Influenza Differential Diagnoses

Due to the fact that there are overlapping symptoms caused by other viruses with upper respiratory infections diagnosing influenza A or B infection based only on the basis of clinical criteria is difficult. Other viruses may present influenza-like symptoms such as adenoviruses, enteroviruses, and paramyxoviruses. Upper respiratory infections and influenza, are also more frequently occurring in the winter. As a consequence doctors and ER personal encounter both in.

Influenza pneumonia must also be distinguished from: Other viral pneumonia, bacterial pneumonia, other noninfectious causes of respiratory issues such as heart failure, chronic obstructive pulmonary disease, aspiration pneumonitis and pulmonary edema.


The transmission of the flu occurs when particles of respiratory droplets get into the air by sneezing, coughing or talking. They can land on objects and contaminate them for a few days. Or it gets in contact with the mucus membranes of another person via their nose, eyes or mouth. The typical incubation period for the flu is two days.

Clinical Presentation

The most frequent presenting symptoms are:

  • Acute onset fever
  • Myalgia
  • Arthralgia
  • Anorexia
  • Headache
  • Dry cough
  • Malaise
  • Fatigue
  • Weakness
  • Chest discomfort

Severe symptoms can include:

  • Altered mental status
  • Seizures
  • Hypoxia
  • Tachypnea
  • Decreased urination

Physical Exam

Clinicians should examine for fever, myalgia, arthralgia, cough, pharyngitis and rhinorrea. The skin may be hot to the touch, dry or diaphoretic. Flushing may also sometimes be seen directly on the face at the exam. Other findings on the examination may include things such as pain on eye motion, photophobia, conjunctival injection and nonexudative pharyngitis. Upon auscultation of the lungs, rhonchi or scattered rales may be heard.


As stated upper respiratory infections (URIs) can be misdiagnosed with influenza. The flu generally has rapid onset with a high fever, headache, dry cough and achy muscles and joints. URIs tend to have a more gradual onset, they have a productive cough and do not have a fever. Another one that is commonly confused with the flu is infectious mononucleosis. Infectious mononucleosis, however, is distinctive as it presents with posterior cervical lymphadenopathy

Diagnostic tests

Clinicians should look at a diagnostic test if the flu is indicated based on their exam.

  • Rapid antigen detection testing: has the quickest results but the sensitivity of the test itself is rather low and it will not exclude flu in a symptomatic
  • Reverse transcriptase- polymerase chain reaction (RT-PCR): The most sensitive test and can differentiate between Type A and B influenza.
  • Viral culture: Can confirm a diagnosis, but commonly used for surveillance of a viral outbreak.


For healthy individuals who have no complications due to the flu, antivirals are not necessary. Antivirals are recommended for patients who are hospitalized with influenza, high risk of complications or otherwise present with a progressive and severe illness. Ideally, in those cases, antiviral treatment should be started within two days of symptom onset or as soon as it is possible.