Despite advances in medical science, pneumonia remains a major cause of morbidity and mortality. In 2017, 49,157 patients in the United States died from the disease.1 Pneumonia can be classified as community-acquired, hospital-acquired, or ventilator-associated. Another category, healthcare-associated pneumonia, was included in an earlier Infectious Diseases Society of America (IDSA) and American Thoracic Society (ATS) guideline but was removed from the 2016 guideline because there was no clear evidence that patients diagnosed with healthcare-associated pneumonia were at higher risk for harboring multidrug-resistant pathogens.2 This review is the first of 2 articles focusing on the management of community-acquired pneumonia (CAP). Here, we review CAP epidemiology, microbiology, predisposing factors, and diagnosis; current treatment and prevention of CAP are reviewed in a separate article.
Definition and Epidemiology
CAP is defined as an acute infection of the lungs that develops in patients who have not been hospitalized recently and have not had regular exposure to the health care system.3 A previously ambulatory patient who is diagnosed with pneumonia within 48 hours after admission also meets the criteria for CAP. Approximately 4 to 5 million cases of CAP are diagnosed in the United States annually.4 About 25% of CAP patients require hospitalization, and about 5% to 10% of these patients are admitted to the intensive care unit (ICU).5 In-hospital mortality is considerable (~10% in population-based studies),6 and 30-day mortality was found to be as high as 23% in a review by File and Marrie.7 CAP also confers a high risk of long-term morbidity and mortality compared with the general population who have never had CAP, irrespective of age.8
Numerous microorganisms can cause CAP. Common causes and less common causes are delineated in Table 1. Until recently, many studies had demonstrated that pneumococcus was the most common cause of CAP. However, in the CDC Etiology of Pneumonia in the Community (EPIC) study team’s 2015 prospective, multicenter, population-based study, no pathogen was detected in the majority of patients diagnosed with CAP requiring hospitalization. The most common pathogens they detected were rhinovirus (9%), followed by influenza virus (6%) and pneumococcus (5%).9 Factors considered to be contributing to the decrease in the percentage of pneumococcus in patients diagnosed with CAP are the widespread use of pneumococcal vaccine and reduced rates of smoking.10,11
Most people diagnosed with CAP have 1 or more predisposing factors (Table 2).12,13 Patients who develop CAP typically have a combination of these predisposing factors rather than a single factor. Aging, in combination with other risk factors, increases the susceptibility of a person to pneumonia.
Clinical Signs and Symptoms
Symptoms of CAP include fever, chills, rigors, fatigue, anorexia, diaphoresis, dyspnea, cough (with or without sputum production), and pleuritic chest pain. There is no individual symptom or cluster of symptoms that can absolutely differentiate pneumonia from other acute respiratory diseases, including upper and lower respiratory infections. However, patients presenting with the constellation of symptoms of fever ≥ 100°F (37.8°C), productive cough, and tachycardia is more suggestive of pneumonia.14 Abnormal vital signs include fever, hypothermia, tachypnea, tachycardia, and oxygen desaturation. Auscultation of the chest reveals crackles or other adventitious breath sounds. Elderly patients with pneumonia report a significantly lower number of both respiratory and nonrespiratory symptoms compared with younger patients. Clinicians should be aware of this phenomenon to avoid delayed diagnosis and treatment.15