What are the measurements and interpretations in a Pulmonary Function Test (PFT), and how are they used to assess lung function?

From the Guidelines

Pulmonary function tests (PFTs) should include spirometry, lung volume measurements, and diffusion capacity to assess lung function, with the FEV1/FVC ratio being a key indicator of obstructive or restrictive patterns 1. The main measurements in a PFT include:

• Spirometry values: FEV1 (forced expiratory volume in one second), FVC (forced vital capacity), and their ratio (FEV1/FVC)
• Lung volumes: TLC (total lung capacity), RV (residual volume), and FRC (functional residual capacity)
• Diffusion capacity (DLCO) to assess oxygen movement from the lungs into the bloodstream When interpreting PFT results, it is essential to compare the patient's values to predicted values based on age, height, sex, and ethnicity, with results typically reported as percentages of predicted values 1. Key points to consider when reading a PFT include:
• A reduced FEV1/FVC ratio (<0.7) suggests obstructive disease, while a normal ratio with reduced volumes indicates restrictive disease
• The flow-volume loop shape provides visual clues, with a scooped-out expiratory curve suggesting obstruction and a proportionally reduced curve indicating restriction
• Bronchodilator response (>12% and >200mL improvement in FEV1) helps identify reversible airway obstruction Understanding these measurements allows for proper diagnosis of conditions like COPD, asthma, interstitial lung disease, and neuromuscular disorders affecting respiration 1. It is crucial to follow a standardized approach to reporting PFT results, including the use of validated parameters and the display of normal limits for each test value 1. In clinical practice, airflow resistance is rarely used to identify airflow obstruction, but it may be useful in patients who are unable to perform a maximal forced expiratory maneuver 1. The most recent and highest-quality study recommends that only FVC, FEV1, and FEV1/FVC need to be routinely reported, with the addition of slow VC and FEV1/VC in patients with suspected airflow obstruction 1.

From the Research

Measurements in Pulmonary Function Tests (PFTs)

• Spirometry: measures forced expiratory flows and mobilisable volumes, allowing for the definition of obstructive defects 2
• Static lung volumes: assessed using body plethysmography, allowing for the definition of restrictive defects and thoracic hyperinflation 2
• Diffusion capacity: measured using the transfer coefficient of the lung, allowing for the assessment of alveolar-capillary wall and capillary blood volume 2
• Arterial blood gas: used to search for respiratory insufficiency 2
• Methacholine challenge test: used to diagnose asthma in cases of diagnostic uncertainty 2
• 6-minute walk test: used to assess walking performance, search for arterial desaturation, and quantify dyspnoea complaint 2

Interpretation of PFT Results

• Comparison to reference values: results are compared to those of a healthy reference population to determine normal, low, or high parameters 3
• Identification of ventilatory impairments: potential obstructive or restrictive ventilatory impairments are identified, which could indicate specific chronic respiratory or extra-respiratory diseases 3
• Assessment of severity: the severity of identified ventilatory impairments or reductions in diffusion capacity is assessed 3
• Evaluation of bronchodilator response: if bronchodilator testing is performed, its response is evaluated 3
• Comparison to previous results: significant changes in PFT parameters over time are noted by comparing current results with previous ones, if available 3

Challenges and Controversies in PFT Interpretation

• Selection of reference standards: the choice of reference standards can affect PFT interpretation 4
• Arbitrary choice of cut-off values: the choice of cut-off values can lead to differences in interpretation 4
• Clinical context: PFT interpretation depends on the clinical context, and multiple challenges can influence interpretation 4
• Controversies in PFT analysis: controversies can lead to important differences in interpretation, particularly in the diagnosis of obstructive ventilatory defect, restrictive ventilatory defect, gas transfer defect, and flow-volume loop abnormalities 4