- Spectrum of illness may vary from mild and self-limited to severe pneumonia.
- Onset often gradual with delayed presentation
- Sore throat and hoarseness may precede cough by a week or more, giving biphasic appearance to illness (uncommon in Legionella, less common in Mycoplasma, Streptococcus pneumoniae, and Haemophilus influenzae).
- Dry cough
- Low-grade fever (usually early in illness)
- Sinus congestion
- Altered mental status
- General appearance usually nontoxic, unless extremely ill
- Diminished breath sounds
- Crackles or wheezing
- Bronchial breath sounds
- Percussion dullness and egophony less sensitive but more specific for pneumonia
- Pharyngeal erythema (without exudates)
- Retropharyngeal lymphoid granulation
- Usually more severe disease in older adults, and more common in the elderly who also have concomitant medical problems
- Elderly patients are less likely to exhibit respiratory symptoms with pneumonia and may present with altered mental status or history of falls.
Diagnostic Tests and Interpretation
ImagingInitial Imaging Approach
- Multiple unreliable laboratory methods for diagnosis including culture, antigen detection, serology, PCR
- Leukocyte count usually normal or low, but may be mildly elevated
- Blood cultures recommended if toxic and requiring ICU admission; otherwise not likely to be helpful
- Culture has traditionally been the gold standard diagnostic method (1):
- Many limitations include technical complexity, limited availability, and variable yield (1)
- Most easily cultured in HL or HEp2 cells (culture is 10–80% sensitive and >95% specific) (2)
- Testing with microimmunofluorescence (MIF) is recommended by the Centers for Disease Control (CDC), as enzyme immunoassay testing is less specific. However, MIF testing is not standardized for C. pneumoniae and may also lack specificity and sensitivity (2):
- 4-fold increase in IgG titer diagnostic of acute infection (10–100% sensitivity) (2)
- Presence of IgM antibody (≥1:16) (1)
- Single IgG titers are discouraged (1)
- Complement fixation for Chlamydia is widely available but cannot distinguish C. pneumonia from Chlamydophila psittaci.
- PCR from pharyngeal swab or bronchioalveolar lavage specimen (30–95% sensitivity, >95% specificity) (2)
- Patients with suspected community-acquired pneumonia (CAP) who are more than mildly ill should be evaluated with a chest x-ray (CXR) (2)[A]. The CXR may be abnormal even in clinically mild disease.
- Variable radiographic abnormalities include unilateral and bilateral infiltrates and pleural effusions. Single, subsegmental funnel-shaped or circumscribed infiltrate is common.
Although serology is 95% specific, definitive diagnosis requires a positive culture or PCR testing (2
- Other causes of atypical pneumonia, including M. pneumoniae and L. pneumophila
- Other bacterial causes of pneumonia, including S. pneumoniae. H. influenzae, Moraxella catarrhalis, and Staphylococcus aureus
- Respiratory viruses: Adenovirus, influenza A, influenza B, parainfluenza virus, and respiratory syncytial virus
- Endemic fungal pathogens: Blastomycosis, coccidioidomycosis, histoplasmosis
- Bioterrorism agents: Anthrax, plague, tularemia
- Conditions that mimic CAP: Acute respiratory disease syndrome, atelectasis, idiopathic pulmonary fibrosis, neoplasm, pulmonary embolus, sarcoidosis, congestive heart failure
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