Reference Equations Used to Predict Pulmonary Function: DISCUSSION
the number of reference studies and equations available in the literature, surprisingly few equations are widely used. Three studies accounted for 85 percent of the equations used for standard spirometric indices. It was not uncommon for institutions to use a different reference equation for each spirometric parameter. While we are not aware of studies on the effects of this practice, it seems likely to increase the uncertainty that prediction equations will match the clinical population. In general, we recommend reference equations for spirometric indices be selected from a single study which has been matched to the patient population. This should be relatively easy for spirometry, but is more difficult for lung volumes, Deo, and airway resistance because there are few studies available which include complete pulmonary function tests.
Two studies account for 83 percent of the lung volume equations (Table 3) and five for 84 percent of the Deo (Table 4). The greater number of Deo equations in use may reflect the larger interlaboratory differences for Deo than spirometry or lung volumes, though a preliminary study suggests academic centers do not consistently choose reference equations which match the Deo values produced in their laboratories. While all reference equations should be demonstrated to show a reasonable match with the individual laboratory’s instruments and clientele, this is especially important in Deo where the variability has been demonstrated to be so great. We believe laboratories should assure that prediction equations for Deo match the data produced in their facilities by comparing measurements made on 20 to 30 healthy subjects with predicted values from several different equations.
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Of those who responded to the question on the adjustments made for ethnic differences, roughly half made no adjustment at all while the majority of those who did simply reduced white values by a fixed percentage which varied from institution to institution. These results confirm our ipipression that there is no clear consensus on whether ethnic adjustments are necessary, which ethnic groups require some sort of adjustment, and how adjustment should be approached if it is necessary. Ethnic differences in pulmonary function parameters are best documented in blacks and Asians, but studies of ethnic differences in lung function are frequently confounded when the different ethnic groups are not studied with the same equipment and techniques. Interlaboratory variability in lung function measurements due to technical factors is large in comparison to the magnitude of ethnic differences and could mask or accentuate ethnic differences. However, comparisons among blacks, Asians and whites have generally shown whites to have the largest lung volumes, blacks to have the smallest, and Asians to have intermediate lung volumes, after adjustment for body size. People of mixed race have been found to have intermediate values. Flow rates have usually been found to be about the same in all three ethnic groups. Studies differ on whether or not proportional adjustments of white equations are an acceptable method of dealing with the differences. At the moment, it is acceptable to use either proportional differences or population specific equations to deal with ethnic differences. It is not, however, acceptable to ignore them.
Reference equations from studies done in Europe are not used frequently in the United States and Canada. The summary equations reported by the standardization project of the European Community for Coal and Steel were rarely cited in this survey. The pattern of the prediction equations in use has changed from that reported from a similar survey conducted in 1968. Equations from larger studies using current standardized methods and newer technology are now more frequently cited.
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The replies to our survey suggest the availability of automated systems—and manufacturers default settings—may also influence the choice of equations. Forty-seven institutions responding to our survey returned manufacturer-supplied computer sheets listing the equations used. Three institutions replied they used software provided by the manufacturer and did not know what equations it was based on. These responses raise the possibility that the delay between the publication of good reference equations and their use in practice is, in part, a function of the time it takes for such information to find its way into manufacturers’ computer software.
