Diagnostics & Procedures · Topic 6 of 16

Pulmonary Function Tests

Diagnostics · pulmonary function

Pulmonary Function Tests

Read the FEV₁/FVC ratio first: low means obstruction. Restriction needs a low TLC, and the DLCO tells lung tissue from chest wall.

Key numbers

Obstruction: low FEV₁/FVC ratioFEV₁/FVC < 0.70

Restriction requires a low TLCTLC < LLN

Bronchodilator response: > 10% of predicted> 10% of predicted

01 · Defined byObstruction: low FEV₁/FVC ratio

Airflow obstruction = a reduced FEV₁/FVC ratio: post-bronchodilator below 0.70 by the GOLD fixed ratio, or below the lower limit of normal (z-score < −1.645) by the 2022 ERS/ATS standard.

02 · Defined byRestriction requires a low TLC

Restriction is defined by a total lung capacity below the lower limit of normal (traditionally < 80% predicted). A low FVC with a normal ratio only suggests restriction until lung volumes are measured.

03 · Defined byBronchodilator response: > 10% of predicted

A significant bronchodilator response is a rise in FEV₁ or FVC of more than 10% of predicted (2022 ERS/ATS), replacing the older ≥ 12% and ≥ 200 mL from baseline that GINA still uses.

What it is

Core idea

PRISm: normal ratio, reduced FVC or FEV₁Preserved ratio impaired spirometry (PRISm) = normal FEV₁/FVC with a reduced FVC (or FEV₁); it may be restriction, early obstruction or a nonspecific pattern — measure TLC — and carries higher mortality.

Causes & risk factors

Why it happens

High DLCO: polycythemia, alveolar hemorrhage, asthmaDLCO rises above predicted with polycythemia, alveolar hemorrhage (red cells in the alveoli bind carbon monoxide) and asthma; anemia lowers DLCO, so always correct it for hemoglobin.

Presentation

What you'll see

Flow-volume loop localizes upper airway obstructionFixed upper airway obstruction (tracheal stenosis, goiter) flattens both limbs; variable extrathoracic obstruction (vocal fold paralysis) flattens the inspiratory limb; variable intrathoracic obstruction (tracheomalacia) flattens the expiratory limb.

Scooped expiratory limb means obstructionOn the flow-volume loop, obstruction gives a scooped-out (concave) expiratory limb; restriction gives a small loop whose flow is normal or high for the lung volume because fibrotic lungs recoil strongly.

Diagnosis & monitoring

Work-up

Mixed defect: low ratio plus low TLCA mixed obstructive–restrictive defect = FEV₁/FVC below the lower limit of normal plus a TLC below the lower limit of normal; obstruction with a low FVC needs lung volumes to detect it.

The rules

Know these

Normal FEV₁ and FVC: useful negativesFEV₁ is the most reproducible spirometric value: a normal FEV₁ makes fixed obstructive disease unlikely, and a normal FVC makes restrictive disease unlikely.

Obstructive or restrictive pattern?

Commonly confused

FEV₁/FVC	Reduced (< 0.70 or < LLN)
TLC	Normal or increased
Residual volume	Normal or increased (air trapping)
Flow-volume loop	Scooped-out expiratory limb
Examples	COPD, asthma, bronchiectasis

Commonly confused

Tell apart

Low DLCO: emphysema, fibrosis, pulmonary vascular disease must knowDLCO falls when the alveolar–capillary surface is lost: emphysema (with obstruction), interstitial lung disease (with restriction), and pulmonary vascular disease such as pulmonary hypertension or embolism (with normal spirometry).

Restriction with normal DLCO: extraparenchymal causeRestriction with a normal DLCO points outside the lung parenchyma — obesity, kyphoscoliosis or other chest wall disease, pleural disease, or neuromuscular weakness — whereas parenchymal restriction (pulmonary fibrosis) lowers DLCO.

Isolated low DLCO: vascular disease or anemiaA low DLCO with normal spirometry and lung volumes suggests pulmonary vascular disease (pulmonary hypertension, pulmonary embolism), anemia (correct for hemoglobin), or emphysema with preserved lung volumes.

Fixed 0.70 ratio overcalls obstruction in eldersFEV₁/FVC falls normally with age, so the fixed 0.70 cutoff can label healthy older adults as obstructed; the age-adjusted lower limit of normal avoids this, while GOLD keeps 0.70 for simplicity.

More detail

Reference

Show 7 reference facts

ERS/ATS 2022 grades impairment by z-score rather than percent predicted: mild −1.65 to −2.5, moderate −2.51 to −4.0, severe below −4.1; GOLD grades COPD airflow limitation by FEV₁ % predicted.

Body plethysmography measures lung volumes more accurately than nitrogen washout or helium dilution in airflow obstruction, because dilution methods miss trapped gas that does not communicate with the airways.

Methacholine challenge is for suspected asthma with normal spirometry: a 20% FEV₁ fall at a dose (PD20) below 25 mcg confirms airway hyperresponsiveness, and a PD20 above 400 mcg excludes it.

An acceptable spirogram has a forceful start, no cough and at least 6 seconds of exhalation; the two best FEV₁ and FVC efforts should agree within 5% or 100 mL, whichever is larger.

The ATS recommends race-neutral (GLI global) reference equations, because race-specific equations underestimate disease severity in non-White patients.

Maximal inspiratory and expiratory pressures assess respiratory muscle strength; a fall in vital capacity of more than 25% from upright to supine suggests significant diaphragm weakness.

After lung resection the DLCO is low because the lung is smaller, but it corrects to normal or above when adjusted for alveolar volume (DLCO/VA).

Pearl

Ratio for obstruction, TLC for restriction, DLCO for where the problem lives.

Sources · Merck Manual Professional — Airflow, Lung Volumes, and Flow-Volume Loop (Wood KL, rev. Apr 2026) · GOLD 2026 Pocket Guide to COPD Diagnosis, Management, and Prevention · Merck Manual Professional — Measurement of Gas Exchange (DLCO section; Wood KL, rev. Apr 2026) · Ora J et al. Pulmonary Function Tests: Easy Interpretation in Three Steps. J Clin Med 2024;13:3655 (PMC11242573; summarizes the 2022 ERS/ATS standard) · Vanderbilt Internal Medicine Residency Handbook (VIM Book) — Pulmonary Function Tests · Merck Manual Professional — Tests of Respiratory Muscle Function (rev. Apr 2026) · Stanojevic S et al. ERS/ATS technical standard on interpretive strategies for routine lung function tests. Eur Respir J 2022;60:2101499 (claims confirmed via Merck and the PMC review) · Ben Saad H et al. How to interpret parameters of routine lung function tests in 2023? Tunis Med 2023 (PMC11157243; summarizes the 2022 ATS/ERS DLCO algorithm) · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).