What is the difference between a Pulmonary Function Test (PFT) and spirometry?

Difference Between PFT and Spirometry

Spirometry is one component test within the broader category of pulmonary function testing (PFT), which encompasses multiple distinct measurements of lung function. 1

Spirometry: A Single Component

Spirometry specifically measures:

• Forced vital capacity (FVC) - the total volume exhaled during a forceful breath 1
• Forced expiratory volume in 1 second (FEV1) - volume exhaled in the first second 1
• FEV1/FVC ratio - used to identify airflow obstruction 1
• Flow-volume curves - graphical representation of expiratory airflow 1

Spirometry evaluates dynamic lung function through forced expiratory maneuvers and can be performed in primary care offices with portable equipment. 2

Comprehensive PFT: Multiple Test Components

A complete pulmonary function test battery includes spirometry plus additional measurements that spirometry alone cannot provide. 1 The full PFT panel typically includes:

Additional Components Beyond Spirometry:

• Lung volume measurements (via plethysmography or gas dilution techniques):

  • Total lung capacity (TLC) - required to diagnose restrictive defects 3
  • Residual volume (RV) - identifies air trapping 1
  • Functional residual capacity (FRC) 1

• Diffusing capacity (DLCO) - measures gas exchange across the alveolar-capillary membrane, assessing parenchymal lung disease 1

• Bronchodilator response testing - evaluates reversibility of airflow obstruction 1

• Arterial blood gas analysis - identifies respiratory insufficiency 3

Clinical Implications of the Distinction

Spirometry alone cannot diagnose restrictive lung disease - it can only suggest restriction when vital capacity is reduced, but confirmation requires measurement of total lung capacity through plethysmography or gas dilution. 3

Key diagnostic limitations of spirometry-only testing:

• Cannot distinguish between true restriction and pseudorestriction from severe obstruction 1
• Cannot quantify hyperinflation or air trapping 1
• Cannot assess gas exchange abnormalities that may exist despite normal spirometry 1
• Cannot measure static lung volumes needed for complete characterization of ventilatory defects 1

Practical Application

For initial screening and monitoring of obstructive diseases like asthma or COPD, spirometry provides the essential measurements (FEV1, FVC, FEV1/FVC ratio). 1, 2

However, when clinical suspicion exists for:

• Interstitial lung disease
• Neuromuscular weakness
• Chest wall disorders
• Unexplained dyspnea with normal spirometry
• Discordance between symptoms and spirometry results

Then comprehensive PFT including lung volumes and diffusing capacity becomes necessary for accurate diagnosis. 1

The American Thoracic Society recommends that interpretation of any pulmonary function testing should include quality assessment, comparison to reference values, identification of ventilatory defect patterns, and correlation with clinical context - whether performing spirometry alone or comprehensive PFT. 1