Think it's Swine Flu? Don't Go For Rapid Test

Doctors report the rapid flu test is less sensitive to the new H1N1 strain.

Rapid flu tests typically used for the seasonal influenza tests did not work particularly well when used to flag H1N1 cases in two Connecticut schools, researchers found.

Dr. J.R. Sabetta, of Greenwich Hospital reported the rapid flu test worked just 47 percent of the time after testing 63 schoolchildren with flu symptoms, of whom 49 were later confirmed to have novel H1N1 infections.

Sabetta and his colleagues wrote in the Sept. 25 issue of the CDC's Morbidity and Mortality Weekly Report, claiming the results of his test "affirm recent CDC recommendations against using negative [rapid test] results for management of patients with possible 2009 pandemic influenza A (H1N1) infection."

The cases arose early in the current outbreak of H1N1 flu, in late April and early May. At both schools, children came down with flu symptoms after overnight field trips -- 63 children eventually underwent diagnostic testing at Greenwich Hospital.

Physicians at the hospital completed a standard symptom survey for each child and took nasopharyngeal swabs.

Samples were sent to the state public health lab for gold-standard molecular analysis, while rapid testing for viral antigens with the Remel Xpect influenza A/B kit took place at the hospital.

No antiviral drugs were given prior to sample collection.

In the 49 children found to have H1N1 flu by polymerase chain reaction (PCR) testing, 53 percent had negative results on the rapid test.

Sabetta and colleagues found no correlations between the false negatives and such factors as severity of symptoms or the lag between onset and testing, although the number of patients was relatively small.

The researchers also found no notable difference in rapid test sensitivity for children who met CDC criteria for influenza-like illness versus those who didn't. The criteria include fever of at least 100 F plus cough and/or sore throat in the absence of another known illness.

Connecticut experience was in line with earlier findings of poor sensitivity for current rapid flu tests, even for non-H1N1 seasonal strains, according to an editorial accompanying the report.

Doctors Hesitant to Bet on Rapid Flu Test

Other reports have found that the novel H1N1 virus frequently escapes detection with tests originally developed around seasonal flu strains.

"The results of these studies and the findings in this report affirm that a negative result for this rapid test does not rule out 2009 pandemic influenza A (H1N1) virus infection in an individual with symptoms consistent with influenza," according to the editorial note.

Factors that might depress the sensitivity of rapid tests include improper specimen collection and handling and long intervals between the time symptoms begin and the time the health care worker took the sample, the editors noted.

But Sabetta and colleagues asserted that they had followed the label instructions in collecting and processing samples.