Pneumonia

The signs and symptoms listed earlier suggest pneumonia to the physician. A chest radiograph is used to confirm the diagnosis (see Chest radiograph of a nursing home patient with pneumonia. The white area in the lung is the pneumonia. The white area in the middle and on the left is the heart and major blood vessels. (Courtesy of the author.) accompanying images). The next step is to determine the microbial cause of the pneumonia. To do this a blood sample is collected for culture. Only 6 to 10 percent of these cultures will be positive. If a sputum sample can be coughed up it is submitted to the laboratory for culture. When examined under the microscope sputum contains many white blood cells and few squamous epithelial cells (these are cells that line the mouth). Examination of the sputum under the microscope can often suggest the infecting microorganism. For example, if only pairs of small round cells that stain with a common solution are seen in a sputum specimen with lots of white blood cells, then the most likely cause of the pneumonia is Streptococcus pneumoniae.

Culture of the sputum and identification of the microorganisms isolated usually requires two to three days. Isolation of a specific microorganism allows the physician to select an antibiotic that will be most effective against this microorganism. Unfortunately most elderly persons cannot produce a sputum specimen for culture. Under these circumstances the attending physician has to choose antibiotics based on the most likely cause of the pneumonia. Some patients who are critically ill as a result of the pneumonia may have a diagnostic bronchoscopy. Here a small tube is passed down the trachea to the bronchi and samples of respiratory secretions are aspirated for culture. Occasionally a lung biopsy is necessary for diagnosis. Other tests that can be used in the diagnosis of pneumonia include a urine sample to detect Legionella antigen.

Antibodies can be detected to a variety of microorganisms that cause pneumonia. Two samples of blood are obtained, one early on in the course of the illness and the other two to six weeks later. Such tests are currently used to diagnose infection with Mycoplasma pneumoniae, Chlamydia pneumoniae, Legionella species, and a variety of viral infections. A four-fold or more increase in the amount of antibody against a specific microorganism between the acute and convalescent phase serum samples indicate that that microorganism caused the pneumonia.