Differential Diagnosis for Pneumonia

The following differential diagnosis is organized into categories to help guide the diagnostic process for pneumonia, considering viral, typical, and atypical forms.

• Single Most Likely Diagnosis

  • Streptococcus pneumoniae (Typical Bacterial Pneumonia): This is often the most common cause of community-acquired pneumonia and should be considered first, especially in patients with classic symptoms such as high fever, chills, and productive cough.
  • Influenza Virus (Viral Pneumonia): During influenza season, this should be a top consideration, especially with symptoms like sudden onset of fever, cough, and muscle pains.

• Other Likely Diagnoses

  • Mycoplasma pneumoniae (Atypical Pneumonia): Common in younger adults and those living in close quarters, presenting with symptoms like cough, fever, and headache, but often less severe than typical bacterial pneumonia.
  • Chlamydophila pneumoniae (Atypical Pneumonia): Similar to Mycoplasma, it affects a wide range of ages and presents with mild symptoms, making it a common cause of atypical pneumonia.
  • Respiratory Syncytial Virus (RSV) and other Viruses: In certain populations, especially the elderly, young children, and those with compromised immune systems, RSV and other viruses like adenovirus, parainfluenza, and human metapneumovirus can be significant causes of pneumonia.

• Do Not Miss Diagnoses

  • Legionella pneumophila (Atypical Pneumonia): Although less common, Legionella can cause severe pneumonia, especially in older adults, smokers, and those with chronic lung disease. It's crucial to consider this due to its high mortality rate if not treated promptly.
  • Pneumocystis jirovecii Pneumonia: In immunocompromised patients, particularly those with HIV/AIDS, this opportunistic infection can be life-threatening and must be considered early in the diagnostic process.
  • Tuberculosis (TB): While not typically considered a form of pneumonia in the traditional sense, TB can present with similar symptoms and is critical to diagnose due to its public health implications and the need for specific treatment.

• Rare Diagnoses

  • Hantavirus Pulmonary Syndrome: A rare but potentially deadly viral infection, typically associated with exposure to rodents or their droppings.
  • SARS-CoV-2 (COVID-19): Although not rare globally, in specific contexts or during certain times, COVID-19 might be less likely but is crucial to diagnose due to its significant impact on public health and the need for isolation and specific treatments.
  • Q Fever (Coxiella burnetii): A zoonotic disease that can cause atypical pneumonia, usually related to exposure to infected animals or contaminated products.
  • Psittacosis (Chlamydophila psittaci): Caused by contact with infected birds, this is a rare cause of atypical pneumonia but should be considered in individuals with a relevant exposure history.