Community-acquired pneumonia laboratory findings

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

Overview

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

Routine Tests

Findings in routine blood tests are based on the severity of the disease and the cause, they can include the following:

• Leukocytosis with left shift (in cases of bacterial pneumonia)
• Leukopenia (in cases of atypical pneumonia)
• Eosinophilia (in cases of eosinophilic pneumonia)
• Anemia
• Hyponatremia
• Thrombocytopenia
• Elevated BUN
• Findings of lactic acidosis (decreased HCO₃, increased lactic acid levels)
• ABG: may show hypoxia and/or hypercapnea

Sputum Gram Stain and Culture

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

Other Laboratory Tests

Urine Antigen Test

• Usually used to diagnose Legionella pneumomnia.
• Useful also in the diagnosis of pneumonia caused by Streptococcus pneumoniae, with a sensitivity of 74.6%.
• The Legionella urine antigen can be detected 24 hours after the onset of the symptoms.
• The more sever the Legionella disease the higher the sensitivity of the urinary antigen test.
• Urinary antigen testing is not recommended for the diagnosis of pneumococcal pneumonia in children due to the high rate of false-positive results.

Polymerase Chain Reaction

• Polymerase chain reaction (PCR) is more useful in the diagnosis of viral and mycoplasma pneumonia.

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.

Procalcitonin

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

Infectious Diseases Society of America/American Thoracic Society consensus recommendation on diagnostics test for etiology of community-acquired pneumonia in adults.

“

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.