1. Throw away out-of-date spirometers
If you have an old spirometer, with no option to read or upload flow-volume or volume-time curves, please ditch it. Without seeing the graphical traces, you cannot accurately detect spurious or inconsistent results and you can’t be sure whether the blow from the patient is sufficient. You will run the risk of missing important diagnoses.
2. Avoid spirometry during an infection or COPD exacerbation
Spirometry for diagnosing COPD needs to be done when a patient is at their best to avoid over-diagnosis – that’s why we bronchodilate beforehand. Never carry out spirometry during a respiratory infection or exacerbation. Ideally wait for six weeks after an infection to allow it to resolve, especially if making a first diagnosis. Use your discretion for patients who continuously exacerbate or require long-term steroid maintenance. Spirometry for suspected asthma should be done pre- and post-bronchodilator and often needs repeating as it can be normal at a planned visit if symptoms are absent.
3. Calibrate your spirometer
An instrument is only as accurate as its calibration. Although annual calibration should be carried out for instruments, best practice advises all operators to calibrate the spirometer prior to each session.
4. Check exhalations meet BTS criteria
There must be full-volume blows with good effort to meet BTS criteria. At least three good blows are required and at least two blows must have a vital capacity within 5% or 100ml of each other. Each forced manoeuvre must last six seconds minimum. Coughing is okay at the end but not at the beginning.
5. The flow-volume curve slope is concave in obstructive disease
Typically a flow-volume curve – as shown in the flow-volume curve diagram below – should have a high peak and then a gradual consistent slope down. In an obstructive defect, the peak is often lower and the sloping decline tends to have a concave shape inwards toward the centre of the axis.
6. In restrictive disease the flow-volume slope is steeper
In restrictive defects the peak may be normal or reduced but the slope tends to be steeper and terminates earlier – see diagram, below. If the curves don’t match the numbers, consider technical error or other pathology and seek specialist help if you cannot interpret it.
7. Look for a slow rise in the volume-time curve in obstructive disease
The volume-time curve should have a quick rise to a reasonable volume and then a plateau. In obstruction, the rise is quite slow and reaching a plateau often takes time. See the volume-time diagram below.
8. A quick rise in the volume-time curve suggests restrictive disease
In restriction, the rise on the volume-time curve is quick, but to a much smaller volume than normal and then it reaches a plateau. See the volume-time diagram right.
9. Use the first blow as a practice
Many patients will not be able to perform the test adequately and will need to be trained, which may take many attempts and be time consuming. It may help to use the first blow as a practice to reduce the chance that results will be skewed. Bring patients back for another appointment if you run out of time, rather than using unacceptable results. If it’s tricky to get a result you are happy with or the clinical picture and results don’t fit, refer to the local hospital lung function or chest clinic.
10. Ensure you have the right training
Everybody who carries out spirometry should have training and ongoing assurance of quality. The Association for Respiratory Technology and Physiology provides a high level of certification that might be suitable for a GP taking a special interest, or developing a diagnostic service. You could also ask your local hospital lung physiologist what spirometry training days they hold.
Dr Azhar Saleem is a GP in south London with a special interest in respiratory and allergy medicine
Dr Noel Baxter is a GP in south London and co-lead of the London Respiratory Team
The Primary Care Respiratory Society UK (PCRS-UK) is the UK-wide professional society committed to improving respiratory care in primary care. PCRS-UK is a registered charity, led by their members through a range of committees and faculties dedicated to meeting the vision of ‘optimal respiratory care for all’, providing education, policy support and research. As a member you’ll have unlimited access to a wealth of specialist respiratory care information, expertise and resources, plus practical everyday tools to help you make a difference in respiratory care. For more information about PCRS-UK and how to join, go to pcrs-uk.org/join.