How predicted spirometry values are calculated
Measured FEV1 and FVC are compared against age-, height-, and sex-based predicted values using simplified linear regression equations, then expressed as a percentage of predicted.
This tool uses simplified reference equations suitable for quick screening. For diagnostic-grade reporting, results should be calibrated against a validated predicted-value set such as GLI-2012, matched to the patient's ethnicity.
Normal values & pattern classification
| Pattern | FEV1/FVC ratio | FVC % predicted |
|---|---|---|
| Normal | ≥ 70% | ≥ 80% |
| Obstructive | < 70% | ≥ 80% |
| Restrictive | ≥ 70% | < 80% |
| Mixed | < 70% | < 80% |
Clinical use
Spirometry patterns broadly distinguish obstructive lung disease (asthma, COPD — airflow limitation with a preserved or increased lung volume) from restrictive disease (interstitial lung disease, chest wall or neuromuscular disorders — reduced lung volume with preserved airflow ratio). Severity within obstructive disease (e.g. GOLD staging in COPD) is further graded by FEV1 % predicted. Formal diagnosis requires a properly calibrated spirometer, technically acceptable/repeatable manoeuvres per ATS/ERS criteria, and — for obstructive patterns — a bronchodilator reversibility test.
Frequently asked questions
What is spirometry?
A breathing test that measures how much air you can forcefully exhale (FVC) and how much of that comes out in the first second (FEV1), used to detect and classify obstructive and restrictive lung disease.
What does the FEV1/FVC ratio mean?
It's the proportion of your total forced exhale that comes out in the first second. A ratio below 70% suggests airflow obstruction (as in asthma or COPD); a preserved ratio with reduced volumes suggests a restrictive pattern instead.
What's the difference between obstructive and restrictive patterns?
Obstructive disease (e.g. COPD, asthma) shows reduced airflow with a low FEV1/FVC ratio but often normal or increased lung volume. Restrictive disease (e.g. interstitial lung disease, chest wall disorders) shows reduced lung volumes with a preserved or increased FEV1/FVC ratio.
Is home spirometry as accurate as a hospital test?
Home/portable spirometers can be useful for trend-monitoring, but formal diagnosis requires a calibrated spirometer, technically acceptable and repeatable manoeuvres per ATS/ERS criteria, and often a bronchodilator reversibility test — conditions a home device typically can't fully replicate.
References
- Miller MR, et al. Standardisation of spirometry. Eur Respir J. 2005;26:319–338.
- Quanjer PH, et al. Multi-ethnic reference values for spirometry (GLI-2012). Eur Respir J. 2012;40:1324–1343.
- Global Initiative for Chronic Obstructive Lung Disease (GOLD). Global Strategy for Diagnosis, Management, and Prevention of COPD, 2024 report.