How Pneumonia Is Diagnosed

Pneumonia affects millions of people every year. According to the Centers for Disease Control and Prevention (CDC), more than 400,000 people require evaluation and treatment in an emergency department and more than 50,000 people die from the disease. But complications can be prevented! Finding out you have pneumonia is the first step, then learning what type of pneumonia you have—bacterial, viral, or fungal—is essential for proper treatment.

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Physical Examination

Fever, cough, and shortness of breath could be signs that you have pneumonia. Your doctor with start an evaluation by checking your vital signs.

He or she will they measure your temperature, blood pressure, heart rate, and respiratory rate, and will also check your oxygen level using pulse oximetry. This is done by placing a small device on your finger to estimate the percentage of oxygen in your blood. Low levels of oxygen are concerning and may mean that you need to be put on oxygen.

Using a stethoscope, a doctor will listen to your lungs. They are looking to hear crackling sounds or wheezing. Decreased sounds in one area could mean that pneumonia has formed there. Tapping on your back over that area may help to determine if there is an associated fluid collection or a consolidation. Do not be surprised if you are asked to say the letter "E" out loud. If you have fluid in your lungs, it will sound like "A" when listening through the stethoscope.

Labs and Tests

While the physical exam can raise suspicion for pneumonia, the diagnosis can be strengthened using a variety of tests. Your doctor may or may not use the following tests. Know that most are simple and straightforward on your part—a simple blood draw or sample collection, quick and painless.

Complete Blood Count

A complete blood count is a simple and inexpensive test. A white blood count is one of the blood counts measured. If it is elevated, infection or inflammation is present. It does not specifically let you know if you have pneumonia.


Procalcitonin is a precursor of calcitonin, a protein that is released by cells in response to toxins. It is measured via a blood test. Interestingly, the levels increase in response to bacterial infections but decrease in viral ones. Results are usually positive within 4 hours of bacterial infection and peak within 12 to 48 hours. While it does not let you know what type of bacteria is present, it indicates that antibiotic treatment may be necessary.

Sputum Culture and Gram Stain

The gold standard for diagnosing bacterial infection is culture. Unfortunately, collecting a good-quality sputum sample can be difficult, especially if someone has a dry cough. It often gets contaminated with normal bacteria that live in the respiratory tract.

A sample should be collected before you are treated with antibiotics. You'll be asked to cough up some sputum, with as little saliva as possible. If you are having trouble doing so, a doctor may use a device with a light and tiny camera placed down your throat. He or she will help relax you with medications during the procedure, and there are few side effects outside of a potential slightly sore throat. 

Once collected, a Gram stain is applied to part of the specimen and examined under a microscope. A good-quality sputum sample will show several white blood cells but few epithelial cells. Bacteria will appear red or violet and, based on their appearance, can be categorized as one of two classes of bacteria. Narrowing the diagnosis makes it easier to choose an appropriate antibiotic.

To find out what specific bacteria is causing your illness, your sample will be cultured in Petri dishes. Once the bacteria or fungi grow, it is tested against different antibiotics to see what treatments will be most effective.

The problem is that it may take days to get a definitive culture result. Also, certain bacteria like S. pneumoniae are difficult to grow and a culture can give false-negative results. Because of the challenges in getting a good-quality sample, this test is more commonly used for people in the hospital rather than those living in the community.

Urine Antigen Tests

Bacterial pneumonia caused by S. pneumoniae and Legionella species has a high incidence of complications. Antigens from these bacteria are excreted in the urine. A simple urine test is available to look for these antigens.

The results are rapidly available and studies have shown them to be more accurate than Gram stain or culture. Another advantage of the test is that treatment with antibiotics will not alter the results.

The problem is that urine antigen tests are less accurate in milder cases of pneumonia. It also only tests for one serotype of Legionella although there are many species. Also, unlike with culture, there is no way to use the results to determine what antibiotics would be most effective for treatment.


Some bacteria are difficult to grow in culture and do not have a urine antigen test available for screening. Chlamydia, Mycoplasma, and some Legionella species are atypical bacteria that fall into this category.

There are serologic blood tests that may be able to determine when and if you have been infected. Serology measures antibodies formed against a specific pathogen. IgM antibodies indicate a new infection whereas IgG antibodies usually show that you have been infected in the past. It can sometimes be difficult to know when IgM antibodies transitioned to IgG antibodies.

PCR and Enzyme Immunoassays

It can be difficult to culture a virus. Instead, viral infections are more commonly diagnosed using polymerase chain reaction (PCR) and enzyme immunoassays. To perform any of these tests, a sample must be collected. Depending on what virus is being considered, this sample can be blood, sputum, nasal secretions, or saliva.

PCR is a test that screens for the presence of specific viral or bacterial DNA in a sample. It is an alternative to serology to screen for atypical bacteria. While results are often available in 1 to 6 hours, PCR cannot be performed on site. It must be processed by a laboratory.

Enzyme immunoassays, however, can be performed as a point of care test with results available in 15 minutes to an hour. These immunoassays use antibodies to detect the presence of specific viral antigens and can screen for multiple viruses at one time.  

Pneumonia is a common complication of COVID-19. For COVID-19 testing, the most accurate specimen is collected from the nose. This is the part of the upper respiratory tract where concentrations of the virus may be greatest. A flexible 6-inch cotton swab is inserted into the nose and along the back of your throat where it is left in place for 15 seconds. The same swab is then inserted into the other nostril to maximize how much mucous is collected for the test. Studies are then performed to assess if genetic material from the virus is present.


Imaging studies are often performed before laboratory tests. If you are otherwise healthy, a doctor may treat you for pneumonia based on physical exam and imaging studies alone.

Chest X-Ray

If pneumonia is suspected based on symptoms and physical exam, the standard of care is to get a chest X-ray. A chest X-ray may show an infiltrate, which is a collection of pus, blood, or protein in the lung tissue. It can also reveal other signs of lung disease like cavitations and pulmonary nodules.

Your doctor usually cannot differentiate between bacterial and viral infections based on imaging alone. However, an infiltrate that fills all or most of one or more lobes of the lungs is likely to be bacterial pneumonia caused by S. pneumoniae.

CT Scan

It is possible that a chest X-ray can miss a diagnosis. If your doctor still has a high suspicion for pneumonia after a negative result, she may choose to confirm the diagnosis by CT scan. Generally speaking, a CT scan is more accurate than a chest X-ray although it costs more and exposes you to higher doses of radiation.

The test is performed by placing you flat in a donut-shaped machine that takes pictures. The study is painless and completed in minutes, but it is important to lie still during the test to get the best images.


In severe cases that do not respond to therapy, your doctor may pursue further imaging to look for other causes. This evaluation may include bronchoscopy, where a thin camera is guided through your nose or mouth down into your lungs.

Bronchoscopy visualizes big airways (trachea or windpipe and large bronchi)—not lungs. Your doctor may decide to take some fluid from your airway for culture if your phlegm culture is negative and you are immunosupressed or if you have a chronic illness requiring precise diagnosis of the cause of your pneumonia. Bronchoscopy is almost never done in an otherwise healthy adult with community acquired pneumonia.

Differential Diagnosis

There are other conditions that can have symptoms similar to pneumonia, like bronchitis or congestive heart failure. If someone has asthma, bronchiectasis, or chronic obstructive pulmonary disease (COPD), it could be a flare-up of their known lung disease. In the worst case scenario, it could be a warning sign of lung cancer.

However, don't be alarmed by these possibilities. The best thing for you to do is visit your doctor for a proper diagnosis. In most cases, once diagnosed, pneumonia can be well taken care of.

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Article Sources
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Additional Reading
  • Bartlett JG. Diagnostic Approach to Community-Acquired Pneumonia in Adults. In: Bond S. (ed), UpToDate [Internet], Waltham, MA. Updated December 2, 2019.