Spirometry

Understand

Overview

Spirometry is a basic lung function test that measures the volume of air inhaled and exhaled by the lungs and the speed at which air is blown out during a forced exhalation. The patient breathes through a mouthpiece connected to a spirometer — a device that records airflow and volume in real time — performing a series of breathing maneuvers, the most important of which is the forced exhalation: a deep breath in, followed by the fastest and most complete exhalation possible. The result is a flow-volume curve along with a set of numerical parameters, the two most clinically important being FEV1 (forced expiratory volume in one second) and FVC (forced vital capacity).

The clinical value of spirometry lies in the fact that it is the only widely available test that objectively confirms or rules out airway obstruction — narrowing of the bronchi that impedes airflow, characteristic of asthma and chronic obstructive pulmonary disease (COPD). An FEV1/FVC ratio below the lower limit of normal is the key criterion for diagnosing obstruction, while a restrictive pattern (reduced FVC with a normal FEV1/FVC ratio) instead suggests interstitial lung disease, chest wall deformities, or neuromuscular disorders limiting lung expansion. Unlike simply assessing symptoms or listening to the chest, spirometry provides a numerical, reproducible result that can be compared over time and between tests.

The test is ordered in many different clinical contexts. Most often it's performed in people reporting chronic cough, exertional breathlessness, wheezing, or a feeling of chest tightness, to confirm or rule out asthma or COPD. It's also a routine part of monitoring already-diagnosed lung diseases, assessing response to bronchodilator treatment, pre-surgical evaluation of smokers and people with lung disease, occupational screening in jobs exposed to dust and airway irritants, and monitoring pulmonary toxicity from certain cancer drugs.

On the practical side, the test is quick — performing the maneuvers takes roughly fifteen to twenty-odd minutes, though it requires active patient cooperation and usually several repetitions to obtain three technically acceptable, reproducible curves. Before the test, patients are usually advised to avoid short-acting bronchodilators for a few hours (if the goal is to assess baseline lung function without their effect), avoid intense physical exertion right before the test, and avoid a heavy meal. The result is available immediately after the test as a printout with the flow-volume curve and a table of values compared to predicted norms for the patient's age, sex, height, and ethnicity.

A common misconception is treating spirometry as simple "blowing into a tube" that doesn't require particular technical precision — in reality, the quality of the result depends heavily on how well the maneuver is performed, and errors such as an incomplete initial breath, premature termination of the exhalation, or coughing during the maneuver can significantly lower or distort the result. That's why the guidelines standardizing the test precisely define acceptability and repeatability criteria for the curves, and the experience of the person administering the test directly affects how reliable the result is.

Spirometry is often extended with a bronchodilator reversibility test — repeating the test roughly fifteen minutes after administering a bronchodilator — which assesses the reversibility of obstruction and is one of the criteria differentiating asthma (obstruction usually reversible) from COPD (obstruction usually only partially reversible or irreversible). Variants of the test also include bronchial challenge testing (e.g., with methacholine) in diagnosing asthma with an ambiguous presentation, and cardiopulmonary exercise testing (CPET), which combines spirometry with exercise and gas exchange measurement.

Spirometry remains the first-line test in diagnosing obstructive and restrictive lung disease precisely because it's inexpensive, widely available, non-invasive, and provides an objective, quantitative picture of respiratory function that symptom assessment alone can't fully replace. Its value grows with regular repetition — a single result gives a snapshot of lung status at a given moment, but a series of measurements over time captures the pace of disease progression or the effectiveness of treatment, which has direct bearing on treatment decisions.