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Three numbers, one ratio
Spirometry produces two primary numbers: FVC, everything a maximal blast can exhale, and FEV1, the fraction delivered in the first second. Their ratio is the single most informative value on the report. A healthy chest empties fast (ratio around 0.75 to 0.85); narrowed airways empty slowly (ratio falls); a small stiff chest empties its reduced volume briskly (ratio preserved or high).
Values are judged against predicted for age, sex, height and ethnicity, so read the percent-predicted column, not the raw litres.
Step 1: is there obstruction?
FEV1/FVC below the lower limit of normal (the fixed 0.70 cutoff is the common shorthand) declares obstruction. That one comparison separates the two great families of lung disease before any other number is consulted. If the ratio is preserved, move to Step 3 and think about size instead of speed.
Step 2: how bad, and does it reverse?
With obstruction declared, FEV1 percent-predicted grades severity: mild above 70, moderate 50 to 69, severe 35 to 49, very severe below 35. Then the bronchodilator answers the diagnostic question: an FEV1 improving by at least 12 percent and 200 mL after salbutamol is significant reversibility, the asthma signature; fixed obstruction in the right smoker is COPD. Normal spirometry with a convincing story does not exclude asthma; between attacks the test can be clean, which is what methacholine challenge is for.
Step 3: is there restriction?
A low FVC with a preserved ratio only suggests restriction; spirometry cannot prove it, because a low FVC can also come from gas trapping. Confirmation needs lung volumes: TLC below 80 percent predicted establishes restriction. The differential then splits by where the smallness lives: stiff parenchyma (fibrosis), a stiff or crowded chest wall (kyphoscoliosis, obesity), or muscles too weak to stretch a normal lung (neuromuscular disease, where a supine FVC drop or low maximal pressures betray the diaphragm).
Step 4: the DLCO splits the finalists
Diffusing capacity measures the gas-exchange surface, and it resolves the ties the mechanics leave behind. Obstruction with a low DLCO is emphysema (surface destroyed); obstruction with a normal or high DLCO is asthma. Restriction with a low DLCO is interstitial disease; restriction with a normal DLCO points to the chest wall or muscles, which restrict volume but leave the membrane intact. And a low DLCO with entirely normal mechanics is its own red flag: pulmonary vascular disease, early ILD, or anemia (which the lab corrects for if given a hemoglobin).
The loop is a silhouette
The flow-volume loop repeats the numbers as a shape you can recognize at a glance. Normal expiration rises to a sharp early peak and descends in a straight line. Obstruction scoops that descent into a concave curve as small airways collapse. Restriction shrinks the whole loop but keeps its proportions. And the loop sees what spirometry averages away: a flattened inspiratory limb is a variable extrathoracic obstruction (vocal cord dysfunction), a flattened expiratory limb a variable intrathoracic one, and both limbs flattened into a box is a fixed central obstruction (tracheal stenosis, goiter) that no bronchodilator will touch.
Pearls
- The ratio detects obstruction, the FEV1 grades it, the bronchodilator classifies it.
- Spirometry suggests restriction; only a TLC below 80 percent predicted confirms it.
- DLCO is the tiebreaker: emphysema versus asthma, ILD versus chest wall.
- A flattened inspiratory limb in a 'refractory asthmatic' is the upper airway asking to be examined.
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