Liu et al. BMC Public Health 2010, 10:82 http://www.biomedcentral.com/1471-2458/10/82 recognized as a key component of the metabolic syn- drome in both children and adults [13,14]. Waist cir- cumference cut-off points associated with increased risk have been developed for adult men and women, how- ever relatively less work has been undertaken in children and adolescents. A further shortcoming of research to date is that reference standards have more commonly been developed on Caucasian populations and may have limited usefulness to people from different ethnic and racial backgrounds [15,16]. An increasing body of research has explored ethnic differences in body composition in both children and adults [10], but considerably more work is needed. For example, the International Diabetes Federation uses the 90th percentile as a cut-off for waist circumference to define the pediatric metabolic syndrome but has recom- mended the development of ethnic-, age- and gender- specific normal ranges for waist circumference based on healthy values. In short, the percentiles used as cut-offs for waist circumference should be reassessed when more data are available [14]. A number of studies have developed reference waist circumference percentiles for children and adolescents in different countries [17-23]. To date, three studies have reported age- and gender-specific waist circumfer- ence cut-offs in Chinese children and adolescents, two in Hong Kong Chinese children and adolescents [24], and the third in children from Xinjiang province [25]. However, there are regional differences in the body composition of Chinese. People living in North China are taller and heavier than those living in the South due to a combination of genetic and environmental factors [26]. The development of waist circumference percen- tiles and cut-offs for different groups would be particu- larly valuable. Therefore, the purpose of the present study was to develop waist circumference percentiles for Chinese children from both the North and South of the country, and secondly, to explore the optimal waist circumfer- ence cut-off values for predicting cardiovascular risk fac- tors clustering in this population. Methods Subjects Three cities, Liaoyang in the Northeast and Tianjin in the North of China, and Guangzhou in the South, were involved in this study. Two schools were randomly selected from each city. Height, weight, and waist cir- cumference were measured for all children at each school (aged 6-12 years) and 40% of the participants at each school were selected for the collection of blood samples. Written consent was obtained from both children and their parents and the study protocol approved by the Page 2 of 9 Ethics Review Committee of the National Institute for Nutrition and Food Safety, China Center for Disease Control and Prevention, and the University Human Research Ethics Committee of the Queensland Univer- sity of Technology, Australia. Anthropometric measurements Height was measured to the nearest 0.1 cm in bare feet. Body weight was measured to the nearest 0.1 kg with a balance-beam scale with participants wearing lightweight 2 clothing. The body mass index (kg/m ) was calculated as weight (kg) divided by the square of height (m). Waist circumference was measured to the nearest 0.1 cm at the mid-point between the lower costal border and the top of the iliac crest with the measurement taken at the end of a normal expiration. Cardiovascular risk factors measurement Blood pressure was measured on the study morning using a random-zero sphygmomanometer after the par- ticipant rested for 5 min in a seated position. Two rest- ing blood pressure measurements were taken to the nearest 4 mmHg. A venous blood sample was collected from each participant after an overnight fast. Serum glu- cose concentration, triglycerides, total cholesterol, and high-density lipoprotein cholesterol, and low-density lipoprotein cholesterol was measured. Definition of cardiovascular risk factors clustering Each participant was classified as having cardiovascular risk factors clustering with 3 or more of the following risk factors [16]: (1) systolic blood pressure and/or diastolic blood pressure ≥ 90th percentiles for age, gender and height recommended by the National Heart, Lung, and Blood Institute (U.S.) [27]; (2) triglycerides ≥ 1.7 mmol/L; (3) high-density lipoprotein cholesterol <1.03 mmol/L; (4) low-density lipoprotein cholesterol ≥ 3.4 mmol/L; (5) fast- ing glucose ≥ 5.6 mmol/L [14,28]. Statistical analysis Continuous data were described as means (± standard deviation). The age and gender differences were tested by t-test. Smoothed age- and gender-specific percentiles were constructed using the LMS ChartMaker Pro software package (The Institute of Child Health, London) for the whole population group. Receiver-operating characteristic analysis was used to explore the diagnostic ability of waist circumference to identify the presence or absence of cardi- ovascular risk factors clustering among children who pro- vided blood samples. The gender-specific value which maximized both sensitivity and specificity was regarded as the optimal threshold for predicting cardiovascular risk factors clustering among boys and girls. Then the age- and sex-specific waist circumference cut-off values were read