The Harrison's Practice Preview allows you to view 5 FREE complete topics or conduct a search that delivers abstracts for more than 450 medical topics. For full access, please subscribe today!

Laboratory Tests

Nonspecific studies

• Assessment of the severity of pneumonia and coexisting disease
  • Arterial blood gas
  • Complete blood count
  • Serum electrolyte and glucose measurements
  • Blood urea nitrogen (BUN) and creatinine measurements
    

Sputum stains and culture

• Gram’s stain
  
  • Useful in screening a sputum sample for suitability for culture and in making a presumptive etiologic diagnosis
    • A sputum sample with > 25 white blood cells (WBCs) and < 10 squamous epithelial cells per low-power field is suitable for culture.
    • Significant interobserver variability exists in the interpretation of gram-stained sputum smears.
  • The presence of any gram-positive diplococci has a sensitivity of 100% but a specificity of 0 for a diagnosis of pneumococcal infection.
    • The presence of > 10 gram-positive diplococci per oil-immersion field has a sensitivity of 55% and a specificity of 85% for this diagnosis.
• Other sputum stains that may be helpful in some patients
  
  • Stains for
    • Acid-fast bacilli
    • Pneumocystis
    • Fungi
    • Cytology
  • Rapid antigen testing for viral pathogens (e.g., influenza)
• Culture
  • Results should always be correlated with those of Gram staining.
    • If an organism is isolated from sputum and its morphologic correlate is not seen on Gram staining, the isolate may be colonizing the upper airway.
  • Certain microorganisms, if isolated from sputum, should always be considered pathogens. These include:
    • M. tuberculosis
    • Legionella spp.
    • Blastomyces dermatitidis
    • Histoplasma capsulatum
    • Coccidioides immitis
  • Only about one-third of elderly patients admitted with CAP produce sputum suitable for culture.
    • One-third of these specimens fail to yield a pathogen.

Blood culture

• Blood should be obtained for culture from patients to be treated on an ambulatory basis if they have been receiving antibiotic therapy and have presented because of any of the following:
  • Hyperthermia (body temperature > 38.5°C)
  • Hypothermia (body temperature < 36°C)
  • Homelessness
  • Alcohol abuse
• All patients admitted to the hospital for CAP should have 2 sets of blood cultures done before initiation of antibiotic therapy (positivity rate: 6–20%).
• The most common isolates, in descending order, are S. pneumoniae (~60%), S. aureus , and Escherichia coli .

Detection of antigens of pulmonary pathogens in urine

• L. pneumophila (see Legionellosis)
  
  • Serogroup 1 antigen can be detected in the urine of patients with Legionnaires’ disease by enzyme-linked immunosorbent assay (ELISA).
    • Sensitivity: 69–72% on average, 88–100% in severe disease, 40–53% in mild disease
    • Sensitivity: low in nosocomial Legionnaires’ disease
    • The results may be negative early in the illness, and antigen excretion can be prolonged.
  • This test should be used for patients in whom Legionnaires’ disease is strongly suspected, including those with rapidly progressive pneumonia.
  • The urine antigen test is now the most frequently used diagnostic method for Legionnaires’ disease.
  • Infection with Legionella spp. other than L. pneumophila serogroup 1 gives a negative test result.
• S. pneumoniae
  
  • Urinary antigen detection by ELISA has a sensitivity of 80% and a specificity of 97–100% in patients with bacteremic pneumococcal pneumonia.
    • The antigen may be detected for up to 1 month after the onset of pneumonia, and the results can be available in 15 min.
    • In children, nasopharyngeal carriage of S. pneumoniae can result in a positive urinary antigen test.

Serology

• Detection of IgM antibody or demonstration of a 4-fold rise in titer of antibody to a particular agent between acute- and convalescent-phase serum samples generally is considered good evidence that this agent is the cause of CAP.
• The following etiologic agents often are diagnosed serologically:
  • M. pneumoniae
  • C. pneumoniae
  • Chlamydia psittaci
  • Legionella spp.
  • Coxiella burnetii
  • Adenovirus
  • Parainfluenza viruses
  • Influenza virus A
• The serologic tests include complement fixation, indirect immunofluorescence, and ELISA.
• Separate IgM and IgG antibody detection tests can be performed with the latter 2 assays.
• One difficulty in relying on serology is that a polyclonal antibody response to one agent may result in a 4-fold rise in antibody titer to others.
  • Thus, results may be nonspecific.
• Serologic testing is not recommended for routine use.
  • If agents such as C. burnetii are suspected, serologic testing is necessary.
  • Serology is a useful part of the workup of outbreaks of pneumonia associated with negative blood and sputum cultures.

Polymerase chain reaction (PCR)

• Amplification of the DNA or RNA of microorganisms that are not part of the pharyngeal flora (from microbial cells collected by throat swab) has been used to infer that the implicated microorganism is the cause of pneumonia.
• A multiplex PCR allows detection of DNA of Legionella spp., M. pneumoniae, and C. pneumoniae.
• This test is expensive and is not routinely available.

The Harrison's Practice Preview allows you to view 5 FREE complete topics or conduct a search that delivers abstracts for more than 450 medical topics. For full access, please subscribe today!