How Accurate Is Your Rapid Flu Test?

October 16, 2017

Spoon Feed
Diagnostic accuracy of the digitally interpreted rapid influenza A and B antigen tests (DIA) was better than the traditional rapid flu tests, 77-80% sensitive vs. 53-54% sensitive.  In children compared to adults, the sensitivity was 18.5 points higher for flu A and 32 points higher for flu B with the traditional rapid test; 12 and 25 points higher for the DIA, respectively; 2.7% points higher for NAAT.  The nucleic acid amplification tests (NAATs) were overall about 95% sensitive.  All tests had specificity of about 98%.

Why does this matter?
Relying on traditional rapid antigen tests as a screening tool is ill advised.  There are simply too many false negatives for them to be trusted.  The DIA is better.  The NAATs are best but are expensive.  Antiviral treatment for influenza is controversial, but certain high-risk groups are likely to benefit.  Relying on poor screening tests to make management decisions will lead to under-treatment and potential harm.  At my institution, we use the DIA; NAATs for flu A or flu B are available as well.

One flu over the cuckoo’s nest
This was a systematic review of 124 studies looking at the diagnostic accuracy of rapid influenza tests.  Only 19 dealt with the newer DIA tests; 13 with NAAT.  The pooled sensitivities and specificities are listed above in the Spoon Feed.  Most of the evidence surrounds the older generation flu tests.  The newer generation tests have improved accuracy, though the number of pooled studies for them is lower.  The Liat vs. Alere NAAT brand had about 12% higher sensitivity and specificity.  Industry sponsored studies had 16-22% higher sensitivity than non-industry sponsored.  Interestingly, this study was partially funded by a DIA manufacturer.

The take home points:

• if you need accurate screening for influenza A or B, use the NAAT;
• traditional rapid tests (not DIA) can’t rule out disease and shouldn’t be used for screening but have acceptable specificity to rule in influenza; 
• children have better diagnostic accuracy for all tests, with acceptable sensitivity for clinical screening use of the DIA;
• brand of test makes a difference;
• industry sponsorship may introduce bias.

Source
Diagnostic Accuracy of Novel and Traditional Rapid Tests for Influenza Infection Compared With Reverse Transcriptase Polymerase Chain Reaction: A Systematic Review and Meta-analysis.  Ann Intern Med. 2017 Sep 19;167(6):394-409. doi: 10.7326/M17-0848. Epub 2017 Sep 5.

Another Spoonful

• Journal Watch had a free writeup on this article.
• Here is what the CDC has to say for clinicians and lab directors (both are helpful for clinicians).

Peer reviewed by Thomas Davis, MD.