'Normal' lung function?
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You propose to use the standard deviation score of FEV1 in assessing whether a patient has, or does not have, airway obstruction. |
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It is very commendable that you want to use the standard
deviation score rather than per cent predicted. After all, the
scatter in FEV1 is independent
of the magnitude of the predicted FEV1.
The appropriate procedure is therefore to compute how many standard
deviations the observed value differs from the predicted value
(the standard deviation score (SDS), also called Z-score), as
this index is independent of gender, age and standing height,
and therefore unbiased. In the case of a normal distribution
a SDS <-1.64 signifies that the observed value occurs in
fewer than 5% of healthy subjects. For your convenience, in
SpirXpert software this is translated into a ‘lower
limit of normal’. If you want to refresh your memory on
the subject, read “Expressing results correctly’
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Unfortunately, we do not agree on using the FEV1 for detecting airway obstruction. After all, the FEV1 as well as the VC may be diminished by either an obstructive or a restrictive disorder. However, only an FEV1 (the average expiratory flow during 1 second) which is disproportionately low with respect to the (F)VC (an index of lung volume) signifies expiratory airflow limitation, i.e. obstructive lung disease. Hence, FEV1%(F)VC should be the starting point for assessing airway obstruction. |
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