Laboratory Tests for Diagnosing Infection and Identifying Bacterial Agents
Although antibiotics are available that can destroy a wide spectrum of organisms, it would be preferable to use an antibiotic that can target the specific microorganism causing the pneumonia. Researchers, then, are looking for laboratory tests that would identify the specific organism or virus causing the pneumonia. Unfortunately, people harbor many bacteria, and sputum and blood tests are not always effective in distinguishing between harmless and harmful microscopic agents.
In severe cases, physicians particularly need to use invasive diagnostic measures to identify the infecting agent.
A sputum sample coughed from the lungs will yield physical information that will help the physician determine severity. In addition, only a sputum sample will reveal the infecting organism.
Typically, The physician first asks the patient to cough as deeply as possible to produce an adequate sputum sample. (A shallow cough produces a sample that usually only contains normal mouth bacteria.) A person who is not able to cough sufficiently may be asked to inhale a saline spray that helps produce an adequate sputum 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 the following indications:
- Presence of blood (an indication of infection).
- Color and consistency. If the sputum is opaque and colored yellow, green, or brown, then infection is likely. Clear, white, glistening sputum indicates no infection.
Blood tests may be used for the following:
- White blood cell count. High levels indicate infection.
- Blood cultures. They may be performed for detecting the specific organism causing the pneumonia, but are not often helpful in distinguishing harmful from harmless organisms. They are accurate in only 10% to 30% of cases, and 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 (immune factors that target specific foreign invaders), but it is not clear if they are accurate.
- Polymerase Chain Reaction. In some difficult cases, a polymerase chain reaction (PCR) may be performed. A PCR is able to make multiple copies of the genetic material (the RNA) of a virus or bacteria
A urine test (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. It may not be very useful in diagnosing S. pneumoniae as a cause of pneumonia in children, since the organism is so common anyway in this population, whether they have pneumonia or not.
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.
- A test performed on whole blood samples that uses a technique called polymerase chain reaction (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 Legionnaire’s disease.
- Specialized tests called DNA probes are being developed to detect these organisms in respiratory secretions.
- In addition, special stains and cultures are required to detect fungal infections
It may reveal the following:
- White areas in the lung called infiltrates, which indicate infection.
- Complications of pneumonia, including pleural effusions (fluid around the lungs) and abscesses.