• lung function
• children

We read with great interest the recent paper by Nystad et al¹ on the feasibility of spirometric tests in preschool children using candle blowing incentives, in support of recent publications.^2–5 As there is a dearth of spirometric reference data for this age group, we value the additional regression equations derived. However, we have several questions concerning this study.

The regression formulae presented were based on 603 children, of which 476 (78.9%) were reported as having “asthmatic symptoms” or “parental smoking habits”. It would be interesting to stratify the results, analysing healthy and non-healthy populations separately.

The actual age distribution of the preschool population in table 1 ranged from 4.1 to 4.8 years (that is, age 4 years). This narrow age distribution may explain the high r values of the linear regressions shown in table 4. Evaluating younger and older children may decrease the r values of logarithmic regression. Linear regressions should be used cautiously since parameters may appear to be too low in older children and “negative” in those who are younger (fig 3).

The “candle blowing” incentives were assumed to facilitate technically correct spirometric tests in the young children. We found that such incentives induced premature termination of forced vital capacity (FVC) which led to lower values than with other methods.^2,3 If this is not the case, how do the authors explain the lower FVC values compared with those of Eigen et al,³ while the forced expiratory volume in 1 second (FEV₁) values were similar (fig 3)?

Acceptance criteria for correct FVC curves are vague in the absence of expiration time and “end of test” criteria.^4,5 Inclusion of curves with a difference of 10% between the two best curves should be avoided on the basis of standard recommendations and previously published data (<5% difference only).^3,6

In view of the increasing interest in lung function in preschool children, resolving these questions would help to standardise spirometric parameters in this age group.