Nasal swab swine flu test can be misleading


swineflu outbreakThe rise in the current outbreak of swine flu, especially in the United States after schools opened, has created a lot of questions around spread, detection and control of swine flu.  I shared my personal experience about the rapid swine flu test, which is also called nasopharyngeal swab test.  My test results came in negative and my flu went away in couple of days, but there are questions about capability of detection of swine flu by the swab test.  Let us look at the concerns in brief:


What are RIDT test for detection of swine flu?

RIDT stands for Rapid Influenza Detection Test.  Center of Disease Control (CDC) has recommended a number of different laboratory diagnostic tests that can detect the presence of influenza viruses in respiratory specimens, including direct antigen detection tests, virus isolation in cell culture, or detection of influenza-specific RNA by real-time reverse transcriptase-polymerase chain reaction (rRT-PCR). Different test differ in terms of their sensitivity and specificity in detecting influenza viruses.  Also difference arises in their commercial availability, the amount of time needed from specimen collection until results are available, and the tests ability to distinguish between different influenza virus types (A versus B) and influenza A subtypes (e.g. novel H1N1 versus seasonal H1N1 versus seasonal H3N2 viruses). More details can be found at CDC website.





nasopharyngeal-swab-method1What is the current issue with nasal swab test?

Two Connecticut schools found that rapid test for H1N1 virus were not fool proof in detecting swine flu virus cases.  Dr. J.R. Sabetta, Greenwich hospital and his colleagues reported in 25th September issue of the CDC’s Morbidity and Mortality Weekly Report, that results of his study “affirm recent CDC recommendations against using negative [rapid test] results for management of patients with possible 2009 pandemic influenza A (H1N1) infection.”  Let us look at some key findings:

·         Study reports rapid test worked 47% of time, based on test conducted on 63 school children, which had flu symptoms after overnight field trips

·         All children were subjected to nasal swab test.  Liquid solution collected with swab test was sent for molecular test to state public lab.  At the same time, solution was tested at the hospital for viral antigens with the Remel Xpect influenza...

A/B kit.

·         49 children, who showed negative in viral antigen test, exhibited H1N1 infection via test conducted at the state lab, which amounts to 57% of the tested population.  The test that state lab conducted is called polymerase chain reaction test (PCR) and is considered as gold standard for detection of swine flu.

·         Though the study was conducted on small population of 67 students, researchers did not find any correction between false negatives and factors such as severity of symptom or lag between onset of flu and testing


What was the cause for failure of swab test to detect H1N1 virus?

It is difficult to pin down the exact cause for failure of swab test, but more than likely

·         improper specimen collection and handling and

·         long intervals between the time symptoms begin and the time the health care worker took the sample,

could be the leading cause. 


I think I got lucky that the negative result shown by the rapid test was not false in my case.  Otherwise Connecticut studies reveal that the person detected negative for H1N1 by rapid test may not be free from swine flu.  I hope hospitals and health insurance agencies are realizing this fact and double checking the samples collected by the nasal swab test.  I wish, I had known this earlier, I would like to say that if you happen to be in a situation that you have to go for swab test for H1N1 virus, please ask your doctor about PCR test along with the rapid test.


Healthy Swabbing!



13 Responses

  1. health forum says:

    Nasopharyngeal swab swine flu test can be misleading | Healthy Living is a good blog post, I was wondering if I could use this blog on my website, I will link it back to your website though.

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