Selecting reference values
There is ample choice of prediction equations. Globally, however, only a limited number is widely used. The software package SpirXpert offers a large range of options for adults, and separately for children and adolescents. The software also has facilities to easily compare sets of prediction equations from the screen. In addition it is very easy to switch from one prediction equation to the other and assess how this affect the evaluation of a concrete pulmonary function test.
One should at least consider the following points when making a choice:
- How well do predicted values fit the data from a representative sample of healthy women and men, and boys and girls, who never smoked. Always use equipment that has been certified as meeting the ATS and ECCS/ERS minimum requirements. Perform spirometric tests professionally, as unsatisfactory patient cooperation and suboptimal instruction and supervision will lead to systematically underestimating true FEV1 and (F)VC. If your results indicate that in healthy subjects measured values are systematically lower than predicted, check critically whether something is wrong with your equipment or how you perform the tests.
- Often in children and adolescents, notably in young school boys and older adolescent boys, predicted values tend to be a bit low, so that there is a trend for the difference between measured and predicted value to be age-related. This appears to be more prominent in the prediction equations due to Zapletal and Knudson than those of Wang or Quanjer.
- How abrupt or smooth is the transition from predicted values for adolescents to those of adults? The transition trends to be smooth if predictions according to Quanjer et al. (up to age 18 yr) are combined with the ECCS/ERS reference values adjusted by the SpirXpert group, or with those due to Crapo.
The SpirXpert software is associated with the databases ‘Demo adults’ and ‘Demo adolescents’; these comprise longitudinal data that can be helpful in judging which set of reference values best fits the observed values. Patients ‘Modal man’ and ‘Modal woman’ (SpirXpert Demo adults) are artificially constructed: between 6-18 year their height is the median of that found in the present Dutch population, remaining unchanged until age 65 year. ‘Observed’ values have been computed according to the revised ECCS/ERS equations (adults), and according to Quanjer et al. (children, adolescents).