Table. 4—

Summary of the usage of reference values

Item	Reference values
General	Predicted values should be obtained from studies of “normal” or “healthy” subjects with the same anthropometric (e.g. sex, age and height) and ethnic characteristics of the patient being tested
	Height and weight should be measured for each patient at the time of testing
	If possible, all parameters should be taken from the same reference source
	When comparing selected reference equations with measurements performed on a sample of healthy subjects in a laboratory, it is suggested to choose the reference equation that provides the sum of residuals (observed – predicted computed for each adult subject, or log observed – log predicted for each subject in the paediatric age range) closest to zero
	When using a set of reference equations, extrapolation beyond the size and age of the investigated subjects should be avoided
	For each lung function index, values below the 5th percentile of the frequency distribution of values measured in the reference population are considered to be below the expected “normal range”
Spirometry	In the USA, ethnically appropriate NHANES III reference equations published in 1999 for those aged 8–80 yrs, and the equations of Wang et al. 29 for children aged <8 yrs are recommended
	In Europe, the ECCS combined reference equations published in 1993 8 are often used for 18–70-yr-old people, and those from Quanjer et al. 30 for paediatric ages
	Currently, a specific set of equations is not recommendable for use in Europe. A new Europe-wide study to derive updated reference equations for lung function is needed
	Table 1 includes reference equations published from 1995 to August 2004
Lung volumes	No specific set of equations can be recommended
	In practice, many USA and European laboratories use the reference equations for TLC, FRC and RV recommended by the 1995 ATS/ERS workshop 7 or by the ECCS in 1993 8
	Table 2 reports studies on reference equations published from 1993 to August 2004
Diffusing capacity	No specific set of equations is generally recommended
	Commonly used equations appear to be those by the ECCS in 1993 38 and those of Crapo and Morris 40. In Europe, equations from Cotes et al. 41, Paoletti et al. 42 and Roca et al. 43 are also used
	Table 3 shows studies on reference equations published from 1995 to August 2004

NHANES: National Health and Nutrition Examination Survey; ECCS: European Community for Coal and Steal; TLC: total lung capacity; FRC: functional residual capacity; RV: residual volume; ATS: American Thoracic Society; ERS: European Respiratory Society.