Laboratory Evaluation
Generally the etiologic agent of CAP cannot be determined solely on the basis of clinical signs and symptoms or imaging studies. Although routine microbiological testing for patients suspicious for CAP is not necessary for empirical treatment, by determining the etiologic agent of the pneumonia, a clinician will be able to narrow the antibiotics from a broad-spectrum empirical regimen to specific pathogen-directed therapy. Determination of certain etiologic agents causing the pneumonia can have important public health implications (eg, Mycobacterium tuberculosis and influenza virus) [24].
Sputum Gram Stain and Culture
Sputum Gram stain is an inexpensive test that may identify pathogens that cause CAP (eg, S. pneumonia and Haemophilus influenzae). A quality specimen is required. A sputum sample must contain > 25 neutrophils and < 10 squamous epithelial cells/low power field on Gram stain to be considered suitable for culture.
The sensitivity and specificity of sputum Gram stain and culture are highly variable in different clinical settings (eg, outpatient setting, nursing home, ICU). Reed et al’s meta-analysis of patients diagnosed with CAP in the United States showed the sensitivity and specificity of sputum Gram stain (compared with sputum culture) ranged from 15% to 100% and 11% to 100%, respectively [24]. In cases of proven bacteremic pneumococcal pneumonia, positive cultures from sputum samples were positive less than 50% of the time [25].
For patients who cannot provide sputum samples or are intubated, a deep-suction aspirate or bronchoalveolar lavage through a bronchoscopic procedure might be necessary to obtain pulmonary secretion for Gram stain and culture. Besides bacterial culture, sputum samples can also be sent for fungal and mycobacterial cultures and acid-fast stain if deemed clinically necessary.
Blood Culture
Because the positivity rate of blood culture in patients who are suspected to have pneumonia but not exposed to antimicrobial agents is disappointingly low (5%–14%), blood cultures are no longer recommended in patients hospitalized for CAP. Another reason for not recommending blood culture is positive culture rarely leads to changes in antibiotic regimen in patients without underlying diseases [26]. However, high-risk patients, including patients with severe CAP or in immunocompromised patients (eg, patients with neutropenia, asplenia or complement deficiencies) should have a blood culture done [24].
A multinational study published in 2008 examined 125 patients with pneumococcal bacteremic CAP versus 1847 patients with non-bacteremic CAP [27]. Analysis of the data demonstrated no association of 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 U.S. 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%) [28,29].
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.
Polymerase Chain Reaction
There are several FDA-approved polymerase chain reaction (PCR) tests commercially available to assist clinicians in diagnosing pneumonia. PCR test of nasopharyngeal swabs for diagnosing influenza have become standard in many medical U.S. facilities. The great advantage of using PCR to diagnose influenza is 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 [30].
Biologic Markers
Two biologic markers—procalcitonin and C-reactive protein (CRP)—can be used in conjunction with history, physical examination, laboratory tests and imaging studies to assist in the diagnosis and treatment of CAP [24]. Procalcitonin is a peptide precursor of the hormone calcitonin that is released by parenchymal cells into the bloodstream resulting in increased serum level in patients with bacterial infections. In contrast, there is no remarkable proclacitonin level increase with viral or noninfectious inflammation. The reference value of procalcitonin in the blood of an adult individual without infection or inflammation is < 0.15 ng/mL. In the blood, procalcitonin has a half-life of 25 to 30 hours. The quantitative immunoluminometric method (LUMI test, Brahms PCT, Berlin, Germany ) is the preferred test to use because of its high sensitivity [31].
A 2012 Cochrane meta-analysis that involved 4221 patients with acute respiratory infections (with half of the patients diagnosed with CAP) from 14 prospective trials found the use of procalcitonin test for antibiotic use significantly decreased median antibiotic exposure from 8 to 4 days without an increase in treatment failure, mortality rates in any clinical setting (eg, outpatient clinic, emergency room), or length of hospitalization [32]. A prospective study conducted in France on 100 ICU patients showed that increased procalcitonin from day 1 to day 3 has a poor prognosis factor for severe CAP whereas decreasing procalcitonin levels is associated with a favorable outcome [33].
CRP is an acute phase protein produced by the liver. CRP level in the blood increases in response to acute infection or inflammation. Use of CRP in assisting diagnosis and guiding treatment of CAP is more limited in part due to its poor specificity. A prospective study conducted on 168 consecutive patients presented with cough showed that a CRP > 40 mg/L had a sensitivity and specificity of 70% and 90%, respectively [34].