# 3149
The Japanese, who generally regard influenza far more seriously than a lot of other societies, have taken the extra step of PCR testing a number of H1N1 patients who initially tested negative for Influenza A using the rapid influenza tests routinely found in doctors offices.
About 30% (range 12%-47%) of people they double-checked actually had the H1N1 virus.
This isn’t a complete surprise (see Case Counts, Testing, And Feeding the Beast) , although this is the first preliminary data I’ve seen on the sensitivity of the rapid influenza test on this novel H1N1 virus.
As I wrote in that blog:
The first test, usually applied in a doctor’s office when someone presents with ILI (Influenza-like-Illness) is a rapid Influenza test.
In order for most samples to be `sent up the chain’ for additional testing, a patient must first test positive for influenza A using a rapid test kit.
This rapid test is designed to show if someone tests positive for the Influenza A or B virus, but generally not the type.
The rapid tests vary in terms of sensitivity and specificity when compared with viral culture or RT-PCR. Product insert information and research publications indicate that:
- Sensitivities are approximately 50-70%
- Specificities are approximately 90-95%
Sensitivity is defined as the ability of a test to correctly identify individuals who have a given disease or condition.
Here in the United States, trying to detect all swine flu cases is a moot point, and so the lack of sensitivity of these rapid tests no longer really impacts us with this outbreak.
But for countries trying to utilize these tests to detect, and contain, the spread of the H1N1 virus (or H5N1 for that matter) it does pose a problem.
My thanks to Shiloh on Flutrackers for posting this article from the Yomiuri Shimbum.
30% of new-flu cases initially tested negative
The Yomiuri Shimbun
About 30 percent of people infected with the new strain of influenza initially tested negative in a commonly used screening procedure for the new flu, according to a survey by the National Institute of Infectious Diseases.
The error reportedly is due to variances in the amount of virus in a person's body after symptoms first appear.
The test almost always proved accurate when conducted the day after symptoms first appeared.
But in cases where the test was performed on the day symptoms first manifested, or two days later, about 40 percent of people infected with the virus were judged not to be infected.
The institute points out that negative test results do not necessarily mean a person is not infected with the new flu. The Health, Labor and Welfare Ministry has instructed medical institutions to use the test kit at appropriate times.
The testing kits, which are used at medical institutions and public health centers, includes cotton swabs used to obtain mucus from inside the nose and other places. Changes in the color of mucus-bearing test paper show about 10-15 minutes later whether infection exists and whether it is type A or type B virus.
