Results
The final study population consisted of 1,866 healthy children and adolescents, and 919 (49.2%) were male. Regarding race, 64.4% were declared as mixed race, 19.6% as white, 12.5% as black and 2.8% as Asian descendent. A total of 97% were born in Cuiabá. Their nutritional statuses were classified as 73.4% normal weight, 9.4% overweight, 6.3% at risk of being overweight, 6.9% obese, 2.7% underweight and 1.2% extremely underweight.26 This classification was performed using WHO Anthros and WHO Anthros Plus version 3.2.2. A total of 39.2% of all participants had a family history of diabetes, and 65.5% and 22.4% reported a family history of hypertension and obesity, respectively. Cardiovascular diseases, such as myocardial infarction and stroke, were reported by 17.6% and 12.3%, respectively. Frequency histograms of the serum levels of TC (Figure 2), HDL-c (Figure 3), non-HDL-c (Figure 4), LDL-c (Figure 5) and triglycerides (Figure 6) for all age analyzed for all groups after excluding the outliers are shown in Figures 2, 3, 4, 5, and 6 respectively.
Figure 2  Histograms by age group for total cholesterol (mg/dL), after excluding outliers, with coupled Gaussian curves.
Figure 3  Histograms by age group for HDL cholesterol (mg/dL), after excluding outliers, with coupled Gaussian curves.
Figure 4  Histograms by age group for nHDL cholesterol (mg/dL), after excluding outliers, with coupled Gaussian curves.
Figure 5  Histograms by age group for LDL cholesterol (mg/dL), after excluding outliers, and coupled Gaussian curves.
Figure 6  Histograms by age group for triglycerides (mg/dL), after excluding outliers, with coupled Gaussian curves. 
The RIs for TC were calculated for 3 age groups: 1-2 years, 3-8 years and 9-12 years. Four age groups were calculated for HDL-c: 1 year, 2 years, 3 years and 4-12 years. All children and adolescents were grouped into a single age group for nHDL-c. Two age groups were defined for LDL-c (1-8 years and 9-12 years), and 3 age groups were used for TG (1 year, 2-5 years and 6-12 years). The proposed RIs (±2 SD) for TC were 89-196 mg/dL for 1-2-year-olds, 102-205 mg/dL for 3-8-year-olds and 103-212 mg/dL for 9-12-year-olds. The proposed RIs (±2 SD) for HDL-c were 18-49 mg/dL for 1-year-olds, 22-55 mg/dL for 2-year-olds, 24-61 mg/dL for 3-year-olds and 27-69 mg/dL for 4-12-year-olds. The proposed RI (±2 SD) for nHDL-c was 57-155 mg/dL for 1-12-year-olds. The proposed RIs (±2 SD) for LDL-c were 43-135 mg/dL for 1-8-year-olds and 45-140 mg/dL for 9-12-year-olds. The proposed RIs (±2 SD) for TG were 26-176 mg/dL for 1-year-olds, 27-135 mg/dL for 2-5-year-olds and 17-134 mg/dL for 6-12-year-olds. The mean, standard deviation, maximum and minimum and 1st and 3rd quartiles values of each parameter evaluated in their respective age groups are shown in Table 1. Many researchers have emphasized the need to consider gender in the determination of the RI for laboratory parameters for the pediatric population. Statistical significance in this study was observed for some parameters in specific age groups, but these differences would not likely have a relevant clinical impact (Table 2).
Table 1  Estimated parameters for the sample distribution of children and adolescents included in the study to establish the reference intervals for lipids after excluding the outliers SD: standard deviation; RI: reference interval.
Table 2  Statistical significance observed for the HDL-cholesterol (HDL-c), non-HDL-cholesterol (n-HDL-c), LDL-cholesterol (LDL-c) and triglyceride (TG) parameters in specific age groups by gender 
The following 75th and 95th percentiles were observed for each age interval. The values obtained for TC were: 1 to 2 years, 160 mg/dL and 189 mg/dL; 3 to 8 years, 170 mg/dL and 199 mg/dL; 9 to 12 years, 176 mg/dL and 205 mg/dL, respectively. For the nHDL c, the only age group 1 to 12 years, this percentiles values were 122 mg/dL and 150 mg/dL, respectively. For the LDL-cholesterol, the values corresponding to the percentiles above, aged 1 to 8 years and 9 to 12 years, were 104 mg/dL and 132 mg/dL; 106 mg/dL and 139 mg/dL, respectively. For the triglycerides, the values corresponding to these percentiles were: 1 year, 127 mg/dL and 189 mg/dL; 2 to 5 years, 98 to 139 mg/dL; 6 to 12 years, 92 mg/dL and 139 mg/dL. The 10th percentiles for HDL-c were 24 mg/dL, 28 mg/dL, 32 mg/dL and 36 mg/dL for children 1, 2, 3 and 4-12 years old, respectively. The clinical decision limits for each parameter, calculated according to the NHLBI,5 are shown in Tables 3 and 4.
Table 3  Desirable, borderline–high and high serum lipid concentrations (mg/dL) for children and adolescents from Cuiabá, Brazil
Table 4  Desirable, borderline and low serum HDL-cholesterol concentrations (mg/dL) for children and adolescents from Cuiabá, Brazil