Flu tests miss many cases, U.S. CDC confirms

By Maggie Fox, Health and Science Editor

WASHINGTON (Reuters) - Current quick tests for flu miss many cases of the new pandemic H1N1 strain, researchers at the U.S. Centers for Disease Control and Prevention reported on Thursday.

The accuracy of the tests ranged from just 40 percent to 69 percent in detecting swine flu, the CDC team reported.

The findings confirm the CDC's warnings that instant tests performed on the spot in doctor's offices and clinics are not highly worthwhile for diagnosing H1N1 infections.

The three popular tests were better at detecting seasonal influenza, catching more than 80 percent of H3N2 infections, the CDC found. But in general for patients, the agency advises treating based on symptoms and the knowledge that the virus is circulating in a community.

"The recent appearance and worldwide spread of novel influenza A (H1N1) virus has highlighted the need to evaluate commercially available, widely used, rapid influenza diagnostic tests," the researchers wrote in the CDC's weekly report on death and disease.

The team at CDC's influenza division tested three popular diagnostics -- BinaxNow, made by Inverness Medical Innovations, Becton Dickinson's Directigen EZ Flu A+B test and Quidel's QuickVue.

They used 65 respiratory specimens collected during April and May that had previously tested positive either for swine flu or one of the seasonal flu viruses.

All had been evaluated using a slower but highly accurate test called real-time reverse transcription-polymerase chain reaction, or rRT-PCR, which checks for the genetic material of the virus.

"The results showed that, although the (rapid tests) were capable of detecting novel A (H1N1) virus from respiratory specimens containing high levels of virus, the overall sensitivity was low," the CDC team wrote.

The QuickVue test detected 69 percent of the H1N1 cases, the Directigen test found 49 percent and BinaxNow caught just 40 percent.

FUTILE TO COUNT

None can distinguish among the different types of flu -- they just tell a patient and a doctor if one of the flu viruses is there.

In its latest update, the World Health Organization last week reported 162,230 confirmed cases of the pandemic H1N1 virus and 1,154 deaths, but the CDC says more than a million people in the United States alone have been infected. Most public health officials have given up on trying to get precise counts of how many people are infected.

The CDC's study gives one reason why: even if patients go to the doctor to be tested, and not all do, the instant test does not do a good job of detecting swine flu.

Many companies have seen an opportunity to make better quick, point-of-care flu diagnostics.