Lung function measurements in children - copsac

lung symptoms, 19% said it was the control twin and 27% said both twins had
comparable symptoms. However, there is a significant risk for recall bias and the MZ
pairs could be mixed up since the hospitalization was on average 7 years ago and
parents were generally most unsure of the details. Furthermore, it would be impossible
to make a subgroup analysis restricted to the twin pairs where the parents’ memories
and data agreed. The study group would be very small.
It is known that male sex could be a risk factor for RSV hospitalization (140-142). In
the whole twin cohort the boys had a higher prevalence of severe RSV infection
compared to the girls; odds ratio 1.20; CI [1.07-1.33] as previously presented by
Thomsen et al. (2008) (115), but there was a random overrepresentation of females in
our study (subgroup).
The discordant MZ twins in our study were born slightly earlier (mean gestational age
35.5 weeks and mean birth weight 2369 grams) than the MZ twins in the overall twin
cohort (mean gestational age 36.1 weeks (SD 2.5) and birth weight 2498 grams (SD
549) (115). The differences were small and not significant; therefore it seems that the
discordant twins were representative of MZ twins in general.
On the other hand it may bias the result because premature infants as a population have
its own risk factors for impaired lung function.
The distinction between possible "wheezers" and “bronchiolitis” may be inaccurate in
this as in other studies. The distinction between wheezers and bronchiolitis is normally
related to severity of wheeze and concurrent clinical signs of lower airway infection.
Our samples included the latter, i.e. first hospitalization for severe wheeze with
concomitant signs of lower airway infection.
The highest incidence of hospitalization for RSV-bronchiolitis is often reported in the 0-
1-yr-olds (113, 114, 143, 144). However, the reported mean age may be biased from
excluding older children from such analyses. For example Sigurs et al. reported mean
age of RSV hospitalization to be 3.5 months excluding infants older than 12 months
(98, 98). Fjaerli et al. reported admission age median to be 6 months in children under
two years old in their study (140). One study included children up to 2 years and
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