Spirometry Interpretation

What is spirometry?

Spirometry is a method of assessing lung function by measuring the volume of air that the patient is able to expel from the lungs after a maximal inspiration. It is a reliable method of differentiating between obstructive airways disorders (e.g. COPD, asthma) and restrictive diseases (e.g. fibrotic lung disease).

Aside from being used to classify lung conditions into obstructive or restrictive patterns, it can also help to monitor disease severity. This guide aims to provide a basic approach to spirometry interpretation.

Spirometry provides several important measures including:

  • Forced expiratory volume in 1s (FEV1) – the volume exhaled in the first second after deep inspiration and forced expiration, similar to PEFR
  • Forced vital capacity (FVC) – the total volume of air that the patient can forcibly exhale in one breath
  • FEV1/FVC – the ratio of FEV1 to FVC expressed as a percentage

Values of FEV1 and FVC are expressed as a percentage of the predicted normal for a person of the same sex, age and height.

Reference ranges

  • FEV1: >80% predicted
  • FVC: >80% predicted
  • FEV1/FVC ratio: >0.7

Patient details

Confirm the patient’s details:

  • Name
  • Age
  • Gender
  • Height
  • Ethnicity

Age, gender, height and ethnicity are used to calculate predicted normal values for the patient.

Assess quality of results

Three consistent volume-time curves are required, of which the best two curves should be within 5% of each other.

The best of the three consistent readings of FEV1 and FVC should be used in your interpretation.

The expiratory volume-time graph should also be smooth and free from abnormalities caused by:

  • Coughing during expiration
  • Extra breath during expiration
  • Slow start to forced expiration
  • Sub-maximal effort


Obstructive pattern

  • FEV1 reduced (<80% of the predicted normal)
  • FVC reduced, but to a lesser extent than FEV1
  • FEV1/FVC ratio reduced (<0.7)
Spirometry: Obstructive lung disease


It can be useful to assess reversibility with a bronchodilator if considering asthma as a cause of obstructive airway disease.

Patients should be asked to stop bronchodilator therapy prior to spirometry, to ensure previous treatments do not affect the results (if the patient has severe disease, this would not be advisable):

  • Short acting beta-2-agonists should be stopped 6 hours prior to testing
  • Long acting beta-2-agonists should be stopped 12 hours prior to testing


To assess reversibility, administer 400 micrograms of salbutamol and repeat spirometry after 15 minutes:

  • The presence of reversibility is suggestive of asthma
  • The absence of reversibility suggests fixed obstructive respiratory pathology such as COPD
  • Partial reversibility may suggest a coexisting diagnosis of asthma and another obstructive airway disease (e.g. COPD)


Causes of obstructive lung disease

  • COPD
  • Asthma
  • Emphysema
  • Bronchiectasis / Cystic fibrosis


Restrictive pattern

  • FEV1 reduced (<80% of the predicted normal)
  • FVC reduced (<80% of the predicted normal)
  • FEV1/FVC ratio normal (>0.7)
Spirometry: Restrictive lung disease


Causes of restrictive lung disease

Pulmonary causes:

  • Pulmonary fibrosis
  • Pneumoconiosis
  • Pulmonary oedema
  • Lobectomy/pneumonectomy
  • Parenchymal lung tumours


Non-pulmonary causes:

  • Skeletal abnormalities (e.g. kyphoscoliosis)
  • Neuromuscular diseases (e.g. motor neuron disease, myasthenia gravis, Guillan-Barre)
  • Connective tissue diseases
  • Obesity or pregnancy


1. Spirometry in Practice: A Practical Guide to Using Spirometry in Primary Care 2nd Ed (2005). British Thoracic Society COPD Consortium. Accessed at: [LINK]

2. Dr Colin Tidy. Spirometry. Patient.info. Published 2nd Dec 2016. Accessed on 12th Dec 2017. [LINK]


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