WASHINGTON, August 6, 2009 (AFP) - Commercially available tests for influenza often fail to detect the swine flu virus, which could mean some patients will go without treatment, a US report issued Thursday said.

So-called rapid influenza diagnostic tests (RIDTs), which can return a result within 15 minutes, are especially prone to missing A(H1N1) virus infections when the amount of virus in a specimen is low, the Centers for Disease Control and Prevention (CDC) said.

"Although the RIDTs were capable of detecting novel A(H1N1) virus from respiratory specimens containing high levels of virus, the overall sensitivity was low among all specimens tested and declined substantially as virus levels decreased," the CDC said in its weekly morbidity and mortality report.

"A negative result does not rule out infection with novel influenza A(H1N1) virus," it warned.

Researchers tested three RIDTs -- Inverness Medical BinaxNOW Influenza A&B; Becton Dickinson Directigen EZ Flu A+B; and Quidel QuickVue Influenza A+B -- on 65 respiratory specimens collected in April and May, when the outbreak of a new strain of swine flu was first reported and began its rapid spread.

The specimens used to evaluate the RIDTs had all previously tested positive for either swine flu or seasonal flu.

Of nine specimens that had high amounts of the A(H1N1) virus, one of the RIDTs returned nine positive tests for swine flu and the other two rapid tests returned eight positives.

In 36 specimens that had returned earlier positive tests for swine flu, but had low viral material, the sensitivity of the RIDTs "declined substantially."

That means that "many influenza virus infections will be missed," the report said.

"A negative RIDT result should not be interpreted as indicating the absence of infection," the report said, advising patients to rely on their doctor's diagnosis when seeking care for suspected swine flu.

"Decisions regarding treatment and further testing among patients with negative results from RIDT testing should be based upon clinician suspicion, underlying medical conditions, severity of illness, and risk for complications in those persons suspected of having novel H1N1 virus infection," the CDC said.

BinaxNOW returned positive tests for the novel H1N1 virus 40 percent of the time, Directigen EZ Flu A+B had a 49 percent sensitivity rate, and QuickVue Influenza a 69 percent sensitivity rate.

In June, the CDC said it believed one million people in the United States had already had swine flu, or around 40 times more than the 28,000 cases confirmed by US health authorities at the time.

The highest rates of illness due to swine flu have so far been in people under the age of 25. The median age of people who have been hospitalized in the United States after being infected with the virus is 19, and the median age of those who have died is 37.

The amount of virus present in a specimen is highest in the first three days of illness, and the age of the patient can affect virus levels, with children generally "shedding more virus and for longer periods than adults," the weekly morbidity and mortality report said.

The CDC has included children and young people on a list of 160 million people in the United States who will be first in line for the swine flu vaccine which is still being developed.

Around 120 million doses of the vaccine are projected to be ready by October.