What laboratory tests are recommended to evaluate lung function?

Laboratory Tests to Evaluate Lung Function

Lung function is not evaluated by laboratory blood tests—it is assessed through pulmonary function testing (PFT), which includes spirometry, lung volume measurements, diffusing capacity, pulse oximetry, and arterial blood gas analysis. 1

Core Pulmonary Function Tests

Basic Testing (First-Line Assessment)

• Spirometry is the most essential test, measuring forced vital capacity (FVC), forced expiratory volume in one second (FEV1), and the FEV1/FVC ratio to identify obstructive or restrictive patterns 1
• Pre- and post-bronchodilator spirometry should be performed to assess reversibility of airflow obstruction, with significant response defined as >12% and >200 mL improvement in adults 1, 2
• Pulse oximetry provides immediate assessment of oxygenation status and should be measured at rest; values <94% warrant further pulmonary evaluation 1

Advanced Testing (When Basic Tests Are Abnormal)

• Lung volume measurements using body plethysmography or helium dilution/nitrogen washout are essential to confirm restrictive lung disease when vital capacity is reduced with normal FEV1/FVC ratio 1, 3
• Diffusing capacity for carbon monoxide (DLCO) assesses gas exchange across the alveolar-capillary membrane and is particularly important in parenchymal lung disease 1, 3
• Arterial blood gas (ABG) analysis measures PaO2, PaCO2, and pH to assess adequacy of ventilation and oxygenation; hypercapnia (elevated PaCO2) indicates severe respiratory muscle weakness requiring urgent intervention 1, 4

Specialized Testing for Specific Clinical Scenarios

Respiratory Muscle Weakness Assessment

• Maximum inspiratory and expiratory pressures (MIP/MEP) evaluate respiratory muscle strength, though interpretation is challenging due to wide variability in healthy populations 1
• Peak cough flow (PCF) assesses cough effectiveness; values <160 L/min indicate inadequate clearance, while <270 L/min suggests ineffective cough 1
• ABG with PaCO2 measurement is critical because hypercapnia signals advanced respiratory muscle weakness (typically when strength is <40% predicted) and requires immediate ventilatory support 4

Small Airways and Flow Assessment

• Forced expiratory flow at 25-75% of vital capacity (FEF25-75%) may detect small airway dysfunction, though its clinical utility remains debated 1
• Flow-volume loops provide visual assessment of airflow patterns throughout the respiratory cycle 1

Recommended Testing Algorithm

Initial Evaluation (All Patients)

• Spirometry with pre- and post-bronchodilator measurements 1
• Pulse oximetry at rest 1

If Spirometry Shows Obstruction (FEV1/FVC <0.70)

• Assess bronchodilator reversibility 1, 2
• Consider lung volumes if hyperinflation suspected 1, 3
• Measure DLCO to assess parenchymal involvement 1, 3

If Spirometry Suggests Restriction (Low FVC, Normal FEV1/FVC)

• Lung volume measurement is mandatory to confirm true restriction (reduced total lung capacity below 5th percentile) 1, 2, 3
• Measure DLCO to differentiate parenchymal from chest wall/neuromuscular causes 1, 3

If Respiratory Muscle Weakness Suspected

• ABG analysis to detect hypercapnia 4
• MIP/MEP measurements 1
• Peak cough flow 1

Critical Clinical Considerations

Safety and Contraindications

• Defer PFT in patients with active hemoptysis, pneumothorax, recent myocardial infarction, unstable angina, severe respiratory distress, or inability to cooperate 5
• Maintain SpO2 >90% (with supplemental oxygen if needed) throughout testing 5
• Continuous pulse oximetry monitoring is mandatory in borderline cases 5

Common Pitfalls to Avoid

• Do not rely on stated height—measure height with a stadiometer at each visit, as height affects predicted values 1
• Do not use FVC alone to diagnose restriction; total lung capacity measurement is required for definitive diagnosis 1, 2, 3
• Do not interpret "excessively good" pulmonary function at face value in patients with skeletal abnormalities or severe disease, as prediction equations may be inaccurate 1
• Recognize that hypoxemia may be a pseudo-marker of hypercapnia in thoracic cage restriction; ABG is needed to detect CO2 retention 1

Quality Assurance

• Ensure at least three acceptable spirometry maneuvers with the two best FVC and FEV1 values within 150 mL 1
• For DLCO, report average of at least two grade A maneuvers repeatable within 2 mL/min/mm Hg 1
• Use race/ethnic-specific reference equations when available; if not, apply adjustment factor of 0.88 for Black patients and 0.94 for Asian Americans 1