Am Fam Physician. 2022;105(6):625-630
Related Letter to the Editor: Recognizing Differing Evidence in the Literature
Author disclosure: No relevant financial relationships.
Community-acquired pneumonia (CAP) is a common condition with a hospitalization rate of about 2% in people 65 years or older and is associated with a 30-day mortality rate of 6% in hospitalized patients. In studies conducted before the COVID-19 pandemic, a bacterial pathogen was identified in 11% of patients, a viral pathogen in 23% of patients, and no organism in 62% of patients. Certain signs and symptoms can be helpful in diagnosing CAP and selecting imaging studies. Diagnosis is usually made with a combination of history, physical examination, and findings on chest radiography, lung ultrasonography, or computed tomography. Procalcitonin measurement is not recommended. CRB-65 (confusion, respiratory rate, blood pressure, 65 years of age) is a well-validated risk stratification tool in the primary care setting and does not require laboratory testing. For outpatients without comorbidities, treatment with amoxicillin, doxycycline, or a macrolide is recommended (the latter only in areas where pneumococcal resistance to macrolides is less than 25%). In outpatients with comorbidities and inpatients with nonsevere pneumonia, a combination of a beta-lactam or third-generation cephalosporin plus a macrolide, or monotherapy with a respiratory fluoroquinolone is recommended. Patients should be treated for methicillin-resistant Staphylococcus aureus or Pseudomonas infection only if they present with risk factors for those pathogens. All adults 65 years or older or those 19 to 64 with underlying conditions should receive the 20-valent pneumococcal conjugate vaccine alone or the 15-valent pneumococcal conjugate vaccine followed by 23-valent pneumococcal polysaccharide vaccine one year later. The 13-valent pneumococcal conjugate vaccine is no longer recommended for routine administration. The Centers for Disease Control and Prevention recommends vaccination against influenza and SARS-CoV-2 viruses for all adults.
Epidemiology
INCIDENCE
The annual incidence of CAP is 248 cases per 100,000 adults. However, this increases to 634 cases per 100,000 in adults 65 to 79 years of age and 16,430 cases per 100,000 in adults 80 years or older.1
Hospitalization rates for CAP increase with advancing age.1–3 A systematic review of population-based studies found that the rate was 1,830 per 100,000 adults 65 years or older and 199 per 100,000 adults younger than 65 years.2
The hospitalization rate for CAP is nine times higher in people with comorbid chronic obstructive pulmonary disease.3
Overall, the mortality rate for patients hospitalized with CAP is 6% at 30 days, even after initial clinical improvement. In hospitalized patients who do not improve initially or have unresolving pneumonia, the mortality rate is 34% at 30 days.4
There are racial and ethnic disparities in the incidence of CAP. One population-based study found that the annual incidence of CAP was two to four times higher in Black adults than in White adults.5
MICROBIOLOGY
A prospective, multicenter, population-based, active surveillance study sponsored by the Centers for Disease Control and Prevention analyzed radiograph and culture results from 2,488 inpatient adults to determine the incidence and microbiologic causes of CAP requiring hospitalization. An organism was not identified in 62% of these patients. A virus was present in 23% of patients and a bacterium in 11% of patients.1
Another study examined the clinical and laboratory data of 323 inpatient adults with radiographically confirmed CAP and tested sputum and endotracheal aspirates to identify pathogens.6 Samples were tested for 26 bacterial and viral pathogens using culture and polymerase chain reaction analysis. A pathogen was detected in 87% of patients; 56% had bacteria alone, 25% had a combination of bacteria and viruses, and 6% had viruses alone.6 Among bacterial causes, the most common were Haemophilus influenzae (40%) and Streptococcus pneumoniae (36%). Mycoplasma and Legionella species were the most common atypical bacteria, and rhinovirus (13%) and influenza virus (7%) were the most common viral pathogens.6
The incidence of Mycoplasma infection varies cyclically over years, and a species of Legionella is present in 3% of patients hospitalized for CAP.7
SARS-CoV-2 infection has been a major cause of CAP during the pandemic, with data on prevalence continuing to change with emergence of disease variants and patient vaccination status; its contribution as a cause of CAP in the future is unclear.8
False-negative results for viral pathogens are common in CAP. Samples from the lower respiratory tract have a greater diagnostic yield than nasopharyngeal or oropharyngeal samples, but obtaining lower respiratory tract samples is not usually feasible in the outpatient setting.9
Diagnosis
The differential diagnosis of CAP includes asthma or chronic obstructive pulmonary disease exacerbation, bronchitis, congestive heart failure, gastroesophageal reflux disease, lung cancer, and pulmonary embolism.
SIGNS AND SYMPTOMS
Patient-reported symptoms often include cough, subjective fever, chills, sputum production, and dyspnea.
A meta-analysis found that the following clinical signs and symptoms had the highest diagnostic odds ratios for pneumonia: physician's overall clinical impression (diagnostic odds ratio = 11.5), egophony (6.5), any abnormal vital sign (6.0), any abnormal lung finding (3.2), tachypnea (3.1), and measured fever (3.3).10
A systematic review found that adults with an acute respiratory tract infection were unlikely to have CAP if they presented with normal vital signs and normal pulmonary examination findings (negative likelihood ratio = 0.1).11
Fever is not always present in patients with bacteremia.12
Clinicians should determine whether patients meet criteria for severe CAP (Table 113) to inform diagnostic testing and antibiotic choice.14
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