Age-specific per cent predicted
Expressing spirometric indices as a percentage of the predicted value has been engrained into our minds, and therefore many people will not easily part with it. Age, height and gender specific percentage predicted would, of course, be a valid alternative of the standard deviation score. But is it a practical solution?
The table below lists the ‘lower limits of normal’ for FEV1 in males and females, expressed as percentage predicted. They were obtained by subtracting from the predicted value 1.69·0.51 L (males) and 1.71·0.38 L (females), respectively (the 5th percentile limit of the standard deviation score (Z-score) multiplied by the residual standard deviation), and expressing this as a percentage of the predicted value. The lowest and highest percentile limit in the table are 55.3 and 85.2% per cent predicted in males, and 43.0 en 87.1% predicted in females. These percentages can all be conveniently summarized as -1.69·RSD in males and -1.71·RSD in females. A similar table can be prepared for the (F)VC, and another one for FEV1%(F)VC as a function of age.
Age and
height specific lower limit of FEV1 as a percentage of predicted
in Caucasian males and females. |
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| Height (m) | Age males (yr) | Age females (yr) | ||||||||||
| 25 | 35 | 45 | 55 | 65 | 75 | 25 | 35 | 45 | 55 | 65 | 75 | |
| 1.40 | 73.9 | 70.7 | 66.7 | 61.3 | 53.9 | 43.0 | ||||||
| 1.50 | 75.3 | 72.9 | 69.9 | 66.3 | 61.5 | 55.3 | 77.7 | 75.4 | 72.7 | 69.1 | 64.6 | 58.8 |
| 1.60 | 78.2 | 76.4 | 74.1 | 71.4 | 68.1 | 64.0 | 80.6 | 78.9 | 76.8 | 74.3 | 71.3 | 67.4 |
| 1.70 | 80.5 | 79.0 | 77.3 | 75.3 | 72.8 | 69.8 | 82.8 | 81.4 | 79.7 | 78.0 | 75.8 | 73.1 |
| 1.80 | 82.4 | 81.2 | 79.8 | 78.2 | 76.3 | 74.0 | 84.5 | 83.4 | 82.2 | 80.8 | 79.1 | 77.2 |
| 1.90 | 83.9 | 82.9 | 81.8 | 80.5 | 79.0 | 77.2 | 87.1 | 86.4 | 85.6 | 84.7 | 83.6 | 82.4 |
| 2.00 | 85.2 | 84.3 | 83.4 | 82.3 | 81.1 | 79.7 | ||||||
It does not seem attractive to work with such tables when the standard deviation score (the Z-score) provides such a simple and convenient solution.