H1N1 Flu Clinical and Public Health Guidance
Clinician Guidance
Influenza Diagnostic Tests
A number of different laboratory diagnostic tests can be used for detecting 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). These tests differ in their sensitivity and specificity in detecting influenza viruses as well as 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). Serologic tests on paired acute (within 1 week of illness onset) and convalescent (collected 2-3 weeks later) sera can help to establish a retrospective diagnosis of influenza virus infection for epidemiological and research studies. However, such serial serological testing is not routinely available through clinical laboratories. At this time, there are only two FDA authorized assays for confirmation of novel influenza A(H1N1) virus infection, including the rRT-PCR Swine Flu Panel assay; however, other rRT-PCR assays such as laboratory developed tests, not approved by FDA, may be able to detect novel influenza A (H1N1) viruses. Public health laboratories in the U.S. are able to perform the rRT-PCR Swine Flu Panel assay. Confirmation of novel influenza A(H1N1) infection may be necessary for surveillance purposes and for special situations, e.g. severely ill patients, patients with immunocompromising conditions, and pregnant and breast feeding women. State and local jurisdictions dictate the specific influenza assay results required for influenza surveillance.
Rapid Influenza Diagnostic Tests
Rapid influenza diagnostic tests (RIDTs) are antigen detection tests that detect influenza viral nucleoprotein antigen. The commercially available RIDTs discussed in this document can provide results within 30 minutes or less. Thus, results are available in a clinically relevant time period to inform clinical decisions. These assays may be referred to as “point-of care” tests since CLIA-waived RIDTs (not all RIDTs are CLIA waived) may be used in facilities with a certificate of waiver or in locations outside a central laboratory. Commercially available RIDTs can either: i) detect and distinguish between influenza A and B viruses; ii) detect both influenza A and B but not distinguish between influenza A and B viruses; or, iii) detect only influenza A viruses. None of the currently FDA approved RIDTs can distinguish between influenza A virus subtypes (e.g. seasonal influenza A (H3N2) versus seasonal influenza A (H1N1) viruses), and RIDTs cannot provide any information about antiviral drug susceptibility. For detection of seasonal influenza A virus infection in respiratory specimens, RIDTs have low to moderate sensitivity compared to viral culture or RT-PCR. The sensitivities of RIDTs to detect influenza B viruses are lower than for detection of influenza A viruses. The sensitivities of RIDTs appear to be higher for specimens collected from children than specimens collected from adults.
These drugs are called neuraminidase inhibitors and they work by blocking the function of neuraminidase (the N in H1N1), a protein on the surface of flu viruses that allows copies of the virus to be released into the body from the infected host cell.
Laboratory Guidance
Testing for influenza (H1N1) virus
Clinicians should consider testing suspected cases of novel influenza A (H1N1), especially those with severe illness, by obtaining an upper respiratory specimen to test for novel influenza A (H1N1) virus.
Storing specimens
All respiratory specimens should be kept at 4°C for no longer than 4 days.
Shipping clinical specimens
Clinical specimens should be shipped on wet ice or cold packs in appropriate packaging. All specimens should be labeled clearly and include information requested by your state public health laboratory. Suspected case specimens shipped from the state public health laboratory to should include all information required for seasonal influenza surveillance isolate or specimen submission.