How do Cormic Index profiles contribute to differences in spirometry values between White and First Nations Australian children?

Background: Spirometry values of First Nations Australian children are lower than White children. One explanation relates to differences in the sitting-height/standing-height ratio (Cormic Index), as this accounts for up to half the observed differences in spirometry values between White children and other ethnicities. We investigated whether the Cormic Index of First Nations children differs from White children and if this explains the lower spirometry values of First Nations children. 

Methods: First Nations children (n = 619) aged 8–16 years were recruited from nine Queensland communities. Their spirometry and Cormic Index data were compared to that of White children (n = 907) aged 8–16 years from the NHANES III dataset. 

Results: FEV₁ and FVC of First Nations children was 8% lower for children aged 8–11.9 years and 9%–10% lower for children aged 12–16 years. The Cormic Index was statistically lower in the First Nations 8–11.9 years group (median = 0.515, interquartile range [IQR]: 0.506–0.525) compared with White children (0.519, IQR: 0.511–0.527), and this difference was greater in the 12–16 years group (0.505, IQR: 0.492–0.516; 0.520, IQR: 0.510–0.529). Adjusting for age, sex, and standing height, lower Cormic Index of First Nations children accounts for 14% (95% confidence interval [CI]: 7%–21%) of FEV₁ and 15% (95% CI: 8%–21%) of FVC differences in the younger group, and 26% (95% CI: 16%–37%) of FEV₁ and 31% (95% CI: 19%–42%) of FVC differences in the older group. 

Conclusion: Ethnic differences in Cormic Index partly account for why healthy First Nations Australian children have lower spirometry values than White children. As childhood spirometry values impact adult health, other contributing factors require attention.