What laboratory tests are essential for diagnosing and managing bronchial asthma and pneumonia?

Laboratory Tests for Bronchial Asthma and Pneumonia

For asthma, spirometry is the essential objective test—not routine laboratory tests—while for pneumonia, blood cultures, sputum Gram stain and culture, and chest radiography are the core diagnostic studies in hospitalized patients.

Bronchial Asthma Diagnostic Testing

Primary Diagnostic Test

• Spirometry with bronchodilator reversibility testing is the gold standard for asthma diagnosis in patients ≥5 years of age, as it objectively demonstrates airflow obstruction and reversibility 1
• Medical history and physical examination alone are unreliable for establishing asthma diagnosis 1
• Peak flow meters are designed for monitoring, not diagnosis, due to wide variability in devices and reference values 1

Additional Pulmonary Function Testing

• Bronchoprovocation testing (methacholine, histamine, cold air, or exercise challenge) may be useful when asthma is suspected but spirometry is normal or near-normal 1
• A negative bronchoprovocation test is more helpful to rule out asthma than a positive test is to confirm it 1
• Consider bronchoprovocation when FEV1 is ≥70% predicted (excluding respiratory infections within past 4 weeks) 2

Laboratory Tests (Limited Role)

• Routine laboratory tests are NOT essential for asthma diagnosis 1
• Peripheral blood eosinophil count ≥150/μl can identify eosinophilic phenotype or type 2 inflammatory endotype and predict biologic therapy response 2
• Fractional exhaled nitric oxide (FeNO) may support atopic asthma diagnosis but has limited utility in non-atopic asthma 3
• Induced sputum eosinophil count is a gold standard biomarker for airway inflammation assessment and phenotype classification 2
• Serum total IgE may help identify atopic phenotype but is not diagnostic 1

Imaging

• Chest radiograph may be needed to exclude alternative diagnoses but is not routinely required for asthma diagnosis 1

Tests to Avoid

• Serologic testing and cold agglutinin measurements are not useful in initial asthma evaluation 1

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Pneumonia Diagnostic Testing

Essential Laboratory Studies for Hospitalized Patients

Blood Work:

• Two sets of blood cultures should be drawn before antibiotic initiation in all hospitalized patients (yield ~11%, with S. pneumoniae most common) 1
• Complete blood count with differential 1
• Routine blood chemistry: glucose, serum sodium, liver and renal function tests, electrolytes 1
• Arterial blood gas in patients with severe illness or chronic lung disease to assess oxygenation and carbon dioxide retention 1

Respiratory Specimens:

• Sputum Gram stain and culture when purulent sputum can be obtained and processed timely 1
  • Adequate specimen: <10 squamous epithelial cells and ≥25 neutrophils per low-power field 1
  • Collect before antibiotic administration when possible 1
  • Culture results should correlate with predominant organism on Gram stain 1

Imaging:

• Chest radiograph (preferably posteroanterior and lateral views) to identify infiltrates 4
• Consider CT scan in complex cases for greater sensitivity 4

Additional Testing Based on Clinical Context

Pleural Fluid Analysis:

• Diagnostic thoracentesis should be performed when significant pleural effusion (≥10 mm thickness on lateral decubitus film) or loculated effusion is present 1, 4
• Send for: white blood cell count and differential, protein, glucose, LDH, pH, Gram stain, acid-fast stain, and cultures for bacteria, fungi, and mycobacteria 1, 4

Pathogen-Specific Testing:

• Legionella pneumophila serogroup 1 urinary antigen for patients with severe pneumonia or when clinically/epidemiologically suspected 1
• Serologic testing for Mycoplasma pneumoniae, Chlamydia pneumoniae, and Legionella is more useful for epidemiologic studies than individual patient management 1

Outpatient Pneumonia Testing

• Chest radiograph is essential 1
• Sputum Gram stain and culture only if drug-resistant bacteria or unusual organisms are suspected 1
• Pulse oximetry to assess oxygenation, particularly in patients with chronic heart or lung disease 1
• Routine laboratory tests have little value for determining etiology but may have prognostic significance in patients ≥65 years or with coexisting illness 1

Invasive Procedures (Not Routinely Indicated)

• Bronchoscopy with protected brush catheter or bronchoalveolar lavage is not indicated in most pneumonia patients 1
• May be considered in severely ill patients on an individual basis, though retrospective data show outcome is not improved by establishing specific etiologic diagnosis 1

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Critical Interpretation Points

For Asthma:

• A sterile lower respiratory tract culture in absence of recent antibiotic changes strongly suggests pneumonia is not present 4
• Clinical criteria alone (fever, purulent secretions, leukocytosis, infiltrates) have high sensitivity but low specificity for pneumonia 4

Common Pitfall: Do not delay antibiotic administration in pneumonia to obtain diagnostic specimens—collect rapidly but prioritize treatment 1. However, make every effort to obtain cultures before antibiotics when feasible 1.