spirometry
Spirometry is the bread-and-butter lung test: you take the deepest breath you can, seal your lips around a mouthpiece, and blast the air out as hard and as long as you can while a machine records every fraction of a second. Think of it as timing how fast and how completely you can empty a balloon.
From that single forced breath the device plots volume against time and airflow against volume, yielding the key numbers clinicians live by — chiefly the volume blown out in the first second, the total volume blown out, and the ratio between them. These numbers separate the two big mechanical problems of breathing: trouble getting air out (obstruction) and trouble getting air in (restriction).
Spirometry is effort-dependent, so coaching and repeated attempts matter; a result is only trustworthy when the patient gives a maximal, smooth, sustained blow that meets quality criteria. It measures only the air that can be moved at the mouth, so it cannot see air permanently trapped in the lungs — that requires separate lung-volume testing.
A 60-year-old smoker with morning cough blows out 1.8 L in the first second and 3.2 L in total, giving a ratio of 0.56 — below the lower limit of normal, confirming airflow obstruction consistent with COPD.
A single forced blow yields the numbers that distinguish obstruction from restriction.
Spirometry is the single most useful test for diagnosing and grading asthma and COPD, but a normal spirogram does not rule out all lung disease — early interstitial disease or vascular disease can hide behind normal numbers.