Chest
Volume 80, Issue 1, July 1981, Pages 97-99
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Special Communication
Changes in Measured Spirometric Indices: What Is Significant?

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SIGNIFICANCE

The usual definition of a clinically significant difference from normal (an abnormal measured value) is a measured value that statistically would be expected in less than 5 percent of normal subjects. In a Gaussian population, this represents values that are more than 1.65 times the coefficient of variation below the mean. For example, the coefficient of variation for FVC determined in a large group of normal subjects (Table 1) is about 13 percent. A value 21.5 percent (1.65 × 13) smaller than

POPULATION SIGNIFICANCE

The application of this significance criterion to the parameters of spirometry in normal population studies is illustrated in Table 1. Coefficients of variation, using a 30-year-old, 173-cm tall man, are given from the studies of Morris et al,3 Kory et al,4 and Knudson et al.5 The approximate average of the coefficients of variation for FVC, FEV1, and FEF 25-75% are 13, 13, and 25 percent, respectively. Multiplying these coefficients of variation by 1.65 will give significance levels of about

SIGNIFICANT CHANGES IN TIME

One must identify coefficients of variation for multiple measurements within a day (acute changes) and for determinations from week to week (chronic changes) to develop criteria for the interpretation of time sequential changes in values for individual patients.

ACUTE CHANGE

Three studies that have presented data with multiple daily spirometric values are listed in Table 2. McCarthy et al7 tested 12 normal subjects ten times each day with a coefficient of variation calculated as shown in Table 2. Pennock et al8 tested 20 subjects with reversible airways obstruction nine times daily and found values for the coefficient of variation about twice those of McCarthy’s normal subjects. Similarly, Hruby et al,9 testing 15 subjects ten times per day, found that the ranges

CHRONIC CHANGE

To evaluate long-term changes, one must look at the coefficients of variation from week to week. Three studies have been identified in which these data are available (Table 3). In normal subjects significant week-to-week changes are 11 percent in the FVC, 12 percent in the FEV1, and 21 percent in the FEF 25-751 The patients with obstructive disease had greater variability than normal subjects, the variability being 21 percent in FVC, 23 percent in FEV1, and 30 percent in FEF 25-75%. Again, in

APPLICATION

As an example of the use of this type of information, one of the tests frequently used in evaluation of obstructed patients is the measurement of spirometric parameters before and after administration of a bronchodilator. From Table 4 it is apparent that an increase in FVC and FEV1 by more than approximately 12 percent; 11 percent in FVC; 13 percent in FEV1, or an increase in FEF 25-75$ of greater than about 25 percent (23 percent) represents a statistically significant change from the baseline.

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