Age dependence of the FEV1/FVC ratio Philip Quanjer, none 1 August 2012 Gólczewski et al.  argue that the FEV1/FVC ratio is age dependent because the FEV1 and FVC are mathematically linked as follows: FEV1 = A¿FVC + C, `without respect to the other factors, including age and height¿. As FVC is age dependent, the authors expect the FEV1/FVC ratio to be also age dependent. Although the FEV1 and FVC are undeniably closely related in healthy subjects, their reasoning is a simplification of the biological determinants of the relationship between FEV1, FVC and the FEV1/FVC ratio. The FVC is determined by a host of factors, including body dimensions, mechanical properties of the thorax, and muscular force, and is therefore age and height dependent. The FEV1 is a portion of the vital capacity that can be inhaled in one second. Whereas it is undoubtedly related to the size of the vital capacity, important independent determinants are the mechanical properties of intrathoracic airways and lung elastic recoil . It has been shown in numerous publications that FEV1 varies with age and height, and that there are biological reasons for this, but that the coefficients are not the same as those for FVC . This implies that the FEV1/FVC ratio is a function of both height and age. This can be illustrated as follows. Both FEV1 and FVC have often been shown to be a power function of height and age. For example, Brändli et al.  provide the following prediction equations for adult males: ln(FEV1) = 1.9095 * ln(Height) - 0.0037 * Age - 0.000033 * Age² - 8.240. ln(FVC) = 2.1685 * ln(Height) + 0.0030 * Age - 0.000075 * Age² - 9.540. where ln is natural logarithm, Height is in cm, and Age in years. If all coefficients could be estimated without errors, it would follow that ln(FEV1/FVC), which equals ln(FEV1) ¿ ln(FVC), would equal: ln(FEV1/FVC) = -0.259*ln(Height) - 0.0067 * Age + 0.00042* Age² + 1.3. This comes fairly close to the estimated relationship quoted by Brändli et al.: ln(FEV1/FVC) = -0.3144 * ln(Height) - 0.0033 * Age + 1.526. This shows that the FEV1/FVC ratio is non-linearly related to both age and height. The latter was recently corroborated in a study by the Global Lung Function Initiative of >74,000 healthy non-smokers . The same group previously showed that in children and adolescents the FEV1/FVC ratio declines far from linearly: a steep fall until the start of the adolescent growth spurt, followed by a rise before a non-linear fall in adults . 1. Gólczewski T, Lubi¿ski W, Chcia¿owski A. A mathematical reason for FEV1/FVC dependence on age. Respiratory Research 2012, 13: 57. 2. Babb TG, Rodarte JR. Mechanism of reduced maximal expiratory flow with aging. J Appl Physiol 2000; 89 :505-511. 3. http://www.lungfunction.org/publishedreferencevalues.html. 4. Brändli O, Schindler Ch, Leuenberger PH, Baur X, Degens P, Künzli N, Keller R, Perruchoud AP. Letters to the editor. Re-estimated equations for 5th percentiles of lung function variables. Thorax 2000; 55: 172. 5. Quanjer PH, Stanojevic S, Cole TJ et al. Multi-ethnic reference values for spirometry for the 3-95 year age range: the Global Lung Function 2012 equations. Eur Respir J published 27 June 2012 as doi: 10.1183/09031936.00080312. 6. Quanjer PH, Stanojevic S, Stocks J, et al. Changes in the FEV1/FVC ratio during childhood and adolescence: an intercontinental study. Eur Respir J 2010; 36: 1391¿1399. Competing interests No competing interests.