Lung Function Test Results for GPs

Help for GPs

If you are reviewing this page, chances are you have a participant from the Hazelwood Health Study bringing you a letter with lung function results attached for review. Participants will only receive this letter if we identify a pattern that we believe requires further investigation.

The lung function results include spirometry and gas transfer measurements.


Spirometry provides information regarding ventilatory function – movement and volume of air in and out of the lungs.

Spirometry can identify ventilatory function that is within normal limits, obstructed (cannot blow out as quickly as expected) or hints at restricted patterns (volumes are smaller than expected).  Spirometry is also able to identify reversible obstruction by assessing change before and after bronchodilator.

Gas Transfer (DLCO)

The transfer (diffusion) across the lungs of carbon monoxide (DLCO) provides information regarding gas exchange – the ability of gases (such as oxygen) to move across the lungs into the blood.

Cautionary statement regarding lung function reports

Diagnosis of lung disease should not be made based on lung function tests alone.  Diagnoses should be based on a good clinical history (symptoms, smoking history, occupational exposures), imaging, lung function tests and other pathology tests (bloods, biopsies) where relevant. This is why for many of the reports, we have only reported patterns that we are seeing and make no comment regarding diagnosis (unless already known) or we have additional clinical information at hand to assist in interpretation.

What appears in the lung function report

The lung function reports for this study will usually comment on the following components:

  • Test performance if it is considered that the test performance may negatively impact on the findings.
  • The observed ventilatory pattern (spirometry)
  • Bronchodilator response (see below).
  • Whether gas exchange is impaired or not (i.e. DLCO is reduced). Where gas exchange is impaired, whether the broad cause of the gas exchange impairment is identified.

Bronchodilator Response:

  • A significant bronchodilator response is defined as an increase of at least 200mL and 12% in FEV1 and/or FVC between pre and post bronchodilator assessments.1
  • A significant bronchodilator response suggests reversible airflow obstruction
  • Where reversibility is demonstrated, the reversibility may be complete (post bronchodilator spirometry returns to within normal limits) or incomplete (post bronchodilator spirometry remains outside the normal limits)
  • Reversibility of airflow limitation is suggestive of sub-optimal control of airways disease.
  • Clinical correlation with known lung disease or clinical history is required.

Obstructive Defects                                                        

Definition: FEV1/FVC ratio is below the lower limit of normal (LLN).1

Common obstructive pathologiesLess common obstructive pathologies
COPD (emphysema, chronic bronchitis)Bronchiectasis
 Upper airway obstruction
 Foreign body in airway



Post bronchodilator spirometry remains obstructed (with or without reversibility to inhaled bronchodilator). Gas exchange may also be impaired. Consider smoking history, age, other symptoms such as exertional breathlessness.

Chronic bronchitis:

Post bronchodilator spirometry remains obstructed (with or without reversibility to inhaled bronchodilator). Gas exchange is unlikely to be impaired. Consider history of respiratory infections, sputum production, smoking history, etc.


Obstruction on baseline spirometry that returns to within normal limits post bronchodilator.  Gas exchange is usually within normal limits.

Note: In patients with history of poorly controlled asthma, post bronchodilator spirometry pattern may show obstruction.

Consider clinical history and symptoms (tightness in chest, nocturnal symptoms, exertional breathlessness/wheeze) as part of clinical correlation.

When to consider referring on:

Referral to a respiratory or general physician for further evaluation is recommended if:

  • FEV1 <70% predicted
  • FVC is reduced (<LLN) in addition to obstruction
  • Significant bronchodilator response
  • No known history of lung disease
  • Any clinical concern related to airflow obstruction

Possible Restrictive Defects                                                        

Definition: FVC<LLN and FEV1/FVC is within normal limits

A restrictive ventilatory defect is defined by a reduced total lung capacity (TLC) when FEV1/FVC is within normal limits.1 We are unable to measure TLC using spirometry. Hence, a reduced FVC in the presence of a FEV1/FVC within normal limits, may suggest a restrictive pattern, though further lung function tests (measurement of lung volumes via plethysmography) is recommended to confirm this.

Causes of Restrictive Ventilatory Patterns
Interstitial lung diseases (stiff lungs)
Scoliosis/disorders that effect movement of chest wall
Neuromuscular disorders – affecting respiratory ‘pump’ muscles
Pulmonary congestion
Pleural effusions

Interstitial lung diseases: 

Restrictive type pattern on spirometry that is often associated with impaired gas exchange (reduced DLCO). Consider clinical history, occupational history, symptoms.


Generally only impacts spirometry when BMI is very high (>40 kg/m2). Gas exchange generally not impacted. Consider clinical history, smoking history, symptoms (breathlessness, wheeze), nocturnal symptoms, sleep disordered breathing.

When to consider referring:

Consider further evaluation of ventilatory defect (referral to local lung function laboratory for assessment of lung volumes to confirm restrictive defect) if:

  • FVC between LLN and 70% but no evidence of obstruction or gas exchange impairment.

Consider referral to respiratory physician (which will likely include clinical review and further lung function tests to confirm restrictive defect) if:

  • If FVC between LLN and 70%, and gas exchange impairment
  • If FVC is less than 70% predicted with or without gas exchange impairment

Changes in gas transfer (DLCO):

Gas transfer measurement can be impacted by many factors, some of which are listed below: 2,3

Decreased DLCOIncreased DLCO
Anaemia (low Hb, increased COHb)Polycythaemia
High PIO2 (supplemental O2)Low PIO2 (test at altitude)
Decreased pulmonary capillary blood volume
(valsalva manoeuvre)
Increased pulmonary capillary blood volume
(Mueller manoeuvre, exercise)
 Supine posture

Gas transfer can also be impacted by pathophysiological causes: 2,3

Decreased DLCOIncreased DLCO
Reduction in alveolar membrane surface area e.g. emphysema, loss of lung units, incomplete alveolar expansionBlood in alveolar spaces eg. pulmonary haemorrhage
Alveolar membrane thickness increase e.g. pulmonary oedema, interstitial lung diseases.Changes in blood volume / distribution in the lungs e.g. asthma, obesity
Reduction in pulmonary capillary volume e.g. pulmonary hypertension, pulmonary embolus, microvascular destructionIncreased pulmonary capillary volume e.g. redistribution of blood flow following lung resection

Abnormal DLCO results should be interpreted in association with spirometry results and clinical history.

  • The most common cause of reduced gas transfer is COPD (emphysema) – clinical correlation is required.

When to consider referring:

Referral to a respiratory physician for further evaluation is strongly recommended for the following:

  • Known COPD with DLCO <60% predicted
  • DLCO is reduced but spirometry is within normal limits
  • DLCO is reduced with a possible restrictive pattern on spirometry
  1. Pellegrino P, Viegi G, Brusasco V, Crapo RO, Burgos F et al. ATS/ERS task force: Standardisation of Lung Function tests. Interpretative strategies for lung function tests. European Respiratory Journal. 2005; 26(5): p. 948-68.
  2. Borg BM, Thompson BR, O’Hehir RE. Interpreting Lung Function Tests: A Step-by-Step Guide Chichester: Wiley Blackwell; 2014.
  3. Bronchiectasis toolbox. [Online]. 2016 [cited 2017 May 31]. Available from:

Our Partners

Acknowledgement to Country

We acknowledge and pay our respects to the Elders and Traditional Owners of the land where our Study teams are based, particularly the Gunaikurnai peoples of Gippsland.