H1N1 Influenza (Swine Flu) Workup

Updated: Aug 14, 2023
  • Author: Michael Stuart Bronze, MD; Chief Editor: Russell W Steele, MD  more...
  • Print

Laboratory Studies

Outbreaks of H1N1 influenza (swine flu) are common in pigs year-round. Historically, when humans have become infected, it is a result of close contact with infected pigs (but not consumption of pork). In major outbreaks, human-to-human transmission occurs.

In the 2009-2011 outbreak, the WHO raised its pandemic alert level for H1N1 influenza to phase 6, indicating that a global pandemic was identified. 

Phase 6 criteria: In addition to the criteria defi Phase 6 criteria: In addition to the criteria defined in Phase 5, the same virus has caused sustained community-level outbreaks in at least one other country in another WHO region. Courtesy of the WHO.

In the 2009-2010 outbreak in the United States, preliminary testing showed that, in all cases, the viruses had the same genetic pattern. The virus is being described as a new subtype of influenza A/H1N1 not previously detected in pigs or humans.

Clinicians should consider the possibility of H1N1 influenza virus infections in patients who present with febrile respiratory illness. The CDC criteria for suspected H1N1 influenza are as follows [5] :

  • Onset of acute febrile respiratory illness within 7 days of close contact with a person who has a confirmed case of H1N1 influenza A virus infection, or
  • Onset of acute febrile respiratory illness within 7 days of travel to a community (within the United States or internationally) where one or more H1N1 influenza A cases have been confirmed, or
  • Acute febrile respiratory illness in a person who resides in a community where at least one H1N1 influenza case has been confirmed.

In September 2011 the FDA approved a new CDC-developed test to diagnose seasonal flu as well as the influenza viruses that could become pandemic. The Human Influenza Virus Real-Time RT-PCR Detection and Characterization Panel (rRT-PCR Flu Panel) is an in vitro laboratory diagnostic test that can provide results within 4 hours. It is the only in vitro diagnostic test for influenza that is cleared by the FDA for use with lower respiratory tract specimens and will be given at no cost to qualified international public health laboratories.

Laboratories should send all influenza A specimens that they are unable to subtype to the Viral Surveillance and Diagnostic Branch of the CDC's Influenza Division as soon as possible for further diagnostic testing. [29]

Since the outbreak, multiple methods of diagnosing influenza have been reported. Influenza tests may include the following:

  • Molecular tests: These may include conventional reverse-transcriptase PCR, office-based rapid molecular testing, and multiplex molecular platforms that can detect influenza in addition to several other common respiratory pathogens. RT-PCR yields the best performance based on sensitivity and specificity data.
  • Rapid antigen or antibody immunoassays: Although these are available, most choose molecular testing over these immunoassays owing to their lower sensitivity.
  • Viral culture: Although viral culture is available, it not very useful in clinical practice.
  • Serology: Serologic testing is not recommended for diagnosis because paired sera (acute and convalescent) are required, thereby limiting the timeliness of these tests in patient care. They may be more useful in epidemiologic studies.

Viral tracking and research

Internationally, scientists have been collaborating on genetic analysis of current animal and human influenza viruses. These researchers have created a human/swine A/H1N1 influenza wiki to facilitate rapid dissemination of the results of this work. The collaboration is producing insights on the origin of the H1N1 virus and should enable scientists to track its evolution as the outbreak spreads around the world. Information from the National Institute of Allergy and Infectious Disease regarding influenza genome sequencing is available to researchers studying how influenza viruses evolve and those developing new and improved ways to prevent, diagnose, and treat influenza disease. [30, 31]