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Although current antibiotics can destroy a wide spectrum of organisms, it is best to use an antibiotic that targets the specific one making a person sick. Unfortunately, people carry many bacteria, and sputum and blood tests are not always effective in distinguishing between harmless and harmful kinds.
In severe cases, a doctor needs to use invasive diagnostic measures to identify the cause of the infection. These tests are not commonly performed in outpatients. Standard lab tests used to help diagnose pneumonia include:
Sputum Tests. The color of the mucus (sputum) sample coughed up from the lungs can reveal the severity of the disease. Only a sputum sample will reveal the organism causing the infection.
The patient coughs as deeply as possible to bring up mucus from the lungs, since a shallow cough produces a sample that usually only contains normal mouth bacteria. Some people may need to inhale a saline spray to produce an adequate sample. In some cases, a tube will be inserted through the nose into the lower respiratory tract to trigger a deeper cough.
The physician will check the sputum for:
- Blood, which means an infection is present.
- Color and consistency: If it is yellow, green, or brown, an infection is likely.
The sputum sample is sent to the laboratory, where it is analyzed for the presence of bacteria and to determine whether the bacteria are Gram-negative or Gram-positive.
Blood Tests. The following blood tests may be performed:
- White blood cell count (WBC). High levels indicate infection.
- Blood cultures. Cultures are done to determine the specific organism causing the pneumonia, but they usually cannot distinguish between harmless and dangerous organisms. They are accurate in only 10 - 30% of cases. Their use is generally limited to severe cases.
- Detection of antibodies to S. pneumoniae. Antibodies are immune factors that target specific foreign invaders. Antibodies that react with mycoplasma or chlamydia are not present early enough in the course of pneumonia to allow for prompt diagnosis by this method.
- C-reactive protein or procolacitonin. Not generally recommended but may help identify which patients with respiratory symptoms have pneumonia and/or need to be hospitalized.
- Polymerase Chain Reaction (PCR). In some difficult cases, PCR may be performed. The test makes multiple copies of the genetic material (RNA) of a virus or bacteria to make it detectable. PCR is useful for identifying certain atypical bacteria strains, including mycoplasma and Chlamydia pneumoniae, and possibly, Haemophilus influenzae type b, but it is expensive. One study found that using a real-time PCR test may help quickly diagnose Pneumocystitis pneumonia in HIV-positive patients.
Urine Tests. Urine antigen tests for Legionella pneumophila (Legionnaires' disease) and Streptococcus pneumoniae may be performed in patients with severe CAP. The S. pneumoniae test takes only 15 minutes and may identify up to 77% of pneumonia cases and rule out S. pneumoniae infection in 98% of patients. It may not be useful in children.
Invasive Tests. In critically-ill patients with ventilator-associated pneumonia, doctors have tried sampling fluid taken from the lungs or trachea. These techniques enabled the physicians to identify the pneumonia-causing bacteria and start the appropriate antibiotics. However, this made no difference in the length of stay in the ICU or hospital, and there was no significant difference in outcome.
Chest X-Rays and Other Imaging Techniques
X-Rays. A chest x-ray is nearly always taken on a patient admitted to the hospital to confirm a pneumonia diagnosis. This does not need to be repeated if the patient gets better.
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Review Date: 04/13/2010
Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine,
Harvard Medical School; Physician, Massachusetts General Hospital.
Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M.,
Inc.
A.D.A.M., Inc. is accredited by URAC, also known as the American Accreditation HealthCare Commission (www.urac.org)
