Atypical versus Typical Pneumonia

Historically, some clinicians distinguished typical and atypical CAP based on clinical presentation. CAP caused by “atypical” organisms (Mycoplasma pneumoniae, Chlamydia pneumoniae, Legionella species, and viruses) was thought to be characterized by a prior viral-like syndrome (myalgias, arthralgias, and sore throat), subacute time course, nonproductive cough, absence of pleurisy and rigors, lower fever, and absence of consolidation on auscultation compared with CAP caused by “typical” organisms (Streptococcus pneumoniae, Haemophilus influenzae, Staphylococcus aureus, and gram-negative bacteria). When studied systematically, however, these characteristics were no more common in atypical than in typical pneumonias. In addition, comparing laboratory data and chest radiographs failed to discriminate between patients with typical and atypical pneumonias. Clinical demographics may give clues to the cause, but definitive diagnosis depends on laboratory and microbiologic testing to identify a specific pathogen.

Pathogens

Many different organisms cause CAP. Although the isolation rate has decreased, S. pneumoniae (pneumococcus) remains the most frequently identified cause of CAP. Yet despite new diagnostic techniques, the cause of CAP remains unknown in up to 45% of cases. The most common pathogens identified (in descending order of frequency) are S. pneumoniae, H. influenzae, M. pneumoniae, C. pneumoniae, respiratory viruses, and Legionella species. The most common organisms in patients admitted to the intensive care unit (ICU) vary from all patients with CAP. For example, a review of nine studies that included 890 patients with CAP who were admitted to the ICU between 1999 and 2001 reported that the most common pathogens in the ICU population were (in descending order of frequency) S. pneumoniae, Legionella species, H. influenzae, Enterobacteriaceae species, S. aureus, and Pseudomonas species. About 20% of CAP in ICU patients may be due to atypical, most likely Legionella, species.

Because initial CAP treatment is often empirical, knowledge of the most likely pathogens is vital. The causative organism may be suspected based on the patient’s risk factors or comorbid illnesses (Table 65.1). S. pneumoniae, however, remains the most common cause in all groups including those with human immunodeficiency virus (HIV). Severe recurrent pneumonias with S. pneumoniae or H. influenzae suggest an underlying immunocompromised state—for example, HIV infection or multiple myeloma. Legionella infections by non-pneumophila species may also be more common in immunosuppressed patients and warrant assessment of the patient’s immune status. Viruses may cause up to 30% of CAP. Although the most prevalent respiratory virus is influenza, others such as adenovirus, respiratory syncytial virus (RSV), and parainfluenza are also common.

Chest Radiograph

When CAP has a typical clinical presentation, an infiltrate on chest radiograph confirms the diagnosis. The chest radiograph may be normal, however, if taken in the first 24 hours of a bacterial pneumonia or in the setting of severe neutropenia. Likewise, an estimated 30% to 40% of patients with Pneumocystis jiroveci (previously named P. carinii) pneumonia may have normal-appearing chest radiographs at presentation.

Gram Stain and Culture of Sputum

Although its value has been widely debated, a Gram stain and culture of expectorated sputum may establish the cause of CAP and guide initial therapy. Although only 60% to 70% of patients with CAP can produce sputum on hospital admission, these sputum tests are non-invasive and relatively inexpensive. In patients eventually diagnosed with pneumonia resulting from S. pneumoniae, 62% to 89% demonstrate lancet-shaped gram-positive diplococci on initial Gram stain. Alternatively, finding many polymorphonuclear neutrophils and no organisms on Gram stain suggests infection with M. pneumoniae, C. pneumoniae, or Legionella species.

Sputum cultures must be interpreted in light of the findings of the corresponding sputum Gram stain. Isolation of a predominant organism by culture is clinically compelling when compatible with the Gram stain findings. Isolation of a predominant organism that is not part of the normal respiratory flora may be useful in modifying antibiotics. Similarly, failure to find organisms such as S. aureus or gram-negative bacilli on Gram stain or in culture suggests they are truly absent. Because of difficulty in isolation, cultures for M. pneumoniae, C. pneumoniae, Legionella species, and respiratory viruses are not routinely performed.

The clinical value of a sputum Gram stain and culture depends largely on the quality of the sputum specimen. Having > 25 polymorphonuclear neutrophils and < 10 squamous epithelial cells per low-power field can determine whether the sputum is an adequate sample from the lower respiratory tract. Prior antibiotic therapy decreases the usefulness of sputum cultures, because the chance of isolating an organism from sputum decreases by up to 50% after just one to two doses of antibiotic. Bronchoscopy and bronchoalveolar lavage often increase the yield of Gram stain and culture but are usually reserved for patients who do not respond to initial empirical antibiotics, are critically ill, or may harbor resistant or uncommon organisms.