Expressing results correctly
In adults it is incorrect to express the measured value of a spirometric index as a percentage of the predicted value because the scatter is the same for a low or a high predicted value. The correct procedure, therefore, is to compare observed and predicted value, and to assess whether the difference between the two is or is not compatible with the scatter about the predicted value. Let us look at a concrete example of
FEV1 in an adult woman |
|
| Measured FEV1 | 3.87
L |
| Predicted FEV1 | 4.33
L |
| Measured - predicted FEV1 | -0.46
L |
| Scatter about predicted FEV1 | 0.38
L |
The measured value is -0.46/0.38 = 1.21 times the residual standard deviation below the predicted value: the Z-score, also called the standard deviation score (SDS) = -1.21. If the data are normally distributed (this is often not the case in daily life!), then in only 5 per cent of subjects of a reference group the SDS is smaller than -1.64, which marks the lower 5th percentile. In the present case the SDS is -1.21, that is well above the 5th percentile and therefore ‘within the normal range’.
The illustration shows that the scatter in FEV1 expressed as a percentage of the predicted value (left panel) exhibits
a predictable, clear trend in the reference population of Vlaardingen
and Vlagtwedde:
the scatter increases with age. If you held the view that values
below 80% of the predicted value were compatible with obstructive
lung disease, then you would be faced with a relatively large proportion
of false positive test results in elderly subjects, and with some
false-negative test results in younger subjects.