehrotra’s team analyzed data on more than 1.3 million visits to retail clinics from 2000 to 2007 from eight clinic operators. They compared this with national data on visits to emergency departments and primary care physicians offices.

“They were very much less likely than the general population to report having a primary care provider,” Mehrotra said.

He said younger patients tend to use retail clinics, with people aged 18 to 44 accounting for 43 percent of clinic visits while that age group accounts for just 23 percent of primary care office visits.

Almost 90 percent of the time, people came seeking treatment for upper respiratory, sinus and urinary tract infections, sore throats, bronchitis, immunizations, swimmers ear, conjunctivitis and routine diagnostic tests.

The same conditions accounted for 18 percent of visits to primary care physician offices and 12 percent of emergency department visits.

Mehrotra said the study did not address questions about the quality of care, and said future studies should look at quality and whether people are getting needed preventive and follow-up care.

“This will not quell the rancor, but at least people will have some facts to talk about,” he said.

(Editing by Maggie Fox and Vicki Allen)