When my physician discusses my lung function with me, he often mentions “predicted values,” data that he compares to other results. What are they exactly? Why are these results so important?
For lung function tests, results are expressed in terms of litres or litres per second. Parallel to these results there are values expressed in percentages calculated on the basis of predicted values. Predicted values, in turn, are obtained by calculating averages based on tests done on a large number of healthy individuals who have no lung disease. Predicted values vary depending on sex, age, height, weight and even race. One can define predicted values as the values you would have if you were free of disease affecting the respiratory system.
Since the standard value is 100%, values between 80% and 120% are deemed to fall within normal limits. For example, one of the most significant parameters of these tests, the FEV1, measures the severity of the lung disease according to the percentage of the predicted value. Light impairment falls between 65% and 80%; moderate impairment, between 50% and 65%; serious impairment, below 50%; and very serious impairment, below 35%, always in relation to the predicted value.