Where other physiologic parameters, like blood pressure or heart rate, are not depending of other physiological properties, spirometry values can greatly differ depending on patient specific properties like age, gender, ethnicity and height.
To allow an interpretation we have to compare the blown values with the so-called predicted values, average values of a large population of healthy subjects.
The software that comes with most spirometers calculates these predicted values.
Predicted values are used because we do not always have better information at hand. In the ideal situation spirometry values should be compared to historical data of the same patient. Studies have shown that healthy patients will lose up to 25 mL of FEV1 every year from the age of 25. Losing more volume can be pathological.
Comparing the spirometry values only with the predicted values can lead to errors: a patient that has blown 120% of his predicted values and blows 100% of his predicted values one year later may have a very big problem, even if comparison with his calculated predicted values shows no signs of abnormality.
Since historical lung function data of the patient is rarely available, the next best thing is used: predicted values.
Experience has shown that predicted values for spirometry are dynamic as the population changes. Years after publication of predicted values it becomes clear that the values do no longer correspond to the present day population.
In 2012 the Global Lung Initiative published new predicted values that correspond better to the present day population. For the first time in history there was a cooperation between 6 international respiratory societies:
Lung function of more than 74000 healthy non-smoking people, spanning all ages and ethnic groups was measured and from this vast amount af data the new GLI predicted values were calculated. It is advised to use these new GLI predicted values and no longer use other sources of predicted values.
Before the latest GLI predicted values were published other predicted values were widely used. Many of these were based on specific populations. Most commonly used were:
These predicted values are no longer a very good indication for the present day population and should be used with great care.