R. Prasad

Their ability to detect positive cases is low, says a study

CHENNAI: A study to be published in the October issue of the journal Emerging Infectious Diseases has found that using rapid diagnostic tests (the antigen-based testing) for detecting A(H1N1) infection is not advisable because their sensitivity (the ability to detect positive cases) is low.

Of the 144 samples tested positive for the virus through RT-PCR (reverse transcription-Polymerase chain reaction), only 16 could be detected when a rapid diagnostic test was used. This meant a sensitivity of just 11.1 per cent. Rapid diagnostic tests, however, have a high specificity (the ability to pick only the positive cases).

The study found that the rapid tests were able to correctly detect infections only when the viral load was high. The rapid test, used by the researchers of the Institute of Virology, Bonn, was a BinaxNOW Influenza A&B Rapid Test (Inverness Medical, Cologne, Germany).

A study undertaken by another team and published in August in the New England Journal of Medicine found that sensitivity was 51 per cent when the QuickVue Influenza assay rapid diagnostic test was used.

According to the Atlanta-based Centers for Disease Control and Prevention (CDC), the sensitivity of rapid influenza diagnostic tests for detecting A(H1N1) ranges from 10 to 70 per cent. “A negative test result [when the rapid test is used] does not rule out A(H1N1) virus infection.” The sensitivity of rapid diagnostic tests is higher among children as they shed more viruses.

According to the CDC, the lower sensitivity of the A(H1N1) rapid tests could be due to the method of collecting the samples, the quality of specimens, the time of illness onset and the age of the patient. Unlike a rapid test, the turnaround time when RT-PCR is used is low, and there are the additional problems of shipping samples, which will be very high during a pandemic, for the laboratory-based testing. Hence, a faster way of detecting positive cases using a rapid test becomes attractive.

Despite the advantage, the researchers reckon that the rapid diagnostic A(H1N1) tests may produce “misleading results in clinical practice. Application of such assays [test kits] should be discouraged in favour of continued molecular diagnostics.”

Since the sensitivity of a rapid diagnostic A(H1N1) test is low, the CDC recommends that anti-viral therapy be administered, if appropriate, and infection-control measures implemented if clinical suspicion of influenza is high in a patient, even if the patient tests negative through a rapid test.

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