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FEV1% changes in times of quick growth

My 14-year-old son has grown 15 cm in the last 15 months. His FEV1 values in liters increased during that period however, sometimes, his FEV1 values in % were lower than before, sometimes the same. Are the values in percent reliable at all in times of quick growth?


In a lung function test, FEV1% is calculated on the basis of reference values. Formerly, and for the most part also today, the reference values by Zapletal from 1987 are used in Europe. In the USA, in most cases the values by Knudson from 1983 are taken as a reference. Recently, the newer values of the LUNOKID study*) are probably supposed to be used in Germany. What is true for all reference values, though, is that hundreds of healthy test subjects were tested so that an age-related distribution in form of percentiles could be calculated.

For adults, often only the absolute number in liters is considered for the spirometry values. For children, however, this is not possible because of the growth. As you also noticed, the liter numbers have increased during the growth. This is normal. The FEV1% values - being relative values – should remain the same if the lung function is the same. Of course there are fluctuations of the lung function values (form of the day, infections, respiratory symptoms, etc.). However, if the tendency of the FEV1% values is falling during a certain period of time (although the absolute values are increasing), this is a clear sign for a deterioration of the lung function. This is exactly the reason why those relative values in percent were established. Despite all fluctuations that can also happen, these reference values have proven their worth for several decades by now and if in Germany new values (LUNOKID study) are introduced now, this is only an adaption of the values since the “normal child” [healthy test subject] is now taller than in former times. In general, everything stays the same more or less, and it is very unlikely – considering the big number of tested healthy subjects – that major incorrect fluctuations within the reference values would occur.

I hope to have been able to answer your question sufficiently.

Best regards,
Olaf Sommerburg

*) English abstract of the German article: