Liu et al. BMC Public Health 2010, 10:82 http://www.biomedcentral.com/1471-2458/10/82 Table 2 Waist circumference percentiles (cm) by age and gender Age (years) n 3rd 10th 25th 50th 75th 90th 97th Boys 6 7 8 9 10 11 12 Girls 6 7 8 9 10 11 12 2830 152 469 506 421 550 499 233 2699 165 470 522 416 546 440 140 44.3 46.5 46.4 48.7 48.2 50.6 49.5 52.4 49.9 53.5 50.3 54.4 51.7 56.6 45.3 47.3 45.9 48.0 47.1 49.4 47.9 50.5 48.5 51.4 49.3 52.7 50.4 54.5 49.3 51.5 53.6 55.9 57.8 59.4 62.4 49.6 50.4 52.2 53.6 55.0 56.8 59.4 52.6 55.0 57.4 60.5 63.2 65.4 69.3 52.3 53.4 55.6 57.5 59.3 61.9 65.4 57.0 59.6 62.5 66.6 70.3 73.1 77.7 55.6 57.1 60.0 62.4 64.9 68.2 72.9 62.7 66.1 70.0 75.5 79.9 83.1 88.0 59.9 61.9 65.9 69.2 72.4 76.6 82.6 71.2 76.6 83.1 90.3 94.2 96.8 101.0 65.6 68.6 74.4 79.3 83.4 88.2 95.5 than British, Australian and Turkish girls, and lower values than Mexican girls in all age groups except lower values than Australian girls at 7-9 years. Compared with Caucasians, Asians are generally smaller [29], however waist circumference results from the present study con- tradict this expectation. A number of factors may con- tribute to this inconsistency. Firstly, the site of waist circumference measurement has varied between studies and previous research has reported significant differ- ences in waist circumference at different sites [30]. International agreement regarding waist circumference measurement site is required if meaningful comparisons regarding central obesity are to be made between chil- dren from different countries and regions. Secondly, considerable changes in body size and shape, including waist circumference, have occurred in recent decades in both children and adults [26,31,32] and this may be a consideration when comparing data collected in differ- ent periods. For example, Utter et al. indicated that the mean waist circumference in New Zealand adolescents increased from 76.2 cm in 1997/1998 to 89.4 cm in 2005, with increases in waist circumference measure- ments at all points in the distribution [31]. Data from Britain and Australia were collected in 1985 and 1990, respectively, a gap of approximately twenty years from the present study. Compared with other studies of Chi- nese children, both boys and girls in the present study have higher waist circumference values in all age groups however had similar values to 7-year-old children from Xinjiang Province. It is widely accepted that body com- position is influenced by the environment in addition to age, gender and ethnicity [10]. Compared with the population in Northern China, both children and adults Page 4 of 9 in Southern China have a smaller body size [26]. This study also demonstrated differences in body size among children in Guangzhou (Southern China), and Liaoning and Tianjin (Northern China) (data not shown). Accord- ingly, further study is required in more districts of China to generate more representative waist circumfer- ence percentiles. In a sub-sample, the present study also evaluated the threshold value of waist circumference to predict cardio- vascular risk factors clustering using receiver-operating characteristic analysis. The 90th and 84th percentiles were identified as the thresholds for diagnosing a higher clustering of cardiovascular risk factors in Chinese boys and girls, respectively. The threshold of waist circumfer- ence for boys is consistent with the International Dia- betes Federation recommendation [14] and the findings of Maffeis et al. and Ng et al. [15,16], but higher than the two previous studies in Chinese boys. The threshold of waist circumference for girls is lower than the Inter- national Diabetes Federation recommendation and simi- lar to the two studies in Chinese girls [24,25]. The thresholds of waist circumference in the present study for both boys and girls are higher than those for Cauca- sian children [19]. The threshold of waist circumference in previous stu- dies varies according to the definition of cardiovascular risk factors clustering. The two studies from Hong Kong [24] and the study from Xinjiang province [25], pro- posed the 85th waist circumference percentile as the appropriate threshold for predicting cardiovascular risk. However, Katzmarzyk et al. [19] proposed the 50-57th percentiles for white and black boys and girls. A number of factors may explain these differences. Firstly, waist circumference is ethnic-independent [33,34], as well as the sensitivity to cardiovascular risk factors [35,36]. Sec- ondly, the definition of cardiovascular risk factors clus- tering has varied between studies with different groupings of cardiovascular risk factors used. Sung et al. reported the 85th percentile was the optimal threshold for diagnosing the presence of cardiovascular risk factors clustering which was defined as 4 or more of 6 risk fac- tors, including elevated systolic and/or diastolic blood pressure, high triglycerides, low high-density lipoprotein cholesterol, high low-density lipoprotein cholesterol, glucose and insulin. However, if cardiovascular risk fac- tors clustering was defined as 3 or more of the 6 risk factors, the cut-offs were the 74th and 69th percentiles for boys and girls, respectively. Furthermore, the level of each cardiovascular risk factor to define an abnormal level also varies between studies, for example, the 75th percentile [37], outer quintiles [19], and 85th percentiles [24]. In the study by Sung et al. [24], the lower the level to define each individual cardiovascular risk factor, the lower the waist circumference percentile. Risk factors