Re: Swine Flu
Interim Guidance on Specimen Collection and Processing for Patients with Suspected Swine Influenza A (H1N1) Virus Infection
(Information for Health Care Professionals)
April 28, 2009 06:00 AM ET
Objective: To provide interim guidance on appropriate specimen collection, storage, and processing for patients with suspected swine influenza A (H1N1) virus infection.
Case Definitions for Infection with Swine Influenza A (H1N1) Virus
A
confirmed case of swine influenza A (H1N1) virus infection is defined as a person with an acute febrile respiratory illness with laboratory confirmed swine influenza A (H1N1) virus infection at CDC by one or more of the following tests:
- real-time RT-PCR
- viral culture
A
probable case of swine influenza A (H1N1) virus infection is defined as a person with an acute febrile respiratory illness who is:
- positive for influenza A, but negative for H1 and H3 by influenza RT-PCR, or
- positive for influenza A by an influenza rapid test or an influenza immunofluorescence assay (IFA) plus meets criteria for a suspected case
A
suspected case of swine influenza A (H1N1) virus infection is defined as a person with acute febrile respiratory illness with onset
- within 7 days of close contact with a person who is a confirmed case of swine influenza A (H1N1) virus infection, or
- within 7 days of travel to community either within the United States or internationally where there are one or more confirmed swine influenza A(H1N1) cases, or
- resides in a community where there are one or more confirmed swine influenza cases.
Duration of Viral Shedding
The duration of shedding with swine influenza A (H1N1) virus is unknown. Therefore, until data are available, the estimated duration of viral shedding is based upon seasonal influenza virus infection. Infected persons are assumed to be shedding virus and potentially infectious from the day prior to illness onset until resolution of fever. Infected persons should be assumed to be contagious up to 7 days from illness onset. Some persons who are infected might potentially shed virus and be contagious for longer periods (e.g., young infants, immunosuppressed, and immunocompromised persons).
Testing for Swine Influenza A (H1N1) Virus
All suspected cases of swine influenza A (H1N1) should have upper respiratory specimens collected to test for swine influenza A (H1N1) virus.
Preferred Respiratory Specimens:
The following should be collected as soon as possible after illness onset: nasopharyngeal swab/aspirate or nasal wash/aspirate. If these specimens cannot be collected, a combined nasal swab with an oropharyngeal swab is acceptable. For patients who are intubated, an endotracheal aspirate should also be collected. Specimens should be placed into sterile viral transport media (VTM) and immediately placed on ice or cold packs or at 4°C (refrigerator) for transport to the laboratory. Recommended infection control guidance is available for
persons collecting clinical specimens and for
laboratory personnel.
Swabs
Ideally, swab specimens should be collected using swabs with a synthetic tip (e.g., polyester or Dacron®) and an aluminum or plastic shaft. Swabs with cotton tips and wooden shafts are not recommended. Specimens collected with swabs made of calcium alginate are not acceptable. The swab specimen collection vials should contain 3ml of viral transport medium (e.g., containing, protein stabilizer, antibiotics to discourage bacterial and fungal growth, and buffer solution), such as M4RT or the
BD Universal Viral Transport System
.
Storing Clinical Specimens
All respiratory specimens should be kept at 4°C until they can be placed at -70°C. If a -70°C freezer is not available, specimens should be kept at 4°C, preferably no longer than 1 week.
Shipping Clinical Specimens
Clinical specimens should be shipped on dry ice in appropriate packaging.
All specimens should be labeled clearly and include information requested by your state public health laboratory. Suspect case specimens shipped from the state public health laboratory to CDC should include all information required for seasonal influenza surveillance isolate or specimen submission.
Recommended Tests
Real-time RT-PCR for influenza A, B, H1, H3 at a State Health Department Laboratory is recommended. Currently, swine influenza A (H1N1) virus will test positive for influenza A and negative for H1 and H3 by real-time RT-PCR. If reactivity of real-time RT-PCR for influenza A is strong (e.g., Ct
<30) it is more suggestive of a novel influenza A virus. Confirmation as swine influenza A (H1N1) virus is performed at CDC currently, but may be available in state public health laboratories soon.
Other Influenza Tests
Rapid Influenza Antigen Test
Some commercially available rapid tests can distinguish between influenza A and B viruses. A patient with a positive rapid test for influenza A may meet criteria for a probable case (
see above). These tests have unknown sensitivity and specificity to detect human infection with swine influenza A (H1N1) virus in clinical specimens, and have suboptimal sensitivity to detect seasonal influenza viruses. Therefore, a negative rapid test could be a false negative and should not be assumed a final diagnostic test for swine influenza infection.
Immunofluorescence (DFA or IFA)
These tests can distinguish between influenza A and B viruses. A patient with a positive for influenza A by immunofluorescence may meet criteria for a probable case (
see above). Immunofluorescence depends upon the quality of a clinical specimen, operator skills, and has unknown sensitivity and specificity to detect human infection with swine influenza A (H1N1) virus in clinical specimens. Therefore, a negative immunofluorescence could be a false negative and should not be assumed a final diagnostic test for swine influenza infection.
Viral Culture
Isolation of swine influenza A (H1N1) virus is diagnostic of infection, but may not yield timely results for clinical management. A negative viral culture does not exclude infection with swine influenza A (H1N1) virus.
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