Pneumonia - Diagnosis

Laboratory Tests for Diagnosing Infection and Identifying Bacterial Agents

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.

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In severe cases, a doctor needs to use invasive diagnostic measures to identify cause of the infection. Standard lab tests used to help diagnose pneumonia include:

Sputum Tests. Looking at the mucus (sputum) sample coughed up from the lungs tells the doctor how sever the disease is. Only a sputum sample will reveal the infecting organism.

The patient coughs as deeply as possible. (A shallow cough produces a sample that usually only contains normal mouth bacteria.) Some patients may need to inhale a saline spray to help them produce an adequate sample. In some cases, a tube will be inserted through the nose down into the lower respiratory tract to induce a deeper cough.

The physician will check the sputum for:

Blood, which means there is an infectin
Color and consistency -- if it is yellow, green, or brown, infection is likely

A good sputum sample is sent to the laboratory for analysis to look for the presence of bacteria and determine if they are gram-negative or 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 detect the specific organism causing the pneumonia, but they usually can not distinguish between harmless and dangerous organisms. They are accurate in only 10% to 30% of cases. Their use should generally be limited to severe cases.
Detection of antibodies to S. pneumoniae. Researchers are using specialized techniques to detect antibodies to S. pneumoniae. Antibodies are immune factors that target specific foreign invaders. It is unclear if these techniques are accurate.
Polymerase Chain Reaction (PCR). In some difficult cases, PCR may be performed. A test makes multiple copies of the genetic material (the RNA) of a virus or bacteria so it becomes detectable.

Urine Tests. A urine test called NOW can detect S. pneumonia within 15 minutes. It may identify up to 77% of pneumonia cases and may rule out the infection in 98% of patients who do not have S. pneumonia. However, it may not be very useful in diagnosing S. pneumoniae as a cause of pneumonia in children.

Laboratory Tests for Less Common Organisms

If uncommon organisms, such as Legionella, Mycoplasma, and Chlamydia organisms, are strongly suspected, more advanced laboratory tests may be used:

Specialized techniques can detect antibodies to the organisms in blood samples, but these antibodies, such as those responding to Mycoplasma or Chlamydia, are not present early enough in the course of pneumonia to permit prompt diagnosis and treatment.
PCR is useful for identifying certain atypical strains, including Mycoplasma and Chlamydiapneumoniae and possibly Haemophilus influenzae type b, but it is expensive.
A urine test can be used to diagnose some cases of Legionnaires' disease.
Specialized tests called DNA probes are being developed to detect these organisms in respiratory secretions.