NEW YORK — Doctors fail to diagnose the flu in the vast majority of young children, depriving them of medicines that could shorten their illness and keep them from spreading it to others, a study suggests.

Flu infections were missed in four out of five preschoolers who were treated for flu symptoms at a doctor's office or emergency room and in about three-quarters of those who were hospitalized.

"Many of the children did not have a test performed and few of the children were sent home with a specific diagnosis of influenza," said Dr. Katherine Poehling, of Vanderbilt University Medical Center in Nashville, Tenn., who led the government-funded study.

If more doctors used a rapid flu test, more cases of flu could be detected and steps taken to prevent its spread, the researchers suggested. About a third of the children would have been candidates for medicines like Tamiflu, which work better to ease symptoms when given early, they said.

Their study is published in today's New England Journal of Medicine. Two of the researchers report receiving grant support and consulting fees from spray flu vaccine maker MedImmune Inc.

Over four years, the researchers did their own lab tests on young children who went to the doctor or were hospitalized with flu-like symptoms such as cough, runny nose and fever. Doctors didn't have the results but could have done their own test. A rapid test takes less than 30 minutes.

The study's findings were presented earlier to a CDC panel which recommended flu shots be expanded this year to children ages 2 to 5 to reduce doctor and ER visits.

Flu shots were already recommended for those 6 months to 23 months.