How Pneumonia Is Diagnosed

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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 symptoms of pneumonia. Your healthcare provider with start an evaluation by checking your vital signs and taking a detailed medical history. They will 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 practitioner will listen to your lungs. They are listening to 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.

Common symptoms of pneumonia

 Illustration by JR Bee, Verywell 

Labs and Tests

While the physical exam can raise suspicion for pneumonia, the diagnosis can be strengthened using a variety of tests. Your healthcare provider 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 four 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 healthcare provider may use a device with a light and tiny camera placed down your throat. They 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 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 one to six 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 six-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 healthcare provider 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 practitioner 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 healthcare provider 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 healthcare provider 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 practitioner may decide to take some fluid from your airway for culture if your phlegm culture is negative and you are immunosuppressed or if you have a chronic illness requiring a 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 healthcare provider for a proper diagnosis. In most cases, once diagnosed, pneumonia can be well taken care of.

Frequently Asked Questions

  • Do you need a chest X-ray to diagnose pneumonia?

    It is recommended that any suspected case of pneumonia be confirmed with an X-ray. If the X-ray is inconclusive or more information is needed, other tests may be ordered, such as a CT scan. However, in some cases, a practitioner may prescribe antibiotics based on symptoms and a physical exam without a chest X-ray.

  • Can pneumonia be mistaken for other illnesses?

    Yes. There are other conditions with similar symptoms, some of which can be differentiated with a chest X-ray, blood tests, pulmonary functions tests (PFTs), and other investigations. These include:

  • How do you get a sputum culture to test for bacterial pneumonia?

    You may be asked to cough into a specimen cup to provide a sample of phlegm to be tested. If you’re unable to cough up a sample, your healthcare provider may perform a bronchoscopy in which a tube is inserted into your airway and a sample is sucked or swabbed out.

9 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Centers for Disease Control and Prevention. Pneumonia.

  2. Regunath H, Oba Y. Community-acquired pneumonia. In: StatPearls [Internet].

  3. Chmielewski PP, Strzelec B. Elevated leukocyte count as a harbinger of systemic inflammation, disease progression, and poor prognosis: a review. Folia Morphol (Warsz). 2018;77(2):171-8. doi:10.5603/FM.a2017.0101

  4. Meisner M. Update on procalcitonin measurements. Ann Lab Med. 2014;34(4):263-73. doi:10.3343/alm.2014.34.4.263

  5. MedlinePlus. Sputum culture.

  6. Andrade DC, Borges IC, Ivaska L, et al. Serological diagnosis of pneumococcal infection in children with pneumonia using protein antigens: A study of cut-offs with positive and negative controls. J Immunol Methods. 2016;433:31-7. doi:10.1016/j.jim.2016.02.021

  7. Centers for Disease Control and Prevention. Overview of yesting for SARS-CoV-2 (COVID-19).

  8. Wootton D, Feldman C. The diagnosis of pneumonia requires a chest radiograph (X-ray)–yes, no or sometimes? Pneumonia. 2014;5(S1):1-7. doi:10.15172/pneu.2014.5/464

  9. Black AD. Non-infectious mimics of community-acquired pneumonia. Pneumonia (Nathan). 2016;8:2. doi:10.1186/s41479-016-0002-1

By Tanya Feke, MD
Tanya Feke, MD, is a board-certified family physician, patient advocate and best-selling author of "Medicare Essentials: A Physician Insider Explains the Fine Print."