Lung Function Interpretation
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Direct answer
Numbers only become a diagnosis when three questions are answered in order: is the FEV1/FVC ratio below 0.7 (obstruction — or below the lower limit of normal, the newer preferred standard), is the total lung capacity reduced below about 80% of predicted (restriction, which spirometry alone can only suggest), and what does the diffusion capacity for carbon monoxide (DLCO) add? An obstructive pattern with reduced DLCO points to emphysema or pulmonary vascular disease; restriction with low DLCO suggests interstitial lung disease; restriction with a normal or high DLCO suggests neuromuscular or chest-wall causes; and a high DLCO accompanies asthma, obesity and alveolar haemorrhage. Severity in airway obstruction is graded on the per cent-predicted FEV1: 80% and above mild, 50-79% moderate, 30-49% severe, below 30% very severe (the GOLD grades).
What you must remember
- Obstruction: FEV1/FVC below 0.7 (fixed-ratio) or below the lower limit of normal; both FEV1 and FVC fall but FEV1 falls more, the flow-volume loop shows a scooped expiratory limb, and TLC is normal or increased with air trapping (raised RV, raised FRC).
- Restriction: proportionately reduced FEV1 and FVC with a normal or high ratio; suspected on spirometry, confirmed only by a reduced TLC (below about 80% predicted or below LLN) — a low FVC alone can reflect air trapping, not restriction.
- GOLD spirometric grades for COPD: grade 1 mild FEV1 at least 80% predicted, grade 2 moderate 50-79%, grade 3 severe 30-49%, grade 4 very severe below 30% (all with FEV1/FVC below 0.7 post-bronchodilator).
- Reversibility: a rise of at least 12% and 200 mL in FEV1 after 200 micrograms of inhaled salbutamol (per older ATS/ERS criteria; newer guidance considers more than 10% of predicted), documenting an asthma component.
- DLCO patterns: reduced in emphysema, interstitial lung disease, pulmonary embolism and anaemia; normal in asthma and chest-wall or neuromuscular disease; increased in alveolar haemorrhage, polycythaemia and obesity; correct for haemoglobin before calling it abnormal.
- Flow-volume loop signatures: flattened inspiratory limb — extrathoracic obstruction (vocal cord palsy, goitre); flattened expiratory limb — intrathoracic obstruction; both flattened — fixed tracheal stenosis; a saw-toothed pattern suggests dynamic upper-airway collapse.
- Bronchoprovocation: a positive methacholine PC20 below 8 mg/mL supports asthma when baseline function is normal; the six-minute-walk test adds functional severity.
- Reference equations matter: Indian populations have different predicted volumes (Indian multi-centric 2015 equations or GLI adapted sets); using Caucasian equations systematically misclassifies Indian patients.
Three patients, three patterns, reasoned out
A 58-year-old smoker reports FEV1 1.6 L (52% predicted), FVC 3.0 L (78%), ratio 0.53, TLC 115% predicted, DLCO 55%. Every number agrees: obstruction with air trapping, loss of alveolar surface, COPD with an emphysema component and no meaningful bronchodilator response. Contrast a 45-year-old with progressive breathlessness and dry cough: FEV1 1.9 L (62%), FVC 2.5 L (60%), ratio 0.76, TLC 68%, DLCO 48% — proportionate volume loss with a preserved ratio and a genuinely reduced TLC confirms restriction, and the crushed DLCO localises it to the alveolar-capillary membrane, fitting interstitial lung disease. The third patient is the trap: myasthenia with FEV1 55%, FVC 54%, ratio 0.98, TLC 70% but DLCO 95% — restriction without membrane disease, the signature of a bellows problem rather than a lung problem. Read as triples — ratio, capacity, diffusion — and the pattern names itself.
Where students slip
The fixed 0.7 ratio overcalls obstruction in the elderly (whose ratios run high) and undercalls it in the young, which is exactly why lower-limit-of-normal interpretation exists — mention both, grade on the one your syllabus uses. The second classic is diagnosing restriction from a low FVC: in air trapping the FVC falls because the RV swells, so "restriction" without a measured TLC is provisional wording, and the report should say so. Third, quoting severity from the wrong column — COPD grades use per cent-predicted FEV1, never the absolute litres, and asthma control is a clinical judgement, not a number. And the exam-ready detail examiners reward: DLCO corrected for haemoglobin before interpretation, because anaemia alone can drag a normal lung's diffusion into the "reduced" box.
Frequently asked questions
How is obstruction defined on pulmonary function testing?
FEV1/FVC below 0.7 post-bronchodilator (or below the lower limit of normal), with air trapping shown by increased RV and TLC on full lung volumes.
Why can spirometry alone not diagnose restriction?
Because a reduced FVC may reflect air trapping rather than small lungs — total lung capacity by body plethysmography or gas dilution is required to confirm true restriction.
What does a reduced DLCO with an obstructive pattern suggest?
Emphysema or coexisting pulmonary vascular disease; asthma characteristically preserves DLCO, sometimes even raises it.
What are the GOLD spirometric grades?
Grade 1 FEV1 at least 80% predicted, grade 2 (50-79%), grade 3 (30-49%), grade 4 (below 30%), all requiring a post-bronchodilator FEV1/FVC below 0.7.
What does a flattened inspiratory limb of the flow-volume loop indicate?
Extrathoracic upper-airway obstruction such as vocal cord palsy or goitre — the expiratory limb alone flattening means intrathoracic obstruction.