Spirometry Test Quality Control includes examination of test values and evaluation of both the volume-time and flow-volume curves for evidence of technical errors. In this unit you will explore some key features to ensure that your spirometric readings are accurate and repeatable.

Reproducibility criteria refers to the closeness of measurements; is there variability that would suggest the patient has not taken a maximal inspiration, not given maximal force to their expiration, or not exhaled a maximal volume? Remember, it is only possible to achieve consistent measurements if you are performing optimal technique; if the subject repeatedly takes a sub maximal inspiration, they are going to have different measurements on every attempt.

Unfortunately, there are various reproducibility guidelines and this can cause confusion, but for the purpose of quality assured diagnostic spirometry, the following reproducibility criteria applies.

Achieve 3 relaxed vital capacity measurements within 5% or 100ml of each other

Achieve 3 forced vital capacity measurements within 5% or 100ml (FEV1 and FVC)

Repeatable peak flow (within 40L/min) and superimposable shape of the flow volume loop

If you achieve these criteria on the three best measurements (forced and base), you can be confident that the patient has put in maximal effort. Another useful quality control check is the comparison of the relaxed vital capacity to the forced vital capacity measurement; remember these are essentially the same measurement, just performed under different conditions. Therefore, if there is a significant difference it’s likely because one manoeuvre has been performed with sub maximal effort.

If the Relaxed Vital Capacity measurement and Forced Vital Capacity measurement differ by more than 10%, it is likely that one or other test was performed with sub maximal effort.

Of course, there is an exception. In a patient with significant obstructive lung disease, the relaxed manoeuvre might be greater owing to dynamic compression of the airways during the forceful manoeuvre.

Also always check that the PEF is within 5% as this helps determine patient effort – after all, varying the PEF may impact on the FEV1 and FVC values.