CDC Cautions Against Relying on Rapid Tests to Spot H3N2v

Steven Fox

August 13, 2012

August 13, 2012 — Negative results from rapid influenza diagnostic tests (RIDTs) for influenza A (H3N2v) should not be considered reliable evidence of lack of infection, according to an update published in the August 10 issue of the Morbidity and Mortality Weekly Report.

In a report published earlier this year, the US Centers for Disease Control and Prevention (CDC) highlighted 13 cases of influenza A (H3N2)v virus infection in humans that occurred between July 2011 and April 2012.

In the August update, the CDC says additional cases of the illness have been reported beginning in July 2012, and that as of August 9, a total of 153 cases of H3N2v have shown up in 4 states: Indiana (120), Ohio (31), Illinois (1), and Hawaii (1).

All the affected people have reported either direct or indirect contact with swine, mostly at agricultural fairs. The CDC says all the viruses identified since July 2011 are similar, in that they have all contained the M gene from the influenza A (H1N1)pdm09.

About 93% of evaluable cases have occurred among individuals aged 18 years or younger, with a median age of 7 years.

The CDC notes that the recent outbreaks have underscored the need to validate the accuracy of RIDTs routinely used in clinical settings.

To that end, the CDC researchers checked the accuracy of 7 commercially available tests using 7 different H3N2v viral strains.

The rapid diagnostic tests evaluated were:

  • BinaxNOW (Alere),

  • Directigen (Becton-Dickinson),

  • FluAlert (SA Scientific),

  • QuickVue (Quidel),

  • Sofia (Quidel),

  • Veritor (Becton-Dickinson), and

  • Xpect (Remel).

The CDC reports that only 4 of the 7 RIDTs tested (Directigen, Sofia, Veritor, and Xpect) detected all 7 of the H3N2v strains. The BinaxNOW test spotted 5 of the 7 strains, and the QuickVue test found 3 of the 7. The FluAlert test detected only 1 of the viral samples in this test.

"In this evaluation of seven RIDTs, the ability to detect H3N2v virus varied substantially among the tests," the authors write, adding, "This evaluation emphasizes the fact that a negative RIDT result should not be considered as conclusive evidence of lack of infection with influenza A (H3N2)v [virus]."

The CDC also notes that during 2011 there was some evidence of limited human-to-human transmission of H3N2v, and the report cautions that could occur again in the current outbreak. "Enhanced surveillance for influenza H3N2v virus infection is indicated, especially in regions and states with confirmed H3N2v cases," the authors write.

"Results from RIDTs, both positive and negative, always should be interpreted in the broader context of the circulating influenza strains present in the area, level of clinical suspicion, severity of illness, and risk for complications in a patient with suspected infection," they conclude.

The CDC urges that specimens from patients with suspected H3N2v be sent to public health labs so the diagnosis can be confirmed using the CDC Flu Real-Time Reverse Transcription Polymerase Chain Reaction Dx Panel.

The authors have disclosed no relevant financial relationships.

Morb Mortal Wkly Rep. 2012;61:1-3. Full text

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