Hospital-acquired pneumonia laboratory findings

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Editor(s)-in-Chief: C. Michael Gibson, M.S., M.D. ; Philip Marcus, M.D., M.P.H.; Associate Editor(s)-in-Chief: Alejandro Lemor, M.D. [1]

Overview

Current guidelines recommend a combination of chest X-ray, laboratory data as well as clinical judgment in diagnosis and management of community acquired pneumonia. Laboratory tests include CBC, metabolic panel, sputum gram-stain and culture, serology for mycoplasma, chlamydia, and legionella. Additional test include bronchial samples and HIV testing for special conditions.

Laboratory Findings

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)

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For Level of evidence and classes click here.