In tutorial 7, we explore how to define normal in spirometry, including reference equations that are used, the different methods to compare our patient to their normal, and the FEV1/FVC ratio and how this differs with age.

Key messages:

• Defining normal in spirometry is not a straightforward as other diagnostic tests that we perform, and is affected by factors such as age, height, sex and ethnicity.
• Reference equations are used to predict a patient’s normal, and once we have generated these predicted values we can begin by comparing the patient’s measured results with their predicted values.
• In most major guidelines, the FEV1/FVC ratio is defined as abnormally reduced when it falls below 0.7, but we need to remember that the ratio declines with age, and so in some circumstances a patient might have abnormal test results according to the guidelines, but normal test results when compared to their normal population.