Microbiology

Although many bacteria, viruses, fungi, and protozoa can cause CAP, most cases are caused by relatively few pathogens. In >50% of cases, a specific etiology is never determined.

• Typical bacterial pathogens include S. pneumoniae, Haemophilus influenzae, Staphylococcus aureus, and gram-negative bacteria such as Klebsiella pneumoniae and Pseudomonas aeruginosa.
  • – The incidence of pneumococcal pneumonia is decreasing because of the increasing use of pneumococcal vaccines.
• Atypical organisms include Mycoplasma pneumoniae, Chlamydia pneumoniae, Legionella spp., and respiratory viruses (e.g., influenza viruses, adenoviruses, human metapneumovirus, respiratory syncytial viruses).
  • A virus may be responsible for a large proportion of CAP cases that require hospital admission, even in adults.
  • Of CAP cases, 10-15% are polymicrobial and involve a combination of typical and atypical organisms.
  • The incidences of cases due to M. pneumoniae and C. pneumoniae are increasing, particularly among young adults.
• Involvement of anaerobes, which play a significant role in CAP only when aspiration precedes presentation by days or weeks, often results in significant empyemas.

Epidemiology

CAP affects >5 million adults each year in the United States, 80% of whom are treated on an outpatient basis. CAP causes >55,000 deaths annually and is associated with an overall yearly cost of ∼$17 billion.

• The incidence of CAP is highest at the extremes of age (i.e., <4 and >60 years).
• Risk factors for CAP include alcoholism, asthma, immunosuppression, institutionalization, and an age of ≥70 years (vs 60-69 years).
• Many factors-e.g., tobacco smoking, chronic obstructive pulmonary disease, colonization with methicillin-resistant S. aureus (MRSA), recent hospitalization or antibiotic therapy-influence the types of pathogens that should be considered in the etiologic diagnosis.

Clinical Manifestations

Pts frequently have fever, chills, sweats, cough (either nonproductive or productive of mucoid, purulent, or blood-tinged sputum), pleuritic chest pain, and dyspnea.

• Other common symptoms include nausea, vomiting, diarrhea, fatigue, headache, myalgias, and arthralgias.
• Elderly pts may present atypically, with confusion but few other manifestations.
• Physical examination often reveals tachypnea; increased or decreased tactile fremitus; dull or flat percussion reflecting consolidation and pleural fluid, respectively; crackles; bronchial breath sounds; or a pleural friction rub.

Diagnosis

Both confirmation of the diagnosis and assessment of the likely etiology are required. Although no data have demonstrated that treatment directed at a specific pathogen is superior to empirical treatment, an etiologic diagnosis allows narrowing of the empirical regimen, identification of organisms with public safety implications (e.g., Mycobacterium tuberculosis, influenza virus), and monitoring of antibiotic susceptibility trends.

• CXR is often required to differentiate CAP from other conditions, particularly since the sensitivity and specificity of physical examination findings for CAP are only 58% and 67%, respectively.
  • CT of the chest may be helpful for pts with suspected postobstructive pneumonia or suspected cavitary disease.
  • Some radiographic patterns suggest an etiology; e.g., pneumatoceles suggest S. aureus.
• Sputum samples must have >25 WBCs and <10 squamous epithelial cells per high-power field to be appropriate for culture. The sensitivity of sputum cultures is highly variable; in cases of proven bacteremic pneumococcal pneumonia, the yield of positive cultures from sputum samples is ≤50%.
• Blood cultures are positive in 5-14% of cases, most commonly yielding S. pneumoniae. Blood cultures are optional for most CAP pts but should be performed for high-risk pts (e.g., pts with chronic liver disease or asplenia).
• Urine antigen tests for S. pneumoniae and Legionella pneumophila type 1 can be helpful.
• PCR of nasopharyngeal swabs has become the standard for diagnosis of respiratory viral infection and is also useful for detection of many atypical bacteria.
• Serology: A fourfold rise in titer of specific IgM antibody can assist in the diagnosis of pneumonia due to some pathogens; however, the time required to obtain a final result and difficulty of interpretation limits the clinical utility of serology.
• Biomarkers: Serial measurements of C-reactive protein (CRP) levels in serum may help identify worsening disease or treatment failure. Measurement of serum procalcitonin (PCT) levels can help distinguish bacterial from viral infection, determine the need for antibacterial therapy, or determine when to discontinue treatment.