While chest X-ray and pulse oximetry are routine tests among all patients with suspected pneumonia, additional laboratory tests (such as blood culture and gram sputum, sputum blood culture and gram stain, urinary antigen tests) are optional tests unless their results might alter the treatment plan.
Laboratory Tests
Specific indications for additional testing in patients with pneumonia
Diagnostic Test
Indications
Sputum Culture
ICU admission
Antibiotic therapy failure
Cavitation
Alcoholic patient
COPD
Pleural effusion
Blood Culture
ICU admission
Leukopenia
Cavitation
Severe liver disease
Alcoholic patient
Asplenia
Pleural effusion
Urinary Antigen Test for Pneumococcus
ICU admission
Leukopenia
Antibiotic therapy failure
Severe liver disease
Alcoholic patient
Asplenia
Pleural effusion
Urinary Antigen Test for Legionella
ICU admission
Antibiotic therapy failure
Alcoholic patient
Recent travel (< 2 weeks)
Pleural effusion
Adapted from IDSA/ATS Guidelines for CAP in Adults[1]
Routine Tests
Findings in routine blood tests are based on the severity of the disease and the cause, they can include the following:[2]
Leukocytosis with left shift (in cases of bacterial pneumonia)
However, the general yield of a sputum sample does not exceed 50-60%.
Blood Culture
Blood cultures should be obtained for patients with severe disease, those who require hospitalization, and those who fail antibiotic therapy.
Blood cultures may be positive in cases of hematogenous spread, such as S. aureus pneumonia, and in approximately one fourth of patients with pneumococcal pneumonia.
This image depicts the colonial characteristics displayed by Streptococcus pneumoniae bacterial colonies that were grown on primary isolation medium, consisting of trypticase soy agar containing 5% sheep’s blood, as well as 5mg of gentamicin/ml. Image obtained from CDC PHIL[4]
C-reactive protein (CRP) may be helpful to differentiate between bacterial from viral pneumonia.
It has been reported that CRP is elevated (> 100 mg/L) in cases of bacterial pneumonia.[8]
Procalcitonin
Procalcitonin levels are associated with the severity of the pneumonia.
This biomarker also helps to differentiate between bacterial and non-bacterial disease.[9]
Infectious Diseases Society of America/American Thoracic Society consensus recommendation on diagnostics test for etiology of community-acquired pneumonia in adults. [10]
“
Recommended Diagnostic Tests for Etiology
Patients with CAP should be investigated for specific pathogens that would significantly alter standard (empirical) management decisions, when the presence of such pathogens is suspected on the basis of clinical and epidemiologic clues. (Strong recommendation; level II evidence)
Routine diagnostic tests to identify an etiologic diagnosis are optional for outpatients with CAP. (Moderate recommendation; level III evidence)
Pretreatment blood samples for culture and an expectorated sputum sample for stain and culture (in patients with a productive cough) should be obtained from hospitalized patients with the clinical indications, but are optional for patients without these conditions. (Moderate recommendation; level I evidence)
Pretreatment Gram stain and culture of expectorated sputum should be performed only if a good-quality specimen can be obtained and quality performance measures for collection, transport, and processing of samples can be met. (Moderate recommendation; level II evidence)
Patients with severe CAP, as defined in the guideline should at least have blood samples drawn for culture, urinary antigen tests for Legionella pneumophila and Streptococcus pneumoniae performed, and expectorated sputum samples collected for culture. For intubated patients, an endotracheal aspirate sample should be obtained. (Moderate recommendation; level II evidence)
↑Mandell, L. A.; Wunderink, R. G.; Anzueto, A.; Bartlett, J. G.; Campbell, G. D.; Dean, N. C.; Dowell, S. F.; File, T. M.; Musher, D. M.; Niederman, M. S.; Torres, A.; Whitney, C. G. (2007). “Infectious Diseases Society of America/American Thoracic Society Consensus Guidelines on the Management of Community-Acquired Pneumonia in Adults”. Clinical Infectious Diseases. 44 (Supplement 2): S27–S72. doi:10.1086/511159. ISSN1058-4838.
↑Solomon, Caren G.; Wunderink, Richard G.; Waterer, Grant W. (2014). “Community-Acquired Pneumonia”. New England Journal of Medicine. 370 (6): 543–551. doi:10.1056/NEJMcp1214869. ISSN0028-4793.
↑Musher, Daniel M.; Thorner, Anna R. (2014). “Community-Acquired Pneumonia”. New England Journal of Medicine. 371 (17): 1619–1628. doi:10.1056/NEJMra1312885. ISSN0028-4793.
↑Flanders, Scott A; Stein, John; Shochat, Guy; Sellers, Karen; Holland, Miles; Maselli, Judith; Drew, W.Lawrence; Reingold, Art L; Gonzales, Ralph (2004). “Performance of a bedside c-reactive protein test in the diagnosis of community-acquired pneumonia in adults with acute cough”. The American Journal of Medicine. 116 (8): 529–535. doi:10.1016/j.amjmed.2003.11.023. ISSN0002-9343.
↑Johansson, Niclas; Kalin, Mats; Backman-Johansson, Carolina; Larsson, Anders; Nilsson, Kristina; Hedlund, Jonas (2014). “Procalcitonin levels in community-acquired pneumonia – correlation with aetiology and severity”. Scandinavian Journal of Infectious Diseases. 46 (11): 787–791. doi:10.3109/00365548.2014.945955. ISSN0036-5548.
