Effect of change of reference standard to NHANES III on interpretation of spirometric ‘abnormality’

The American Thoracic Society has recently recommended the use of NHANES III spirometric reference standard in the United States. The objective of this study was to better quantify the well-known ‘problem’ of the change in interpretation of spirometry, as a consequence of the change from the other commonly used reference standards (Morris, Kory, Crapo, Knudson 1976, and Knudson 1983) to NHANES III. This is a cross-sectional study of spirometries of 1,106 non-Hispanic Caucasian American adults, including 234 subjects with obstructive and 228 subjects with restrictive spirometric ‘abnormalities’. A weighted Kappa statistic was used to evaluate the level of agreement between NHANES III and other commonly used reference standards. The level of agreement in assessing the presence of an ‘abnormality’ was poor to moderate – values of Kappa statistic ranged from 0.13 to 0.46. There was however, good to very good level of agreement in assessing the severity of the ‘abnormality’ – values of Kappa statistic ranged from 0.61 to 0.91. This study better quantifies the well-known differences in the interpretation of spirometric ‘abnormalities’ as a consequence of the recommended change of reference standard to NHANES III, which in turn may cause confusion among patients and their treating physicians.