What is the difference between spirometry and pulmonary function test (PFT) in assessing lung function?

Spirometry vs Pulmonary Function Tests (PFTs): Understanding the Difference

Spirometry is a specific component of pulmonary function testing that measures airflow, while complete pulmonary function tests (PFTs) include spirometry plus additional measurements such as lung volumes and diffusing capacity. 1

Key Differences Between Spirometry and PFTs

Spirometry

• Measures airflow parameters during forced breathing maneuvers
• Primary measurements include:
  • FVC (Forced Vital Capacity)
  • FEV1 (Forced Expiratory Volume in 1 second)
  • FEV1/FVC ratio 1
• Primarily detects obstructive lung disorders
• More widely available and can be performed in office settings
• Requires patient effort and cooperation
• Less comprehensive than full PFTs

Complete Pulmonary Function Tests (PFTs)

• Comprehensive assessment that includes:
  1. Spirometry (as described above)
  2. Lung volumes (measured by plethysmography, nitrogen washout, or helium dilution)
  3. Diffusing capacity (DLCO) 1
• Provides more complete evaluation of lung function
• Can detect both restrictive and obstructive patterns
• Typically performed in specialized pulmonary function laboratories
• More time-consuming and resource-intensive

Clinical Applications and Interpretation

When Spirometry Alone Is Sufficient

• Initial screening for obstructive lung diseases (COPD, asthma)
• Monitoring known obstructive lung diseases
• Pre-operative assessment in low-risk patients
• Occupational health screening 1

When Complete PFTs Are Necessary

• Evaluation of suspected restrictive lung disease
• Assessment of diffusion abnormalities
• Evaluation of dyspnea with normal spirometry
• Monitoring interstitial lung diseases
• Pre-transplant or complex surgical evaluations
• Evaluation of pulmonary vascular diseases 1

Diagnostic Value of Different Components

Spirometry Parameters

• Normal FEV1/FVC with reduced FVC suggests restriction
• Reduced FEV1/FVC indicates obstruction
• Reduced FEV1/FVC with reduced FEV1 indicates obstructive disease severity 1

Lung Volumes (Part of Complete PFTs)

• Total Lung Capacity (TLC): confirms restrictive disease when reduced
• Residual Volume (RV): elevated in air trapping conditions
• Functional Residual Capacity (FRC): helps assess hyperinflation

Diffusing Capacity (Part of Complete PFTs)

• Reduced in interstitial lung diseases, emphysema, pulmonary vascular diseases
• Normal DLCO with restrictive pattern suggests chest wall restriction rather than parenchymal disease 2

Common Pitfalls in Interpretation

1. Relying solely on spirometry to diagnose restrictive disease (TLC measurement required)
2. Not considering patient effort and technique when interpreting results
3. Using inappropriate reference values for patient demographics
4. Overlooking the need for DLCO in patients with normal spirometry but unexplained dyspnea 1, 3

Special Considerations

• In patients with systemic autoimmune rheumatic diseases, complete PFTs are recommended for screening and monitoring of interstitial lung disease 1
• For post-hematopoietic stem cell transplantation monitoring, both spirometry and complete PFTs are recommended at specific intervals 1
• In occupational settings, spirometry may be sufficient for routine monitoring, but complete PFTs should be considered when abnormalities are detected 1

By understanding the differences between spirometry and complete PFTs, clinicians can select the appropriate testing strategy based on the clinical scenario, suspected diagnosis, and available resources.