Pneumonia laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1];Associate Editor(s)-in-Chief: Hamid Qazi, MD, BSc [2], Alejandro Lemor, M.D. [3]


Laboratory findings such as leukocytosis are helpful for the diagnosis of bacterial pneumonia or to assess the status of the patient. Sputum samples need to be collected from every patient and gram staining and culture need to be performed to determine the exact pathogen causing the pneumonia. Other tests include urine antigen test, PCR, C-reactive protein, and procalcitonin.

Laboratory Tests

Specific indications for additional testing in patients with pneumonia
Diagnostic Test Indications
Sputum Culture
Blood Culture
Urinary Antigen Test for Pneumococcus
Urinary Antigen Test for Legionella
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]

Sputum Gram Stain and Culture

  • Sputum samples should be obtained of all patients with productive cough.
  • Gram-stain and culture should be performed to assess the causative agent and guide the therapy.
  • In more than 80% of cases of pneumococcal pneumonia the sputum culture is positive.[3]

Blood Culture

  • Blood cultures should be obtained for patients with severe disease, patients that require hospitalization, and patients in which antibiotic therapy failed.
  • Blood culture may be positive in cases of hematogenous spread, such as S. aureus pneumonia, and in around one fourth of patients with pneumococcal pneumonia.

Other Laboratory Tests

Urine Antigen Test [4]

  • Usually used to diagnose Legionella disease.
  • Useful also in the diagnosis of pneumonia caused by Streptococcus pneumoniae, with a sensitivity of 74.6% and an association with worst clinical outcome.[5]
  • The presence of the antigen in urine can be detected in 24 hours since the onset of the symptoms.
  • The severity of Legionella disease increases the sensitivity of the urinary antigen test.
  • Urinary antigen test is not recommended in the diagnosis of pneumococcal pneumonia in children due to the high rate of false-positive results.[6]

Polymerase Chain Reaction

C-Reactive Protein

  • 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.[7]


  • Procalcitonin levels are associated with the severity of the pneumonia and the etiology.
  • This biomarker also helps to differentiate between bacterial and non-bacterial disease.[8]

Community Acquired Pneumonia

The infectious diseases society of America/American thoracic society consensus recommendation on diagnostics test for etiology of community-acquired pneumonia in adults are as follows:[9]

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)

For Level of evidence and classes click here.

Hospital Acquired Pneumonia

Basic Blood Works


Sputum Culture

  • Sputum gram stain and culture have poor yield. Sputum culture provides diagnostics information in roughly 1 in 5 patients only.
  • Sputum cultures generally take at least two to three days, so they are mainly used to confirm that the infection is sensitive to an antibiotic that has already been started.
  • A good sputum sample contains small number of squamous epithelial cells and a large number of PMNs.

Blood Culture

  • Blood cultures are not recommended for the outpatient management of CAP due to the low yield of pathogens.
  • A blood sample may similarly be cultured to look for infection in the blood (blood culture). Any bacteria identified are then tested to see which antibiotics will be most effective.


Oxygen Monitoring

Respiratory Samples for VAP

Bronchial Samples Over Non-bronchial Sample
Advantages Disadvantages
  • Helps in accurate diagnosis and selection of narrow antibiotic regimen. Thus, decreases incidence of antibiotic resistance.
  • Quantitative cultures from nonbronchoscopic specimens have a lower specificity than quantitative cultures derived from bronchoscopic specimens.
  • Obtaining bronchial sample is more invasive and risk of injuries are more compared to the non-invasive methods.

Bronchial Samples

Non-Bronchial Sample

  • Tracheo-bronchial aspiration
  • Mini-bronchoalveloar lavage

Quantitative Culture

  • Tracheobronchial aspiration - > 1 million cfu / mL is
  • Bronchoalveolar lavage - > 10,000 cfu / mL
  • PSB (protected brush sampling) - > 1,000 cfu / mL

Semi-quantitative Culture

  • Report bacterial growth as heavy, moderate, light, or no growth.
  • A moderate to heavy growth is suggestive of ventilator associated pneumonia.
  • More false positive results compared to quantitative cultures.

Special Tests

  • In more severe cases, (bronchoscopy) can be used collect fluid for culture.
  • Special tests can be performed if an uncommon microorganism is suspected (such as testing the urine for Legionella antigen when Legionnaires' disease is a concern).
  • HIV testing should be performed on all patients presenting with CAP (ages 13 to 75) in a medical setting.
  • Respiratory secretions can also be tested for the presence of viruses such as influenza, respiratory syncytial virus, and adenovirus.

Major Points and Recommendations for Laboratory Tests in Adults with Hospital-Acquired, Ventilator-Associated, and Healthcare-Associated Pneumonia [10]

  • All patients with suspected VAP should have blood cultures collected, recognizing that a positive result can indicate the presence of either pneumonia or extrapulmonary infection (Level II).
  • A diagnostic thoracentesis to rule out a complicating empyema or parapneumonic effusion should be performed if the patient has a large pleural effusion or if the patient with a pleural effusion appears toxic (Level III).
  • Samples of lower respiratory tract secretions should be obtained from all patients with suspected HAP, and should be collected before antibiotic changes. Samples can include an endotracheal aspirate, bronchoalveolar lavage sample, or protected specimen brush sample (Level II)

For Level of evidence and classes click here.


  1. 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. ISSN 1058-4838.
  2. 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. ISSN 0028-4793.
  3. Musher, Daniel M.; Thorner, Anna R. (2014). "Community-Acquired Pneumonia". New England Journal of Medicine. 371 (17): 1619–1628. doi:10.1056/NEJMra1312885. ISSN 0028-4793.
  4. Couturier MR, Graf EH, Griffin AT (2014). "Urine antigen tests for the diagnosis of respiratory infections: legionellosis, histoplasmosis, pneumococcal pneumonia". Clin Lab Med. 34 (2): 219–36. doi:10.1016/j.cll.2014.02.002. PMID 24856525.
  5. Zalacain R, Capelastegui A, Ruiz LA, Bilbao A, Gomez A, Uranga A; et al. (2014). "Streptococcus pneumoniae antigen in urine: diagnostic usefulness and impact on outcome of bacteraemic pneumococcal pneumonia in a large series of adult patients". Respirology. 19 (6): 936–43. doi:10.1111/resp.12341. PMID 24976113.
  6. Bradley JS, Byington CL, Shah SS, Alverson B, Carter ER, Harrison C; et al. (2011). "The management of community-acquired pneumonia in infants and children older than 3 months of age: clinical practice guidelines by the Pediatric Infectious Diseases Society and the Infectious Diseases Society of America". Clin Infect Dis. 53 (7): e25–76. doi:10.1093/cid/cir531. PMID 21880587.
  7. 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. ISSN 0002-9343.
  8. 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. ISSN 0036-5548.
  9. Mandell LA, Wunderink RG, Anzueto A, Bartlett JG, Campbell GD, Dean NC, Dowell SF, File TM, Musher DM, Niederman MS, Torres A, Whitney CG (2007). "Infectious Diseases Society of America/American Thoracic Society consensus guidelines on the management of community-acquired pneumonia in adults". Clinical Infectious Diseases : an Official Publication of the Infectious Diseases Society of America. 44 Suppl 2: S27–72. doi:10.1086/511159. PMID 17278083. Retrieved 2012-09-06. Unknown parameter |month= ignored (help)
  10. "Guidelines for the management of adults with hospital-acquired, ventilator-associated, and healthcare-associated pneumonia". American Journal of Respiratory and Critical Care Medicine. 171 (4): 388–416. 2005. doi:10.1164/rccm.200405-644ST. PMID 15699079. Retrieved 2012-09-13. Unknown parameter |month= ignored (help)

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