Effects of changing reference values and definition of the normal range on interpretation of spirometry

Background and objective: Recent spirometry reference studies are arguably the most valid ever performed and the ATS/ERS now recommend the National Health and Nutrition Assessment Survey (NHANES) equations for North America. It is timely to consider adopting these reference values in Australasia; however, an evaluation of the consequences of such a change is required. Methods: We analysed data from 1108 patients tested in two pulmonary function laboratories in public hospitals. Lower limits of normal (LLN) were calculated using European Community for Steel and Coal (ECSC) (1993), Knudson (1983), NHANES (1999) and Health Survey of England (HSE) (2004) equations and used to define restriction (FVC < LLN) and obstruction (FEV1/FVC < LLN). This interpretative strategy was also compared with the GOLD definition of obstruction (FEV1/FVC < 70%). Results: Average age for all patients (50% female) was 60 years (range: 20-91). The mean predicted FVC from NHANES and HSE were similar and consistently higher than those from ECSC and Knudson (average 270 mLs). This translates into a 76% increase in the incidence of 'restrictive' interpretations using NHANES data compared with ECSC and Knudson, and a smaller increase of 40% for HSE. Using FEV1/FVC < 70% to diagnose obstruction in those over 65 years would result in false positive rates of approximately 28%. Using the same definition in a younger group (< 50 years old) results in a false negative rate of approximately 14%. Conclusions: Changing to either NHANES or HSE predicted values will significantly increase the rate of 'restrictive' interpretation, and alter the rate of obstructive findings. The NHANES and HSE data confirm that using FEV1/FVC < 70% to define obstruction causes misdiagnosis in elderly and younger subjects.