MetS to Identify Adolescents With Imparied Glucose Tolerance
MetS to Identify Adolescents With Imparied Glucose Tolerance
Data were obtained from NHANES (1999–2010), a complex, multistage probability sample of the US population. These annual cross-sectional surveys are conducted by the National Center for Health Statistics (NCHS) of the Centers for Disease Control (CDC), with randomly-selected participants undergoing anthropometric and blood pressure measurements, answering questionnaires and undergoing phlebotomy (http://www.cdc.gov/nchs/nhanes.htm). The NCHS ethics review board reviewed and approved the survey and participants gave informed consent prior to participation.
Height, weight, BMI, WC, blood pressure (BP), and laboratory measures of triglycerides, HDL-C, and glucose were obtained using standardized protocols and calibrated equipment. All blood samples used for analyses were obtained following a ≥8 hours fast.
A fasting oral glucose tolerance test was performed on a random sub-set of adolescent participants of NHANES. After their fasting glucose assessment, participants were given 1.75 g/kg of glucola to a maximum of 75 g. Two hours after this ingestion a second blood draw was obtained to assess 2-hour glucose level.
Data from non-Hispanic-white, non-Hispanic-black, or Hispanic (Mexican-American/other Hispanic) adolescents 12-19 y were analyzed. Children <12 y were excluded since fasting values for triglycerides and glucose were only obtained in participants ≥12 y. Participants with known diabetes (T1DM or T2DM) were excluded by eliminating those with self-reported diabetes and those on anti-diabetic medications. Participants were also excluded if they were pregnant or taking antihyperlipidemic medications as these are likely to alter lipid and glucose levels in a manner that may reflect baseline relationships between MetS and glucose tolerance. Individuals taking anti-hypertensive medication were classified as having hypertension.
IGT was classified for a two-hour blood glucose of ≥140 mg/dL. MetS status was evaluated in two manners: 1) A commonly-used pediatric/adolescent adaptation of the Adult Treatment Panel III (ATP III) criteria. Participants had to meet ≥3 of the following 5 criteria: concentration of triglycerides ≥110 mg/dL, HDL-C ≤40 mg/dL, WC ≥90th percentile for age/sex (or ATP III limit of 102 cm for males and 88 cm for females, whichever was lower), glucose concentration ≥100 mg/dL, and systolic or diastolic BP ≥90th percentile (age, height, and sex-specific). Similarly, hypertension was defined as systolic or diastolic BP ≥90th percentile for age, height, and sex. 2). Using a pediatric-adolescent continuous metabolic syndrome severity Z-score. This score is based on a factor analysis of the contributions of individual MetS components on a sex- and race/ethnicity-specific basis. The sex- and race/ethnicity-specific equations have been published previously and are available as an online calculator (http://publichealth.hsc.wvu.edu/biostatistics/mets).
Statistical significance was defined as a p-value < 0.05. Statistical analysis was performed using SAS (version 9.3, Cary, NC). Prevalence rates of MetS were calculated by glucose tolerance category, and compared via chi-square tests. The presence of insulin resistance was categorized based on an elevated fasting insulin ≥16.0 IU/mL, approximately the 95 percentile among lean adolescents in NHANES and used elsewhere previously. Receiver operating characteristic (ROC) analysis was used to assess the ability of the sex- and race/ethnicity-specific MetS Z-score to discriminate IGT. Overall predictive performance was measured by the area under the curve (AUC) of the ROC curve, with AUC of 0.5 and 1.0 indicating no and perfect predictive ability, respectively. Sensitivities and specificities to predict the presence of IGT were compared between the traditional MetS classification and using cut-offs of the MetS severity Z-score. This includes Z-score cut-offs as follows: 1) 1.0 to approximate the prevalence of ATP-III MetS and 2) 0.75 assess the performance of a more liberal definition of MetS. These statistics were done on a sex and race/ethnicity-specific basis. All analyses, except for the ROC analysis, accounted for the survey design of NHANES in producing population-based estimates of basic descriptive statistics as well as prevalences, sensitivities, and specificities.
Methods
Data were obtained from NHANES (1999–2010), a complex, multistage probability sample of the US population. These annual cross-sectional surveys are conducted by the National Center for Health Statistics (NCHS) of the Centers for Disease Control (CDC), with randomly-selected participants undergoing anthropometric and blood pressure measurements, answering questionnaires and undergoing phlebotomy (http://www.cdc.gov/nchs/nhanes.htm). The NCHS ethics review board reviewed and approved the survey and participants gave informed consent prior to participation.
Height, weight, BMI, WC, blood pressure (BP), and laboratory measures of triglycerides, HDL-C, and glucose were obtained using standardized protocols and calibrated equipment. All blood samples used for analyses were obtained following a ≥8 hours fast.
A fasting oral glucose tolerance test was performed on a random sub-set of adolescent participants of NHANES. After their fasting glucose assessment, participants were given 1.75 g/kg of glucola to a maximum of 75 g. Two hours after this ingestion a second blood draw was obtained to assess 2-hour glucose level.
Data from non-Hispanic-white, non-Hispanic-black, or Hispanic (Mexican-American/other Hispanic) adolescents 12-19 y were analyzed. Children <12 y were excluded since fasting values for triglycerides and glucose were only obtained in participants ≥12 y. Participants with known diabetes (T1DM or T2DM) were excluded by eliminating those with self-reported diabetes and those on anti-diabetic medications. Participants were also excluded if they were pregnant or taking antihyperlipidemic medications as these are likely to alter lipid and glucose levels in a manner that may reflect baseline relationships between MetS and glucose tolerance. Individuals taking anti-hypertensive medication were classified as having hypertension.
IGT and MetS Classification
IGT was classified for a two-hour blood glucose of ≥140 mg/dL. MetS status was evaluated in two manners: 1) A commonly-used pediatric/adolescent adaptation of the Adult Treatment Panel III (ATP III) criteria. Participants had to meet ≥3 of the following 5 criteria: concentration of triglycerides ≥110 mg/dL, HDL-C ≤40 mg/dL, WC ≥90th percentile for age/sex (or ATP III limit of 102 cm for males and 88 cm for females, whichever was lower), glucose concentration ≥100 mg/dL, and systolic or diastolic BP ≥90th percentile (age, height, and sex-specific). Similarly, hypertension was defined as systolic or diastolic BP ≥90th percentile for age, height, and sex. 2). Using a pediatric-adolescent continuous metabolic syndrome severity Z-score. This score is based on a factor analysis of the contributions of individual MetS components on a sex- and race/ethnicity-specific basis. The sex- and race/ethnicity-specific equations have been published previously and are available as an online calculator (http://publichealth.hsc.wvu.edu/biostatistics/mets).
Statistical Analysis
Statistical significance was defined as a p-value < 0.05. Statistical analysis was performed using SAS (version 9.3, Cary, NC). Prevalence rates of MetS were calculated by glucose tolerance category, and compared via chi-square tests. The presence of insulin resistance was categorized based on an elevated fasting insulin ≥16.0 IU/mL, approximately the 95 percentile among lean adolescents in NHANES and used elsewhere previously. Receiver operating characteristic (ROC) analysis was used to assess the ability of the sex- and race/ethnicity-specific MetS Z-score to discriminate IGT. Overall predictive performance was measured by the area under the curve (AUC) of the ROC curve, with AUC of 0.5 and 1.0 indicating no and perfect predictive ability, respectively. Sensitivities and specificities to predict the presence of IGT were compared between the traditional MetS classification and using cut-offs of the MetS severity Z-score. This includes Z-score cut-offs as follows: 1) 1.0 to approximate the prevalence of ATP-III MetS and 2) 0.75 assess the performance of a more liberal definition of MetS. These statistics were done on a sex and race/ethnicity-specific basis. All analyses, except for the ROC analysis, accounted for the survey design of NHANES in producing population-based estimates of basic descriptive statistics as well as prevalences, sensitivities, and specificities.
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