Influenza Rapid Tests Show Variable Performance

A study evaluating the performance of 11 US Food and Drug Administration–cleared rapid influenza diagnostic tests (RIDTs) in 23 recently circulating influenza viruses showed that lower concentrations of influenza virus types and subtypes are differentially detected by these tests.

Roxanne Shively, MS, from the Biomedical Advanced Research and Development Authority within the Office of the Assistant Secretary for Preparedness and Response in the US Department of Health and Human Services in Washington, DC, and colleagues report their findings in the November 2 issue of the Morbidity and Mortality Weekly Report.

According to the researchers, commercially available RIDTs are widely used in clinical practice to diagnose influenza because of their simplicity and rapid detection (within 15 minutes) of the influenza virus nucleoprotein (NP) antigen. However, these tests have not been analyzed in a standardized laboratory setting against a panel of representative seasonal influenza viruses.

The current study assessed the analytical performance of the tests in 16 influenza A and 7 influenza B viruses from the 2011-2012 influenza season, provided by the Centers for Disease Control and Prevention.

Swab samples or mock nasal wash specimens were prepared in saline from several dilutions of each virus as indicated for individual RIDTs. Three separate tests were performed for each combination of virus and RIDT. The primary outcome was the positive detection ability of the tests, defined as at least 2 positive results from the 3 tests performed at each dilution for each of the 23 influenza viruses.

The researchers found that although most RIDTs detected viral antigens in samples with higher concentrations, the RIDTs had fewer positive results with lower concentrations of stock NP concentrations (<2 μg/mL); the tests also differed in their abilities to detect each type of influenza virus.

"Each influenza virus had variable levels of positivity with RIDTs, suggesting that several viruses of each type and subtype should be evaluated with each RIDT on a regular basis," Shively and colleagues note.

The authors also point out that nucleoprotein "levels of influenza B virus stocks generally were higher, and the first two dilutions were detected more uniformly than for influenza A viruses."

They add that "[n]o significant performance differences were noted for B/Victoria or B/Yamagata lineages of influenza B viruses."

They also found that the FluAlert Influenza A (SA Scientific) RIDT did not uniformly detect influenza A (H1N1)pdm09 (pH1N1) viruses or other influenza A viruses at high concentrations, whereas 4 RIDTs detected the majority of influenza B viruses in third dilution samples. Only a single RIDT (Directigen EZ Flu A+B, BD) detected at least 50% of all influenza A viruses in third dilution samples.

The authors recommend that "[c]linicians should be aware of the variability of RIDTs when interpreting negative results and should collect test samples using methods that can maximize the concentration of virus antigen in the sample, such as collecting adequate specimens using appropri-ate methods in the first 24–72 hours after illness onset."

The study was not commercially funded. The authors have disclosed no relevant financial relationships.

Morb Mortal Wkly Rep. 2012;61:873-876. Full text