Opportunistic sampling from early childhood centres: a substitute for random sampling to determine lead and iron status of pre-school children?

This report compares the results from two sampling strategies used to determine the prevalence of elevated blood lead concentrations and iron status in 12-36 month old children in Central Sydney. The two methods were stratified random sampling using census collector districts and an opportunistic sampling strategy using client registers at Early Childhood Centres (ECCs). The response rates were 75.3% (n = 718 of whom 198 were aged 12-36 months) and 24.1% (n = 304) respectively. The geometric mean blood lead concentrations were 0.40 and 0.34 mumol/L respectively (p = 0.001). The traditional random sampling prevalence survey identified a significantly higher proportion of children with blood lead concentrations greater than 0.48 (OR = 0.61, 95% CI 0.40-0.93) and 0.72 mumol/L (OR = 0.44, 95% CI 0.21-0.92) compared to the simpler opportunistic survey. The median plasma ferritin concentration for both studies was 19 micrograms/L (p = 0.4). The prevalence of iron depletion, iron deficiency and iron deficiency anaemia was not significantly different between the two studies. In conclusion, opportunistic sampling through ECCs does not appear to be a substitute for the traditional random sampling prevalence surveys of determine the prevalence of elevated blood lead concentrations in pre-school children in Central Sydney. However, opportunistic sampling through ECCs may be an appropriate method for monitoring iron status, in particular iron depletion, in pre-school children in Central Sydney.