eMedicine Specialties > Infectious Diseases > Viral Infections

Influenza: Differential Diagnoses & Workup

Author: Robert W Derlet, MD, Professor of Emergency Medicine, University of California at Davis School of Medicine; Chief Emeritus, Emergency Department, University of California at Davis Health System
Coauthor(s): Christian E Sandrock, MD, MPH, FCCP, Assistant Professor of Clinical Medicine, Division of Pulmonary/Critical Care Medicine, Division of Infectious Diseases, Department of Internal Medicine, University of California, Davis Medical Center; Hien H Nguyen, MD, Assistant Clinical Professor, Division of Infectious Diseases and Pulmonary/Critical Care Medicine, University of California at Davis School of Medicine; Medical Director, Acute Infections Management Service, UC Davis Health System; Ruth Lawrence, MD, Chief, Division of Infectious and Immunologic Diseases, Director of Medical Student Education, Department of Internal Medicine, UC Davis Health System
Contributor Information and Disclosures

Updated: Aug 12, 2009

Differential Diagnoses

Adenoviruses
Japanese Encephalitis
Arenaviruses
Parainfluenza Virus
Cytomegalovirus
Rhinoviruses
Echoviruses
Severe Acute Respiratory Syndrome (SARS)

Other Problems to Be Considered

Acute HIV infection
Hanta pulmonary syndrome (HPS)

Workup

Laboratory Studies

  • Findings of standard laboratory studies such as a CBC count and electrolytes assessment are nonspecific but helpful in the workup of influenza.
  • Leukopenia and relative lymphopenia are typical findings in influenza.
  • Thrombocytopenia may be present.
  • Viral culture
    • The criterion standard for diagnosing influenza A and B is a viral culture of nasopharyngeal samples and/or throat samples.
    • Obtain samples with Dacron swabs and send the samples in appropriate viral transport media (eg, multimicrobe [M4] transport media) to the laboratory to be cultured in several lines of cells. A laboratory diagnosis of influenza is established once specific cytopathic effect is observed or hemadsorption testing findings are positive. Staining the infected cultured cell lines with fluorescent antibody confirms the diagnosis.
    • The process may require 3-7 days, long after the patient has left the clinic, office, or emergency department and well past the time when drug therapy could be efficacious.
  • Direct immunofluorescent tests
    • Some laboratories offer direct immunofluorescent tests on fresh specimens, but these tests are labor-intensive and are less sensitive than culture methods.
    • These tests require specially trained laboratory personnel (people generally not available during all shifts, even in large medical centers) for interpretation.
  • Serologic studies
    • In order to overcome the expensive and time-consuming obstacle of culturing, several serologic tests have become available. In reality, many of these are not bedside tests; generally, 30-60 minutes are required to perform the test's multiple steps.
    • Some rapid tests are performed best in a laboratory, not in the office or emergency department.
    • Disadvantages to performing these rapid diagnostic tests include the cost of the laboratory personnel, the cost of the test itself, and potential false-negative results for influenza A and B. Test sensitivities generally range from 60-70%.
  • Office tests
    • Recently, the US Food and Drug Administration waived federal Clinical Laboratories Improvement Act (CLIA) requirements and approved 3 office tests for diagnosis.
    • Of these, the fastest is the 10-minute QuickVue bedside test, which yields a sensitivity of 70-80%.
    • Because of cost, availability, and sensitivity issues, most physicians diagnose influenza based on clinical criteria alone.

Imaging Studies

  • Chest radiography: In elderly or high-risk patients with pulmonary symptoms, perform chest radiography to exclude pneumonia.
    • Early radiographic findings include no or minimal bilateral symmetrical interstitial infiltrates.
    • Later, bilateral symmetrical patch infiltrates become visible.
    • Focal infiltrates indicate superimposed bacterial pneumonia.

Other Tests

  • Arterial blood gas
    • Severe hypoxemia is present in severe cases of influenza.
    • The A-a gradient may be increased (>35 mm Hg).

Procedures

Patients with physical examination findings compatible with meningitis should undergo lumbar puncture.

More on Influenza

Overview: Influenza
Differential Diagnoses & Workup: Influenza
Treatment & Medication: Influenza
Follow-up: Influenza
References
Further Reading
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References

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Further Reading

Additional resources on influenza are available at Medscape's Influenza Resource Center.

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Keywords

influenza, flu, influenza virus, flu virus, influenzavirus, influenza A, influenza B, influenza C, influenza A subtype H3N2, H1N1, H5N1, H9N2, avian influenza, avian flu, bird flu, upper respiratory tract infection, URTI, severe acute respiratory syndrome, SARS, flu pandemic, Orthomyxoviridae, respiratory syncytial virus, RSV, West Nile virus

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Contributor Information and Disclosures

Author

Robert W Derlet, MD, Professor of Emergency Medicine, University of California at Davis School of Medicine; Chief Emeritus, Emergency Department, University of California at Davis Health System
Robert W Derlet, MD is a member of the following medical societies: American Academy of Emergency Medicine, American Association for the Advancement of Science, Infectious Diseases Society of America, Society for Academic Emergency Medicine, and Wilderness Medical Society
Disclosure: Nothing to disclose.

Coauthor(s)

Christian E Sandrock, MD, MPH, FCCP, Assistant Professor of Clinical Medicine, Division of Pulmonary/Critical Care Medicine, Division of Infectious Diseases, Department of Internal Medicine, University of California, Davis Medical Center
Christian E Sandrock, MD, MPH, FCCP is a member of the following medical societies: American College of Chest Physicians, American Thoracic Society, and Infectious Diseases Society of America
Disclosure: Nothing to disclose.

Hien H Nguyen, MD, Assistant Clinical Professor, Division of Infectious Diseases and Pulmonary/Critical Care Medicine, University of California at Davis School of Medicine; Medical Director, Acute Infections Management Service, UC Davis Health System
Hien H Nguyen, MD is a member of the following medical societies: American College of Physicians, Infectious Diseases Society of America, and Society of Critical Care Medicine
Disclosure: Nothing to disclose.

Ruth Lawrence, MD, Chief, Division of Infectious and Immunologic Diseases, Director of Medical Student Education, Department of Internal Medicine, UC Davis Health System
Ruth Lawrence, MD is a member of the following medical societies: American Association for the Advancement of Science, American College of Physicians, American Society for Microbiology, and Infectious Diseases Society of America
Disclosure: Nothing to disclose.

Medical Editor

Klaus-Dieter Lessnau, MD, FCCP, Clinical Associate Professor of Medicine, New York University School of Medicine; Medical Director, Pulmonary Physiology Laboratory; Director of Research in Pulmonary Medicine, Department of Medicine, Section of Pulmonary Medicine, Lenox Hill Hospital
Klaus-Dieter Lessnau, MD, FCCP is a member of the following medical societies: American College of Chest Physicians, American College of Physicians, American Medical Association, American Society for Artificial Internal Organs, American Thoracic Society, Physicians for Social Responsibility, and Society of Critical Care Medicine
Disclosure: sepracor Ownership interest None

Pharmacy Editor

Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine
Disclosure: eMedicine Salary Employment

Managing Editor

Joseph F John Jr, MD, FACP, FIDSA, FSHEA, Clinical Professor of Medicine, Molecular Genetics and Microbiology, Medical University of South Carolina; Associate Chief of Staff for Education, Ralph H Johnson Veterans Affairs Medical Center
Disclosure: BioMerieux Honoraria Review panel membership; Cubist Honoraria Review panel membership; Pfizer Honoraria Speaking and teaching; Merck Stock dividends stock holdings

CME Editor

Eleftherios Mylonakis, MD, Clinical and Research Fellow, Department of Internal Medicine, Division of Infectious Diseases, Massachusetts General Hospital
Eleftherios Mylonakis, MD is a member of the following medical societies: American Association for the Advancement of Science, American College of Physicians, American Society for Microbiology, and Infectious Diseases Society of America
Disclosure: Nothing to disclose.

Chief Editor

Burke A Cunha, MD, Professor of Medicine, State University of New York School of Medicine at Stony Brook; Chief, Infectious Disease Division, Winthrop-University Hospital
Burke A Cunha, MD is a member of the following medical societies: American College of Chest Physicians, American College of Physicians, and Infectious Diseases Society of America
Disclosure: Nothing to disclose.

 
 
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