Defining Normal Values
One important aspect of pulmonary function testing is defining
what is normal and what indicates the presence of respiratory disease. Results are
commonly interpreted in relation to reference values to determine whether they
lie within a certain normal range. One statistically acceptable approach is to define
as abnormal the lowest 5% of the reference population. For most tests, results from
large numbers of normal patients have been used to generate regression equations
based on age, gender, race, and most importantly, height. Although a number of clinical
laboratories designate values of FVC and FEV1
less than 80% of predicted
as abnormal, this arbitrary standard has no statistical basis and should be avoided.
Assuming a gaussian distribution, the 95% confidence limits can
be derived as 1.65 times the coefficient of variation. The latter is the quotient
of the standard deviation and the mean. For FVC and FEV1
, coefficients
of variation are 12% to 13% in large groups of normal subjects.[19]
A value of 1.65 times these variations indicates that a measurement must be about
21% lower than the mean to be considered abnormal. Because the coefficient of variation
for FEF25%–75%
is about 25% in similar populations, values must
be 40% below the population mean before they are truly considered abnormal.
