Pulmonary Function Tests (PFTs): Spirometry and Lung Volumes

Pulmonary function tests (PFTs) are a standardized battery of non-invasive measurements used to assess how well the lungs move, hold, and exchange air. The two foundational components — spirometry and lung volume measurement — provide complementary data that guide diagnosis, staging, and treatment monitoring across conditions ranging from asthma and COPD to pulmonary fibrosis and occupational lung disease. Understanding what these tests measure, how they differ, and when each is indicated is central to the broader landscape of pulmonary medicine. The American Thoracic Society (ATS) and the European Respiratory Society (ERS) have jointly published the primary interpretive standards that govern clinical PFT use worldwide.

Definition and scope

Pulmonary function testing encompasses a group of physiologic measurements that quantify airflow, lung capacity, and gas transfer. The term is used broadly across the regulatory context for pulmonary medicine, where occupational health standards — including those set by the Occupational Safety and Health Administration (OSHA) under 29 CFR 1910.1043 for cotton dust exposure and related respiratory surveillance programs — mandate spirometric screening for workers in specific exposure categories.

The two primary measurement domains are:

Spirometry: measures the volume and speed of air that can be inhaled and exhaled forcefully. The key outputs are Forced Vital Capacity (FVC), Forced Expiratory Volume in 1 second (FEV1), and the FEV1/FVC ratio.
Lung volumes: measures the static subdivision of total lung capacity (TLC), including Residual Volume (RV), Functional Residual Capacity (FRC), and Inspiratory Reserve Volume (IRV). These require either body plethysmography or gas dilution techniques and are not captured by spirometry alone.

A third component, diffusing capacity of the lung for carbon monoxide (DLCO), is often performed alongside lung volumes to assess the efficiency of gas transfer across the alveolar-capillary membrane, but is classified separately from spirometry and static lung volumes in ATS/ERS guidelines (ATS/ERS Standardisation of Lung Function Testing, 2005 series).

How it works

Spirometry procedure

The patient breathes into a calibrated device — a spirometer — following a defined protocol. The ATS/ERS 2019 technical standards require a minimum of 3 acceptable and repeatable maneuvers, with the two largest FVC and FEV1 values agreeing within 150 mL (ATS/ERS Technical Standard for Spirometry, 2019).

The core spirometric indices:

FVC (Forced Vital Capacity): total volume exhaled with maximum effort after full inhalation. Reduction suggests either obstructive air trapping or restrictive capacity loss.
FEV1 (Forced Expiratory Volume in 1 second): volume exhaled in the first second. The single most reproducible index of airflow obstruction.
FEV1/FVC ratio: the proportion of FVC exhaled in 1 second. A ratio below the Lower Limit of Normal (LLN) — derived from reference equations such as the Global Lung Initiative (GLI) 2012 multi-ethnic equations — indicates obstruction.
FEF 25–75%: mean flow during the middle 50% of the FVC maneuver. Used as a secondary marker of small airways function, though it carries higher variability than FEV1.

Lung volume measurement

Static lung volumes require a separate test. Two principal methods exist:

Body plethysmography (body box): the patient sits inside an airtight chamber and pants against a closed shutter. Pressure-volume relationships quantify FRC directly via Boyle's Law. This method captures all gas in the thorax, including trapped gas behind obstructed airways, making it the reference standard for patients with obstructive disease.
Gas dilution (helium dilution or nitrogen washout): the patient breathes a tracer gas mixture until equilibrium or washout is complete. These techniques measure only communicating lung volumes and systematically underestimate TLC in patients with significant air trapping.

The difference between plethysmographic TLC and gas-dilution TLC — sometimes exceeding 1.0 liter in severe emphysema — is itself diagnostically informative, indicating the degree of non-communicating trapped gas.

Common scenarios

PFTs are ordered across a wide range of clinical presentations. The five most common indications in outpatient pulmonary practice are:

Suspected obstructive disease: evaluating shortness of breath, chronic cough, or wheezing for asthma or COPD. GOLD (Global Initiative for Chronic Obstructive Lung Disease) staging of COPD severity relies on post-bronchodilator FEV1 % predicted, with GOLD Grade 1 defined as FEV1 ≥ 80% predicted and Grade 4 as FEV1 < 30% predicted (GOLD 2023 Report, goldcopd.org).
Suspected restrictive disease: evaluating diffuse parenchymal disease such as pulmonary fibrosis or sarcoidosis. Restriction requires full lung volumes (TLC < LLN); spirometry alone cannot confirm a restrictive defect.
Pre-operative assessment: risk stratification before thoracic or major abdominal surgery. Predicted post-operative FEV1 and DLCO are used as thresholds in resection planning.
Occupational and surveillance programs: OSHA-mandated spirometry for workers exposed to silica, asbestos, coal dust, or cotton dust under standards including 29 CFR 1910.1001 (asbestos) and 29 CFR 1910.1053 (silica).
Monitoring treatment response: serial spirometry in asthma management tracks response to inhaled corticosteroids; serial DLCO tracks progression in connective tissue disease-associated interstitial lung disease.

Decision boundaries

Interpreting PFT results requires applying a structured classification algorithm. The ATS/ERS 2022 interpretive strategy document (Stanojevic et al., ERJ 2022) updated the canonical approach:

Pattern	FEV1/FVC	TLC	Key implication
Obstruction	< LLN	Normal or elevated	Airflow limitation; severity graded by FEV1 % predicted
Restriction	Normal or high	< LLN	Reduced lung size; requires full volumes to confirm
Mixed	< LLN	< LLN	Both components present simultaneously
Non-specific	Normal	Normal, but FVC low	May indicate early restriction, air trapping, or obesity effect

Bronchodilator reversibility is assessed by repeating spirometry 10–15 minutes after administration of a short-acting bronchodilator (typically 400 mcg of salbutamol). A significant response is defined by ATS/ERS as an increase in FEV1 or FVC of at least 12% and 200 mL from baseline — a threshold that supports, but does not confirm, a diagnosis of asthma.

Reference equation selection materially affects whether a value falls within or outside the normal range. The GLI 2012 equations, which incorporate continuous age scaling and cover ages 3 to 95 across 4 ethnic groups, are currently recommended over older fixed-ratio or single-population references (Global Lung Initiative, thoracic.org). Use of a fixed FEV1/FVC cut-off of 0.70 — still embedded in some institutional protocols — over-diagnoses obstruction in older adults and under-diagnoses it in younger patients compared to LLN-based interpretation.

Isolated DLCO reduction with normal spirometry and normal TLC points toward pulmonary vascular disease, early interstitial disease, or anemia — conditions where airflow mechanics are preserved but gas transfer is impaired.

References

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