Spirometry – top tips part 2: Interpretation

This article is the second of two articles designed to support you in your understanding of spirometry. You can read the first part, on the topic of accurately performing spirometry, here. In this article, you’ll learn:

• a step-by-step approach to interpreting spirometry results
• when and how to use reversibility testing top tips for primary care clinicians.

Interpreting results: a step-by-step approach

Remember, the spirometry alone will not give you the diagnosis, rather, it confirms or refutes what you suspect from the clinical history and patient assessment! Use the Global Lung Function Initiative reference values for ethnicity, age, sex, and height. Spirometry software should support this.

1. Assess the FEV₁/FVC (VC) Ratio^[1]

• when assessing the ratio use the highest volume VC or FVC for the calculation
• change in the absolute value post-bronchodilator ratio <0.70 or below age related lower limits of normal (LLN) confirms airflow obstruction
• Z – scores (standard residuals) remove age, height and sex related bias. The further away (to the left) the Z score is from -1.64 the ‘more’ abnormal the result. A percentage of ‘healthy’ individuals will have results in the ‘pink’ zone. Always focus on the patient, not just the numbers.

2. Assess the volumes^[1],[2]

• VC and FVC should be above 80% predicted
• VC and FVC should be within 150mls of each other
• If VC is higher than FVC this can indicate air trapping, and the VC should be used to calculate the ratio.

3. Classify the Pattern^[3]

• Normal – characterised by FVC, FEV_1, and FEV₁/FVC all within the normal range
• Obstructive – characterised by FEV₁/FVC < below lower limit of normal (LLN) This is usually but not always accompanied by FEV₁ < LLN
• Restrictive – characterised by FVC and FEV₁ <LNN accompanied by FEV₁/FVC>LNN, which may indicate restrictive lung disease.

4. Grading Severity (NICE COPD criteria, post-bronchodilator FEV₁)^[1]

• >80% predicted – Mild
• 50–79% – Moderate
• 30–49% – Severe
• <30% – Very Severe.

Reversibility testing: when and how

Reversibility testing is used mainly in suspected asthma, not routinely for COPD diagnosis^[1],^[2]

• administer 400 mcg salbutamol via spacer
• wait 15 minutes, then repeat spirometry
• where there is significant reversibility: FEV₁ increases by ≥12% and ≥200 mL or if (or if the FEV₁ is 10% or more of the predicted normal FEV₁).

Remember – reversibility supports, but does not confirm, asthma diagnosis. Asthma is variable – normal spirometry does not rule it out. Asthma guidelines now focus first line on markers of type 2 inflammation (raised blood eosinophils/ FENO) for diagnosis of asthma.^[4]

Top tips for primary care clinicians

1. Practice coaching – clear, simple instructions improve test quality
2. Use visual flow-volume curves – as these help spot technical errors
3. Check reversibility timing – wait the full 15 minutes post-bronchodilator
4. Be cautious with interpretation – context matters; use spirometry as part of a broader clinical picture
5. Document well – include pre/post results, quality grading, and interpretation in records
6. Refer when needed – unclear results, suspected restriction, or severe abnormalities may require specialist input or full lung function testing
7. Keep up to date – here on Together in Respiratory, or consider undertaking ARTP accredited training.

Any advice given and opinions expressed in this article are those of the author and do not reflect the view of Chiesi Limited (Chiesi).  All content in this article is for informational and educational purposes only.  Although Chiesi strives to always provide accurate information, it is not responsible for and does not verify for accuracy any of the information contained within.

[1]. A guide to performing quality assured spirometry tests. Available at: https://www.brit-thoracic.org.uk/media/70454/spirometry_e-guide_2013.pdf

[2]. Available at: https://my.clevelandclinic.org/health/diagnostics/17833-spirometry

[3]. Haynes JM. Basic spirometry testing and interpretation for the primary care provider. Can J Respir Ther. 2018

[4]. National Institute for Health and Care Excellence. Asthma: diagnosis, monitoring and chronic asthma management. Available at: https://www.nice.org.uk/guidance/ng245