Accurate measurement of height and age for lung function testing
Height should be measured, as self-reported height is unreliable. Differences between actual and self-reported height may be up to 6.9 cm, and are generally largest in elderly subjects [1-6]. The FEV1 and FVC are a function of heightk, where k ~ 2.2. In a 110 cm tall child, or a 180 cm tall adult, a 1 cm error leads to an error in the predicted lung function index of 2% and 1.2%, respectively. Not only should standing height be measured, but the stadiometer should be calibrated every year, and in calculating predicted values height should be entered with 1 decimal accuracy [7,8].
The effect of errors in age on predicted values cannot be so easily estimated because of the variable contribution of the spline in age. If age is systematically underestimated by 0.75 years by rounding off, then the percentage error is as listed in the table.
|Age (yr)||FEV1 %error||FVC %error||FEV1 %error||FVC %error|
|3 vs 3.75||-2.8||-3.4||-2.9||-3.6|
|10 vs 10.75||-1.3||-1.4||-2.6||-2.7|
|15 vs 15.75||-3.4||-2.9||-3.4||-2.9|
|50 vs 50.75||+0.4||+0.4||+0.6||+0.7|
|85 vs 85.75||+0.7||+0.5||+0.9||+1.0|
The errors vary with age, the largest errors occurring in childhood. Therefore, in calculating predicted values, age should be entered with 1 decimal accuracy [7,8].
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