Study: Rapid flu tests do best at high viral loads

Nov 1, 2012 (CIDRAP News) – Commercially available rapid influenza diagnostic tests (RIDTs) detected common virus strains at high concentrations fairly well but performed variably at lower concentrations, an evaluation of all 11 Food and Drug Administration–approved RIDTs showed.

Writing in today's Morbidity and Mortality Weekly Report (MMWR), researchers from the Centers for Disease Control and Prevention (CDC), the Biological Advanced Research and Development Authority, and the Medical College of Wisconsin said they used 23 recently circulating flu viruses under identical lab conditions to assess each RIDT.

The viruses included 4 pandemic 2009 H1N1 (pH1N1) strains, 6 seasonal H1N1 (sH1N1), 6 H3N2 (not the variant swine-origin strains linked to US fairs this summer and fall), and 7 influenza B (4 from the Victoria and 3 from the Yamagata lineages).

Before the 2009 pH1N1 pandemic, evaluations of RIDTs found sensitivities ranging from 27% to 61% compared with real-time reverse-transcription polymerase chain reaction, the gold standard for identifying flu strains, according to the report. And during the pandemic the CDC reported that the sensitivities of three commonly used RIDTs ranged from 40% to 69% and recommended that clinicians use caution when interpreting RIDT results.

RIDTs are simple to use and provide results in less than 15 minutes, today's report said. The kits detect the influenza virus nucleoprotein (NP) antigen.

To ensure a level playing field for their assessment, the investigators used identical viral concentrations for each trial and tested for all 23 viruses with each kit. They tested for each virus at five different concentrations ranging from 10-1 to 10-3.

Results were tabulated in a matrix that scored each vaccine for 20 variables (the four strains—pH1N1, sH1N1, H3N2, and B—multiplied by the five concentrations). Each variable reflected three trials of the kit for each of the viruses within each strain; for example, 21 trials were run for each NP concentration of influenza B, because there were seven test B strains.

The team found that RIDTs overall worked better at higher concentrations and had the fewest positive results at the lowest NP concentrations. The kits generally performed better at detecting influenza B, and the researchers found no significant difference in detecting B/Victoria versus B/Yamagata strains.

All tests detected 100% of viruses on at least 1 of the 20 variables scored. Only two RIDTs, however, scored 100% for 8 variables; the rest scored 100% for 6 variables or fewer.

The QuickVue Influenza A+B kit (Quidel Corp.) detected 100% of B strains at the three highest concentrations, pH1N1 and H3N2 at the two highest concentrations, and sH1N1 at the highest concentration. BD Directigen EZ Flu A+B (Becton, Dickinson and Co.) detected 100% of all four strains at the two highest levels.

Four RIDTs detected most influenza B viruses at the third-highest concentration, but BD Directigen EZ Flu A+B was the only test that detected at least 50% of all influenza A viruses at the third-highest concentration.

One kit (SAS FluAlert Influenza A [SA Scientific]) did not uniformly detect influenza A viruses at high concentrations. It and another test, BinaxNOW Influenza A&B (Inverness Medical), were the only tests to score 100% for only 1 of the 20 variables.

"The findings in this report further emphasize the importance of collecting respiratory specimens when the amount of influenza virus is at its peak (within 24-72 hours of symptom onset)," the article said, pointing out that the highest NP concentration tested might exceed levels found in clinical specimens.

"In addition, given the narrow range of virus concentrations that can be detected by the majority of RIDTs, clinicians should follow best practices for specimen collection and timing to maximize the number of influenza viruses per specimen and improve the clinical utility of the test."

CDC. Evaluation of 11 commercially available rapid influenza diagnostic tests—United States, 2011-2012. MMWR 2012 Nov 2;61(43);873-6 [Full text]

See also:

Aug 2, 2009, CDC report on RIDTs during 2009 pandemic

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