Patients with complaints of dyspnea or physical signs of respiratory distress (tachypnea, use of accessory muscles, wheezing, etc.) should have oxygen saturation measured by pulse oximetry. In young, otherwise healthy, mildly ill, ambulatory patients, no further ancillary testing may be necessary. If a patient requires admission, additional tests recommended include complete blood count and determination of serum electrolyte, blood urea nitrogen, creatinine, and glucose levels. 9 Evaluation of arterial blood gas is indicated if patients are hypoxic or have moderate to severe respiratory distress. No single set of recommendations for diagnostic testing can encompass all patients, and additional ancillary studies should be obtained according to appropriate indications.
In patients with fever, cough, and radiographic abnormalities, the etiology of the infection is confirmed by identification of a pathogenic organism from the blood, sputum, or pleural fluid. Atypical agents may be demonstrated by a variety of sophisticated laboratory techniques, including evaluation of titers from acute and convalescence sera or by direct fluorescent antibody testing. In hospitalized patients with CAP, about 10 percent will have a positive blood culture, most of which are pneumococcal.10 The incidence of positive blood cultures in nonhospitalized patients with CAP is lower, pathogen identification does not alter treatment, and the overwhelming majority of patients respond to empiric antibiotic treatment.11 Thus, blood cultures are recommended only for patients who require hospitalization.
The value of sputum examination in CAP has been debated.9 The yield of a sputum Gram stain is reduced in the 10 to 30 percent of patients unable to produce sputum, in the 15 to 30 percent of patients who have already received antibiotic therapy, and in the 30 to 65 percent of patients who have a negative Gram stain despite a positive culture. Taken together, the sputum Gram stain is positive in only 10 to 40 percent of cases with CAP. The test is noninvasive and inexpensive, however, so it is often advocated.91 and 11 Pneumococcal pneumonia may be suspected in patients with more than 10 gram-positive, lancet-shaped diplococci from an adequate sputum sample. Haemophilus influenzae may also be occasionally demonstrated on Gram stain. There are few indications for performing transtracheal aspiration or bronchial washings in the emergency department.
The differential diagnosis of patients with cough and radiographic abnormality includes a number of disorders, such as lung cancer, tuberculosis, pulmonary embolism, chemical or hypersensitivity pneumonitis, connective tissue disorders, granulomatous disease, and fungal infections.
A chest radiograph may provide clues to the underlying organism.17 Pneumococcal pneumonia typically presents with lobar segmental pneumonia. Occasionally, patients may present with the so-called round infiltrate of pneumococcal pneumonia. In general, patients with bacterial pneumonia are more likely to have unilobar or focal infiltrates than patients with viral or atypical pneumonia. Hilar adenopathy is more common in patients with atypical pneumonia. Pleural effusions occur more commonly in patients with bacterial pneumonia, although occasionally patients with viral pneumonia or atypical agents may have small effusions. Cavitary lesions occur in patients with bacterial or tuberculous lesions. Lung abscesses are rare complications of pneumonia in the antibiotic era, but they occur due to S. aureus or Klebsiella. Pneumonia may mimic the appearance of lung masses particularly when the pneumonia is pneumococcal and staphylococcal. Other atypical pneumonia such as Q fever and tularemia may present with discrete masses.
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