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]

Overview

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	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 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.

Blood Culture

• Blood cultures should be obtained for patients with severe disease, patients that require hospitalization, and patients in which antibiotic therapy failed.

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

• 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

• 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.
• 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.

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 and the etiology.
• This biomarker also helps to differentiate between bacterial and non-bacterial disease.

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:

“

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

• Complete blood count (leucocytosis). In some people with compromised immunity, the white blood cell count may appear deceptively normal.
• Basic metabolic panel
  • Used to evaluate kidney function when prescribing certain antibiotics
  • Hyponatremia in pneumonia is thought to be due to excess anti-diuretic hormone produced when the lungs are diseased (SIADH)

Culture

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.

Serology

• Specific blood serology tests for other bacteria (Mycoplasma, Legionella and Chlamydophila) can be done in conditions with strong suspicion of the causative organisms.

Oxygen Monitoring

• Pulse oximeter
• Arterial blood gas

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

• Broncho-alveolar lavage
• Protected specimen brush

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

“

Arterial oxygenation saturation should be measured in all patients to determine the need for supplemental oxygen. Arterial blood gas should be determined if concern exists regarding either metabolic or respiratory acidosis, and this test generally is needed to manage patients who require mechanical ventilation. These results, along with other laboratory studies (complete blood count, serum electrolytes, renal and liver function), can point to the presence of multiple organ dysfunction and thus help define the severity of illness (Level II). 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.