Diagnostic Difficulties in CAP. It is important to determine whether the cause of CAP is a bacterium, atypical bacterium, or virus, because they require different treatments. In children, for example, S. pneumoniae is the most common cause of pneumonia, but respiratory syncytial virus may also cause the disease. Although symptoms may differ, they often overlap, which can make it difficult to identify the organism by symptoms alone. The cause of CAP is found in only about half of cases.
Nevertheless, in many cases of mild-to-moderate CAP, the physician is able to diagnose and treat pneumonia based solely on a medical history and physical examination.
Diagnostic Difficulties with Hospital-Acquired (Nosocomial) Pneumonia. Diagnosing pneumonia is particularly difficult in hospitalized patients for a number of reasons:
- Many hospitalized patients have similar symptoms, including fever or abnormal x-rays.
- In hospitalized patients, sputum or blood tests often show bacteria or other organisms, but such agents do not necessarily indicate pneumonia.
Medical and Personal History
The patient's history is an important part of making a pneumonia diagnosis. Patients should report any of the following:
- Alcohol or drug abuse
- Exposure to people with pneumonia or other respiratory illnesses (such as tuberculosis)
- History of smoking
- Occupational risks
- Recent or chronic respiratory infection
- Recent antibiotic therapy
- Recent outpatient wound care
- Home infusion therapy or dialysis
- Recent travel
- Exposure to birds, bats, or other animals
Use of the Stethoscope. The most important diagnostic tool for pneumonia is the stethoscope. Sounds in the chest that may indicate pneumonia include:
- Rales, a bubbling or crackling sound. Rales strongly suggest pneumonia.
- Rhonchi, abnormal rumblings indicating that there is sputum in the large airways.
- A dull thud. The physician will use a test called percussion, in which the chest is tapped lightly. A dull thud, instead of a hollow drum-like sound, indicates certain conditions that suggest pneumonia. These conditions include consolidation (in which the lung becomes filled with fluid and pus) and pleural effusion (fluid build-up in the space between the lungs and the lining around it).
Pulse Oximetry. A pulse oximetry test can help determine if a patient needs hospital care. A simple test using a device on the fingertip or earlobe, this determines the amount of oxygen in the blood.
Laboratory Tests for Diagnosing Infection and Identifying Bacteria
Although current antibiotics can attack 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. The Infectious Diseases Society of America/American Thoracic Society (IDSA/ATS) recommends diagnostic testing when it may impact the choice of antibiotic and in patients with a high likelihood of accurate results (sicker patients). Many of these laboratory tests take 4 - 5 days or longer to complete, however, and therapy should be started before results are available.
Although viral pneumonia accounts for more than a third of CAP cases, it can sometimes be difficult to determine if the pneumonia is viral or bacterial in origin.
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 are used to help diagnose pneumonia.
Sputum Tests. A sputum sample may 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 suggests an infection is present.
- Color and consistency: If it is gray, 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. 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 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 (DNA) of a virus or bacteria to make it detectable. PCR is useful for identifying certain atypical bacteria strains, including mycoplasma and Chlamydia pneumoniae, 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 helpful in some patients with severe CAP.
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.
A chest x-ray may reveal the following:
- Complications of pneumonia, including pleural effusions and abscesses
- White areas in the lung called infiltrates, which indicate infection
Other Imaging Tests. Computed tomography (CT) scans or magnetic resonance imaging (MRI) scans may be useful in some circumstances, especially when:
- A lung tumor is suspected
- Complications occur
- Patients do not respond to antibiotics
- Patients have other serious health problems
- Pulmonary embolism is suspected
- X-ray results are unclear
Click the icon to see an image of a CT scan.
CT and MRI can help detect tissue damage, abscesses, and enlarged lymph nodes. They can also detect some tumors that block bronchial tubes. No imaging technique can determine the actual organism causing the infection. However, features on the CT scan of patients with certain forms of pneumonia -- for example, that are caused by Legionella pneumophila -- are usually different from features produced by other bacteria in the lungs.
Invasive Diagnostic Procedures
Invasive diagnostic procedures may be necessary when:
- Patients have AIDS or other immune problems
- Patients have life-threatening complications
- Standard treatments have failed for no known reason
Thoracentesis. If a doctor detects pleural effusion during the physical exam or on an imaging study, and suspects that pus (empyema) is present, a thoracentesis is performed.
- Fluid in the pleura is withdrawn using a long thin needle inserted between the ribs.
- The fluid is then sent to the lab for multiple tests.
Complications of this procedure are rare, but they can include collapsed lung, bleeding, and infection.
Bronchoscopy. Bronchoscopy is an invasive test to examine respiratory secretions. It is not usually needed in patients with community-acquired pneumonia, but it may be appropriate for patients with a severely compromised immune system who need immediate diagnosis, or in patients whose condition has worsened during treatment.
A bronchoscopy is done in the following way:
- The patient is given a local anesthetic, oxygen, and sedatives.
- The physician inserts a fiber optic tube into the lower respiratory tract through the nose or mouth.
- The tube acts like a telescope into the body, allowing the physician to view the windpipe and major airways and look for pus, abnormal mucus, or other problems.
- The doctor removes specimens for analysis and can also treat the patient by removing any foreign bodies or infected tissue encountered during the process.
Click the icon to see an image of a bronchoscopy.
Bronchoalveolar lavage (BAL) may be done at the same time as bronchoscopy. This involves injecting high amounts of saline through the bronchoscope into the lung and then immediately sucking the fluid out. The fluid is then analyzed in the laboratory. Studies find BAL to be an effective method for detecting specific infection-causing organisms.
The procedure is usually very safe, but complications can occur. They include:
- Allergic reactions to the sedatives or anesthetics
- Asthma attacks in susceptible patients
Lung Biopsy. In very severe cases of pneumonia or when the diagnosis is unclear, particularly in patients with a damaged immune system, a lung biopsy may be required. A lung biopsy involves taking some tissue from the lungs and examining it under a microscope.
Lung Tap. This procedure typically uses a needle inserted between the ribs to draw fluid out of the lung for analysis. It is known by a number of names, including:
- Lung aspiration
- Lung puncture
- Thoracic puncture
- Transthoracic needle aspiration
- Percutaneous needle aspiration
- Needle aspiration
This is a very old procedure that is not done often anymore because it is invasive and poses a slight risk for collapsed lung. Some experts argue, however, that a lung tap is more accurate than other methods for identifying bacteria, and the risk it poses is slight. Given the increase in resistant bacteria, they believe its use should be reconsidered in young people.
An infectious disease specialist may need to be consulted in severe or difficult cases.
Ruling Out Other Disorders that Cause Coughing or Affect the Lung
Common Causes of Persistent Coughing. Persistent coughing is nearly always temporary and harmless when other symptoms, such as fever, are not present. The four most common causes of persistent coughing are:
- Chronic bronchitis
- Gastroesophageal reflux disease (GERD)
- Postnasal drip
Other common causes of chronic cough include heavy smoking or the use of blood pressure drugs known as ACE inhibitors.
Acute Bronchitis. Acute bronchitis is an infection in the passages that carry air from the throat to the lung. The infection causes a cough that produces phlegm. Acute bronchitis is almost always caused by a virus and usually clears up on its own within a few days. In some cases, acute bronchitis caused by a cold can last for several weeks.
Chronic Bronchitis. Chronic bronchitis causes shortness of breath and is often accompanied by infection, mucus production, and coughing, but it is a long-term and irreversible condition. The same bacteria and viruses that cause pneumonia can cause an infection in patients with chronic bronchitis. However, infections involve only the airways leading to the lungs, and not the lung tissue itself. The two disorders may share the same symptoms, such as:
- Sputum production
There are significant differences between chronic bronchitis and pneumonia:
- Patients with bronchitis are less likely to have wheezing, shortness of breath, chills, very high fevers, and other signs of severe illness.
- Those with pneumonia often cough up heavy sputum, which may contain blood.
- X-rays of patients with pneumonia show inflammation and other changes in the lung tissue that are not seen in patients with chronic bronchitis.
Asthma. In asthma, the cough is usually accompanied by wheezing and occurs mostly at night or during activity. Fever is rarely present (unless the patient also has an infection). Asthma symptoms from occupational causes can lead to persistent coughing, which is usually worse during the work week. Tests -- the methacholine inhalation challenge and pulmonary function studies -- may be effective in diagnosing asthma.
Other Disorders that Affect the Lung. Many conditions mimic pneumonia, particularly in hospitalized patients. They include:
- Acute respiratory distress syndrome (ARDS)
- Atelectasis, a partial collapse of lung tissue
- Bronchiectasis, an irreversible widening of the airways that is usually associated with birth defects, chronic sinus or bronchial infection, or blockage
- Heart failure (if it affects the left side of the heart, fluid build-up can occur in the lungs and cause persistent cough, shortness of breath, and wheezing)
- Interstitial pulmonary fibrosis, a non-infectious inflammation of the lung that causes damage and scarring
- Lung cancer
- Severe allergic reactions, such as reactions to drugs
Ruling Out Causes in Children. Important causes of coughing in children at different ages include:
- Sinusitis in children 18 months - 6 years