Rapid diagnostic tests for H1N1 not reliable

Published in The Hindu on September 11, 2009

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

Of the 144 samples that tested positive for H1N1 virus using RT-PCR (reverse transcription-Polymerase chain reaction), only 16 could be detected when a rapid H1N1 diagnostic test was used, corresponding to a sensitivity of just 11.1 per cent.

Rapid diagnostic tests, however, have a high specificity (ability to pick only positive cases). However, specificity is low during non-peak influenza period.

The study found that the rapid diagnostic test correctly detected infections only when the viral load was high. Rapid test used by the researchers from the Institute of Virology, Bonn, Germany, was a BinaxNOW Influenza A&B Rapid Test (Inverness Medical, Colonge, 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 QuickVue Influenza assay rapid diagnostic test was used.

According to the Atlanta-based Centres for Disease Control and Prevention (CDC), sensitivity of rapid influenza diagnostic tests for detecting H1N1 ranges from 10-70 per cent. “A negative test result [when rapid test is used] does not rule out influenza A(H1N1) virus infection,” it notes.

The latest study gains importance as very few studies have been done to compare the sensitivity of RT-PCR and rapid diagnostic tests for H1N1 infection.

The limited data that is available also shows that sensitivity of rapid diagnostic tests to detect H1N1 infection is equal to or less than its ability to detect seasonal influenza.

According to CDC, lower sensitivity of H1N1 rapid tests could possibly be due to the method of collecting the samples, quality of specimens, time of illness onset to specimen collection, age of the patient, etc. The sensitivity of rapid diagnostic tests is higher in the case of children as they shed more viruses.

Unlike a rapid test, the turnaround time when RT-PCR is used is low. Also, shipping the samples, which will be very high during a pandemic, for laboratory-based testing can cause additional problems. Hence a faster way of detecting positive cases using a rapid test becomes attractive.

Despite the attractiveness, using a rapid test becomes meaningful and advisable only if it is able to detect most of the positive cases. The researchers conclude that rapid diagnostic 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 H1N1 test is low, the CDC recommends that antiviral therapy should be administered, if appropriate, and infection control measures implemented if clinical suspicion of influenza is high in a patient even though the patient tests negative with a rapid diagnostic test.