At the time of this writing, the World Health Organization (WHO) has changed the phase of pandemic alert for the novel Influenza A (H1N1) virus from Phase 5 to Phase 6. This indicates a global pandemic is underway in which there is little or no immunity in the population. While the upgrade designation is an indication to world leaders to implement strategies that reduce the spread, such response has already occurred in our area. It does NOT mean that the virus is more virulent.
The CDC, California Department of Public Health (CDPH) and our local county health services departments have confirmed the widespread incidence of this infection in our community. In the early stages of the outbreak, local county health services and CDPH asked us to test patients with influenza-like illness (ILI) via rapid antigen methods for Influenza A & B, and to forward specimens that tested positive for Influenza A for further speciation testing with real-time PCR. At that time, we were trying to do quick surveillance and identify if and when the novel H1N1 virus would enter the community. Of the 22 rapid test Influenza A positive specimens that the laboratories of John Muir Health forwarded, 21 were confirmed positive for the novel H1N1 Influenza virus (i.e. 95% specificity). As of June 3, 2009 the CDPH reported 1014 total cases of the novel H1N1 Influenza in California (796 confirmed, 218 probable - pending confirmation).
Now that the virus is definitely here, there is no longer a need to test all people with ILI for the novel H1N1 virus, and the practice of referring for confirmation all rapid screen positive Influenza A specimens has been discontinued. However, county health services departments and CDPH are now seeking to determine the severity of disease and how many hospitalizations it is causing. Thus, testing for the novel H1N1 Influenza virus should continue only on people with severe ILI who are hospitalized regardless of rapid test results. For these cases in which further testing for novel H1N1 Influenza virus is required, please notify the John Muir Health laboratory where the specimen was submitted. MuirLab: Microbiology (925) 692-5670; John Muir Med. Center, Walnut Creek Campus: (925) 947-5285 (or ext. 35285); John Muir Med. Center, Concord Campus: (925) 674-2184 (or ext. 22184).
Use of Rapid Influenza Diagnostic Tests: The reliability of rapid influenza diagnostic tests depends largely on the conditions under which they are used. For detection of seasonal influenza virus infection, the sensitivity of the rapid diagnostic test in use at the John Muir Health laboratories ranges from 70% to around 80% and specificity approaches 95%. While it is reasonable to assume that these rapid diagnostic tests can detect novel H1N1 Influenza A in respiratory specimens as nucleoprotein antigens are highly conserved across Influenza A viruses, data are not available on a large scale to determine the sensitivity of the tests for this novel virus. However, our own experience of 95% specificity (see above) is encouraging.
Interpretation of a positive rapid test result: A patient testing positive for Influenza B likely has been infected with seasonal Influenza B virus or is a false-positive result. Such a patient is unlikely to have novel H1N1 virus infection. There are several possibilities when a patient tests positive for Influenza A by rapid antigen test.
- Might have novel H1N1 virus infection, or
- Might have seasonal Influenza A virus infection, or
- Might have false positive test result.
Interpretation of a negative rapid test result: Novel H1N1 Influenza virus infection cannot be excluded. Further testing and treatment should be based upon clinical suspicion, severity of illness, and risk for complications. If there is no epidemiologic link to confirmed cases of novel H1N1 infection and the patient has mild illness, further testing and treatment are not recommended.
For further questions or concerns regarding the laboratory aspect of influenza testing, please contact MuirLab Microbiology Supervisor, Janet Long, MS, MT (ASCP) SM at (925) 692-5671, MuirLab Microbiology Medical Director, Nicholas Byrne, MD at (925) 947-5356, or John Muir Health Medical Director of Laboratory Services, Barry Latner, MD at (925) 692-5405. Thank you.