The goals with testing are to determine the microorganism causing the pneumonia, to limit its spread to other people, to determine the severity of the pneumonia, and to guide treatment. In a significant number of cases, it is not possible to determine the exact cause of acute pneumonia, and treatment is based upon the person's clinical presentation and medical history, the health practitioner's experience, likely microorganisms present in the community at the time, such as influenza, and on established guidelines for the care of lower respiratory infections. If a person's symptoms do not resolve, then additional testing may be performed to help diagnose less common pneumonia causes.
The search for the cause of symptoms typically begins with a physical exam. As part of the exam, a health practitioner listens to a person's lungs for abnormal breath sounds that may indicate a lung infection. This is often followed with imaging studies. These may include:
- Chest X-ray (radiography)—used to detect and help evaluate the severity of a lung infection
- CT (computed tomography) scan—sometimes use to detect and evaluate a lung infection and to look for other non-infectious causes of a person's symptoms
Depending on the affected person's medical history and the signs and symptoms that are present at the time of the physical exam, a number of laboratory tests may be performed to help make a diagnosis.
General laboratory tests may include:
- CBC (complete blood count)—evaluates the type and number of white blood cells; results may indicate that an infection is present
- BMP (basic metabolic panel)—blood tests for sodium, potassium, and other chemistries to help determine the severity of the illness
- Blood gases—measures the pH and the amount of oxygen (O2) and carbon dioxide (CO2) present in a sample of blood, usually from an artery, in order to evaluate lung function
Examples of tests for suspected bacterial pneumonia include:
- Bacterial sputum culture and gram stain—primary tests ordered to detect and identify the cause of bacterial pneumonia
- Susceptibility testing—performed on pathogenic bacteria grown in culture and identified by testing; used to guide treatment
- AFB smear and culture—ordered when tuberculosis or a nontuberculous mycobacteria infection is suspected
- Blood culture—used to detect septicemia when it is suspected that infection has spread from the lungs to the blood or from the blood to the lungs
- Mycoplasma testing—blood test or special culture to help diagnose a mycoplasma infection
- Legionella testing—blood test for the specific antigen, culture or molecular test to diagnose a suspected Legionella infection
Examples of tests for suspected viral pneumonia include:
- Influenza tests—used to help diagnose an influenza infection (flu) and sometimes to help document the presence of influenza in the community; testing may also be used to identify the type and/or the strain of influenza causing the infection.
- RSV testing—used during the RSV season to help diagnose the infection in people who have moderate to severe symptoms and lower respiratory tract involvement. It is primarily ordered on infants (between the ages of 6 months and 2 years), the elderly, and those with compromised immune systems, such as those who have pre-existing lung disease or who have had an organ transplant.
- Viral culture
- Molecular tests—these tests may be used to identify the presence of bacteria and/or viruses in a respiratory sample by detecting the genetic material (DNA, RNA) of the microorganisms. These types of tests are generally very sensitive and specific and are particularly useful in detecting microorganisms that are difficult to culture.
- Some tests look for a single specific microorganism, such as Mycoplasma pneumoniae. A common method is polymerase chain reaction (PCR).
- Newer methods can simultaneously test for several bacteria, fungi and/or viruses. Examples of these methods include multiplexed PCR and DNA microarrays.
Other types of tests that may be performed depending on the affected person's history and clinical presentation include:
- Pleural fluid analysis—if fluid has accumulated in the space between the lungs and chest wall, the fluid may be tested to help determine the cause of infection.
- Fungal tests—ordered when a fungal infection is suspected; may be a culture, antigen detection or antibody tests or a molecular detection assay