Community-Acquired Pneumonia: Evaluation and Diagnosis
Blood Culture
Because the positivity rate of blood culture in patients who are suspected to have pneumonia but not exposed to antimicrobial agents is low (5%–14%), blood cultures are not recommended for all patients with CAP. Another reason for not recommending blood culture is positive culture rarely leads to changes in antibiotic regimen in patients without underlying diseases.27 However, the 2019 ATS/IDSA guidelines recommend blood culture in patients with severe disease and in all inpatients treated empirically for MRSA or P. aeruginosa.26
A multinational study published in 2008 examined 125 patients with pneumococcal bacteremic CAP versus 1847 patients with non-bacteremic CAP.28 Analysis of the data demonstrated no association between pneumococcal bacteremic CAP and time to clinical stability, length of hospital stay, all-cause mortality, or CAP-related mortality. The authors concluded that pneumococcal bacteremia does not increase the risk of poor outcomes in patients with CAP compared to non-bacteremic patients, and the presence of pneumococcal bacteremia should not deter de-escalation of therapy in clinically stable patients.
Urinary Antigen Tests
Urinary antigen tests may assist clinicians in narrowing antibiotic therapy when test results are positive. There are 2 US Food and Drug Administration–approved tests available to clinicians for detecting pneumococcal and Legionella antigen in urine. The test for Legionella pneumophila detects disease due to serogroup 1 only, which accounts for 80% of community-acquired Legionnaires’ disease. The sensitivity and specificity of the Legionella urine antigen test are 90% and 99%, respectively. The pneumococcal urine antigen test is less sensitive and specific than the Legionella urine antigen test (sensitivity 80% and specificity > 90%).29,30
Advantages of the urinary antigen tests are that they are easily performed, results are available in less than an hour if done in-house, and results are not affected by prior exposure to antibiotics. However, the tests do not meet Clinical Laboratory Improvements Amendments criteria for waiver and must be performed by a technician in the laboratory. A multicenter, prospective surveillance study of hospitalized patients with CAP showed that the 2007 IDSA/ATS guidelines’ recommended indications for S. pneumoniae and L. pneumophila urinary antigen tests do not have sufficient sensitivity and specificity to identify patients with positive tests.31
Polymerase Chain Reaction
There are several FDA-approved polymerase chain reaction (PCR) tests commercially available to assist clinicians in diagnosing pneumonia. PCR testing of nasopharyngeal swabs for diagnosis of influenza has become standard in many US medical facilities. The great advantages of using PCR to diagnose influenza are its high sensitivity and specificity and rapid turnaround time. PCR can also be used to detect Legionella species, S. pneumonia, Mycoplasma pneumoniae, Chlamydophila pneumonia, and mycobacterial species.24
One limitation of using PCR tests on respiratory specimens is that specimens can be contaminated with oral or upper airway flora, so the results must be interpreted with caution, bearing in mind that some of the pathogens isolated may be colonizers of the oral or upper airway flora.32
