Restriction: a small, stiff lung
When you blow into a spirometer, two numbers dominate: the FVC (the total air you can forcibly exhale) and the FEV1 (how much comes out in the first second). In an obstructive disease like COPD the air comes out slowly, so the FEV1/FVC ratio drops. In a restrictive pattern the opposite happens: the lung is small and stiff, so there is less air to begin with. The FVC falls, but the FEV1 falls roughly in step with it, so the ratio stays normal or even high. Air that is in there comes out briskly — there is just less of it.
Patient with suspected ILD — spirometry (% predicted) FVC 2.40 L (62% predicted) -> reduced FEV1 2.00 L (66% predicted) -> reduced FEV1/FVC 0.83 (normal/high) -> NOT obstructed Reading it: Low FVC + preserved (high) ratio = restrictive pattern (confirm with low TLC on lung-volume measurement) Contrast - the same person if they had COPD: FEV1/FVC 0.55 (low) -> obstructed
DLCO: how well gas crosses the membrane
The second fingerprint is the DLCO — the diffusing capacity. The patient inhales a trace of carbon monoxide (which behaves like oxygen but binds haemoglobin so tightly we can measure exactly how much vanished into the blood) and we see how much was taken up. DLCO is really a measurement of the whole gas-transfer apparatus: the surface area of working membrane, its thickness, and the volume of blood available to load. In ILD the membrane is thickened and scarred and surface area is lost, so the DLCO falls.
Here is the elegant part. Restriction tells you the lung is small and stiff. A low DLCO tells you the gas-transfer surface is damaged. Put them together — a restrictive pattern with a disproportionately low DLCO — and you have the physiological signature of ILD. By contrast, restriction with a normal DLCO points away from the lung tissue and toward something pressing on the lung from outside, like obesity, a chest-wall problem, or weak breathing muscles. The DLCO is the test that earns its keep here.
Why oxygen drops with activity first
A thickened membrane can often keep up at rest, when blood lingers in the capillary long enough to fully load with oxygen. But during exercise the heart speeds up and blood races through the capillary faster, leaving less time to cross a slow membrane. That is why a person with early ILD may have a normal resting oxygen level yet desaturate sharply when they walk. The six-minute walk test is built around exactly this: we watch the oxygen saturation fall as the patient walks, which both confirms the problem and gives a baseline to track over time.
These numbers are not just for diagnosis — they are the dials we watch over years. A falling FVC and a falling DLCO are the most useful warnings that an ILD is progressing, often before the person notices much change. Because compliance (the ease of inflating the lung) drops as scarring advances, the work of each breath climbs, and the trend in these tests guides every treatment decision that follows.