Lung function measurements in children - copsac
Lung function measurements in children - copsac
Lung function measurements in children - copsac
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of methods for control of the actual resistance measure for young <strong>children</strong> and not only<br />
the flow and box leakage. Without such proof of accuracy normative values generated at<br />
other centers may not be applicable. Until a mechanical standard becomes available<br />
biological standard (healthy subjects) is the only possible substitute.<br />
We used a standardized protocol <strong>in</strong>clud<strong>in</strong>g standard calibration of flow, box leakage and<br />
<strong>in</strong>ternal box pressure <strong>in</strong> 6 Danish centers at secondary and tertiary referral hospital<br />
departments. The 6 centers <strong>in</strong>cluded <strong>in</strong> the study of center-agreement were spread over<br />
the country, which prevented <strong>measurements</strong> the same day. Therefore the day-to-day<br />
variability reduced the sensitivity by which we could identify outliers among the<br />
centers. The visit order was randomized to ensure a possible difference between the 1 st<br />
and 2 nd visit did not bias the center variation.<br />
In the current study the with<strong>in</strong>-subject SD on the same day and center was 0.01 and the<br />
with<strong>in</strong>-subject SD between centers was 0.02. The small difference between centers<br />
with<strong>in</strong>-subject could be due to different flow and respiratory rates. A higher expiratory<br />
or <strong>in</strong>spiratory flow (turbulence) may cause higher resistance. Every center followed the<br />
same standard operat<strong>in</strong>g procedure where the aim of breath<strong>in</strong>g frequency was 30-45<br />
breaths per m<strong>in</strong>ute and the observers were tra<strong>in</strong>ed to choose technically acceptable<br />
loops with m<strong>in</strong>imal turbulence. We did not report the flow and respiratory rates because<br />
it was not possible with the current software to save the data.<br />
In our previous study, the precision (repeatability) of sRaw <strong>measurements</strong> 9 days<br />
(mean) apart <strong>in</strong> young <strong>children</strong> with asthma (asymptomatic dur<strong>in</strong>g the study period) was<br />
found to have an <strong>in</strong>tra class coefficient of 0.87 (with<strong>in</strong>-subject SD 0.03) for basel<strong>in</strong>e<br />
<strong>measurements</strong> between occasions (50). The higher with<strong>in</strong>-subject SD <strong>in</strong> the previous<br />
study could be expla<strong>in</strong>ed by the asthma status of <strong>children</strong>.<br />
The current study was designed to f<strong>in</strong>d a possible center effect. We were able to account<br />
for any possible observer bias by hav<strong>in</strong>g a center specific observer as well as a common<br />
observer visit<strong>in</strong>g every center. The order of measur<strong>in</strong>g the biological control for the<br />
local and travell<strong>in</strong>g observer was not randomized, but we found no effect of the<br />
<strong>in</strong>vestigator who travelled between the centers on the <strong>measurements</strong> (p-value >0.5)<br />
(table 1). A previous study found significant systematic difference <strong>in</strong> between-observer<br />
variability (7%) <strong>in</strong> sRaw <strong>measurements</strong> (13).<br />
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