Liu et al. BMC Public Health 2010, 10:82 http://www.biomedcentral.com/1471-2458/10/82 directly from the corresponding smoothed percentiles con- structed from the whole population group by the LMS method. In addition, odds ratio was calculated using logis- tic regression analysis adjusted for age to explore the risk of having cardiovascular risk factors clustering among boys and girls who were at the optimal threshold of waist cir- cumference and higher compared with their counterparts. The SAS 8.0 software package was used for analyses. All statistical analyses were two-sided and a p value of < 0.05 was considered statistically significant. Results A total of 5529 children (2830 boys and 2699 girls) aged 6-12 years participated in the study. The descriptive char- acteristics of the sample by age and gender are shown in Table 1. The age- and gender-specific 3rd, 10th, 25th, 50th, 75th, 90th, and 97th percentiles for waist circum- ference are shown in Table 2 and smoothed waist cir- cumference percentile curves are presented in Figure 1. In general, waist circumference increased with age in both boys and girls. Boys had a higher waist circumfer- ence value than girls at every age and percentile level except for the 3rd, 10th, and 25th percentiles at 6 years. Receiver-operating charactistic curves for waist cir- cumference with high cardiovascular risk (3 or more of 5 cardiovascular risk factors) in boys and girls is shown in Figure 2. Table 3 summarizes the optimal threshold of waist circumference for boys and girls. The 90th and 84th percentiles for waist circumference represented the cut-offs for boys and girls, respectively. The odds ratio Table 1 Characteristics of the population by age and gender Page 3 of 9 of a clustering of cardiovascular risk factors among boys at the 90th percentile of waist circumference and higher, and 84th and higher percentiles of waist circumference in girls was 10.349 (95% confidence interval 4.466 to 23.979) and 8.084 (95% confidence interval 3.147 to 20.767) compared with their counterparts. Table 4 shows the age-specific waist circumference cut-off points according to gender. These cut-off points are those above which there is an increased likelihood of being at high risk of cardiovascular risk factors clustering. Discussion This study provides the age- and gender-specific waist circumference reference percentiles for Chinese children aged 6-12 years living in the North and South of the country. Consistent with findings in previous studies [17-21], waist circumference increases with age and boys have a higher value than girls at each age. The age- and gender-related variation of waist circumference also shows similarity to other body dimensions. It is interesting to compare our data with those pre- viously reported in British [21], Turkish [17], Australian [18], Mexican [23], and Chinese children in Hong Kong [24] and the Xinjiang Province [25] (see Figure 3). Boys in the present study had higher waist circumference values than Australian, Turkish, and British boys, and lower values than Mexican boys aged 6-12 years, how- ever were similar to British boys aged 6-7 years. Girls in the present study had higher waist circumference values Body mass index (kg/m2) Waist circumference (cm) 16.5 ± 2.5* 16.5 ± 2.9** 17.0 ± 3.6** 18.3 ± 5.4** 18.6 ± 3.9** 19.5 ± 4.4** 19.5 ± 4.4** 15.9 ± 2.1 15.8 ± 2.1 16.2 ± 2.7 17.0 ± 3.0 17.6 ± 3.6 18.1 ± 3.3 18.7 ± 3.8 54.9 ± 7.2 56.5 ± 7.0** 59.1 ± 8.4** 62.2 ± 10.1** 65.5 ± 10.5** 67.6 ± 11.0** 68.0 ± 11.8** 53.8 ± 5.5 54.0 ± 5.4 56.6 ± 6.6 59.3 ± 7.6 61.3 ± 8.9 62.8 ± 9.0 64.3 ± 9.0 Boys Girls Age (years) 6 7 8 9 10 11 12 Age (years) 6 7 8 9 10 11 12 n 2830 152 469 506 421 550 499 233 2699 165 470 522 416 546 440 140 Height (cm) 120.3 ± 5.0 124.5 ± 5.3** 129.0 ± 6.3 134.6 ± 6.8 138.8 ± 6.8* 143.5 ± 7.3* 147.0 ± 7.2* 119.6 ± 5.0 123.1 ± 5.7 128.7 ± 5.9 133.5 ± 7.1 138.0 ± 7.1 144.3 ± 7.3 148.5 ± 7.3 *P < 0.05,**P < 0.001 for the difference between boys and girls. Weight (kg) 24.1 ± 4.9 * 25.8 ± 5.7** 28.6 ± 8.1** 33.5 ± 10.8 ** 36.4 ± 10.2** 40.6 ± 11.4 ** 42.7 ± 12.5 22.9 ± 4.2 24.0 ± 4.4 27.0 ± 5.6 30.6 ± 7.5 33.9 ± 9.1 38.0 ± 9.1 41.6 ± 10.1