Skinfold thickness is an important index for assessment of body fat composition in children [1]. Although high body mass index (BMI) in children may be an indicator of raised total body fat, it may be inaccurate [2]. Reports suggest that skinfold thickness is associated with adiposity even in non-obese children [3], leading to the assumption that it may be a better predictor of cardiovascular risk [2]. Thus, BMI followed by this measurement may help to correctly identify excess body fat in children [4].

Triceps skinfold thickness (TSFT) measurements are non-invasive and can function independently as predictors of childhood hypertension [5]. We have previously reported (on the same dataset) a positive association of TSFT with BP [6]. Furthermore, TSFT being easier to obtain as compared to skinfold measurements at other sites (removal of clothes not being a measurement requisite), making this technique useful in epidemiological settings. Reference centiles for TSFT for other populations [7,8] may not be applicable to Indian children due to racial differences.

The primary objective of this study was to develop reference centiles for TSFT representative of apparently healthy Indian children and adolescents aged 5-17 years. A secondary objective was to determine TSFT centile cut-offs to predict the risk of childhood hypertension.

This study was part of a multi-centric study with the objective of defining gender- and age-wise waist percentiles for Indian children [9]. The study was performed in schools catering to children of higher socio-economic status in 5 major Indian cities (Delhi, Chennai, Pune, Kolkata and Raipur), one from each of the five zones [10] in India. Data were collected from July 2011 to January 2012. Detailed methodology has been previously detailed [6,9]. The study was approved by the Ethics Committee of the Hirabai Cowasji Jehangir Medical Research Institute.

Data were collected by the same team at each site; equipments were calibrated daily. The mean inter-observer and intra-observer coefficients of variation were <1% for TSFT, height and BP measurements. BMI categories were defined as normal weight, overweight and obese as per adult equivalent values for Asians [11] and height-for-age (HAZ), weight-for-age (WAZ) and BMI-for-age (BAZ) z-scores were computed as deviations from the median [12]. TSFT was recorded using the Harpenden caliper, on the non-dominant upper arm [13]. Average of two readings for all parameters was used for analysis.

Clinical assessments were carried out by pediatricians to confirm good health. BP was recorded as previously detailed [6]. SBP and/or DBP >90th percentile and < 95th percentile was considered as prehypertension. SBP and/or DBP >95th percentile was classified as hypertension [14].

Statistical Methods: All the statistical analyses were performed using SPSS software (version 16.0.2007, SPSS Inc., Chicago, IL). Pearson’s correlation coefficient was computed gender-wise between TSFT, age, height, weight and BMI. All results were expressed as mean (SD) for comparability with other studies. Smoothed gender-specific reference plots showing 5th, 10th, 25th, 50th, 75th, 85th and 95th percentiles were derived using LMS method (LMSchartmaker Pro version 2.4, 2008; by Dr. Huiqi Pan and Dr. Tim Cole), which constructs reference percentiles adjusted for skewness [15]. Each variable of interest was summarized by three smooth curves plotted against age, representing the median (M), coefficient of variation (S) and skewness (L) of the measurement distribution [16]. Models were checked for goodness of fit using the detrended Q-Q plot, Q Tests and worm plots [17]. The LMS method was found to be appropriate to use for this data as the measure of skewness of the data was 1.1 with a standard error of 0.03.

With the help of the LMS chartmaker, we also determined the age- and gender-specific smoothed median triceps skinfold thickness for children who were overweight and obese according to the adult equivalent BMI cut-offs for Asians [8,11,18]. Receiver-operating characteristic (ROC) analysis was performed for identifying gender-wise risk of hypertension.

A total of 13388 children (7596 boys) from the selected schools were enrolled in the study.We excluded 13 outliers (5 children for height, 4 for BMI, and 4 for triceps). Anthropometric and blood pressure data on 13375 children (7590 boys) aged 5-17 years were analyzed for generating TSFT centiles. Table I describes the anthropometric characteristics of the included children. All the children were within reference range for anthropometric parameters, with the means of HAZ, WAZ and BAZ around 0. Using adult equivalent cut-offs for BMI [11, 13], 26% of children were found to be overweight and 14% were obese. 89.5% of the children had normal SBP and 85.9% had normal DBP.

TABLE 1 Baseline Characteristics of the Participants 

Fig. 1 Smoothed percentile curves for triceps skinfold thickness in participants: (a) Boys, and (b) Girls.

TSFT was significantly (P<0.01) associated with age (r=0.18), weight (r=0.54), height (r=0.23) and BMI (r=0.77) in boys as well as in girls [age (r=0.4); weight (r=0.72), height (r=0.44) and BMI (r= 0.83)]. TSFT reference centile curves (5th, 10th, 25th, 50th, 75th, 85th and 95th) are presented for boys and girls in Fig. 1a and 1b, respectively. Corresponding smoothed percentile values are given according to age and gender in Table II and III. The model was considered a good fit as per the shape of the worm plot; the Q statistic curves for L, M & S were within -2 and +2, and the detrended Q-Q plot indicated that the population was approximately normal. The smoothed percentile curves for boys plateaued around the age of 13 years whereas for girls the smoothed curves increased steadily till the age of 18 years (Fig. 1a and 1b). Median TSFT increased by 7% to 9% till the age of 9 years in both genders. After the age of 12 years, median TSFT decreased by 1% to 2% in boys but showed increase by 3% to 4% in girls. Percentiles lower than 50th were more flat than the higher percentiles in boys. On the other hand, increasing trend was seen at lower as well as higher centiles in girls.

Fig. 2 Triceps skinfold thickness percentiles with median TSFT percentile for overweight and obese participants (a) boys and (b)girls.

To judge the efficacy of TSFT centiles in identifying excess body fat and in turn risk of hypertension, ROC analysis was carried out. The optimal cut-off percentile yielding maximal sensitivity and specificity for predicting high BP was 70th TSFT percentile in boys and girls, the values for which are presented in Table II and III. Area under the curve (AUC) was 0.778 (95% CI: 0.753, 0.803) in boys with 68% sensitivity and 78% specificity. In girls, AUC was 0.749 [95% CI (0.713, 0.785)] with 68% sensitivity and 74% specificity.

TABLE II	Triceps Skinfold Thickness (tsft) Percentiles Values for Boys with Cut-off Value 
of 70th percentile of TSFT for Hypertension Risk 

TABLE III	Triceps Skinfold Thickness (tsft) Percentiles Values for Girls with 
Cut-off Value of 70th percentile of TSFT for Hypertension Risk 

Fig. 2a and 2b illustrate the trajectory of the median TSFT curve for children classified overweight and obese according to BMI criteria. For boys, the median skinfold thickness for overweight boys was above the 75th percentile for TSFT, and the median TSFT for obese boys was above the 85th percentile for TSFT across age. For girls, the median skinfold thickness for overweight girls tracked the 75th percentile for skinfold thickness neatly while the median for obese girls was above the 85th percentile for skinfold thickness across age.

The present study provides age- and gender-specific reference percentiles of TSFT for Indian children and adolescents. Reports suggest that the ability of TSFT percentiles to screen for adiposity is similar to that of BMI or waist circumference [19]. Around 15% children in this study had TSFT values above the 75th percentile even when their BMI was within normal range as judged by BMI adult equivalent cut-offs. This suggests that at lower BMI Indian children may have higher body fat percent [20].

In comparison with TSFT percentiles developed for Polish children using Harpenden caliper [7], it was observed that Indian children had similar TSFT values as compared to Polish children upto the 50th centile in boys and upto the 75th centile in girls, after which the TSFT values were higher in Indian children. This may partly be due to the tendency of Asians to have higher body fat than their Western counterparts at lower or similar BMI [21, 22]. TSFT centiles above 50th percentile in Indian boys in the present study were higher than the reference centiles of TSFT in Caucasian boys using Lange calipers whereas the TSFT centile values of girls were similar [8]. The percentage difference between the 50th and 95th percentile for TSFT by Harpenden caliper for Indian children (123.5% for boys, 86.8% for girls) in the current study was compared with Caucasian children as measured by Lange calipers (124.7% for boys, 89.5% for girls). The small magnitude of differences in TSFT centiles may be due to the difference in measurement technique.

The cut-off of 70th percentile of TSFT in our study for predicting hypertension is similar to the cut-off for waist circumference percentile suggested by us for screening for metabolic syndrome risk for both genders [9]. While we have reported cut-offs for TSFT for predicting risk for childhood hypertension in an earlier publication [6], these cut-offs were based on Western data due to absence of Indian reference data. The present cut-offs, based on Indian reference data, are lower than those earlier reported and may be used effectively to screen Indian children with a risk for hypertension.

One of the limitations of the study is the comparability of Harpenden calipers with other instruments for measuring TSFT. Therefore, similar methods of measuring TSFT need to be employed for using these centiles. Another limitation is that TSFT is prone to measurement errors by technicians and it is imperative that trained personnel measure TSFT. Additionally, while our rates of false positivity are comparable to those reported earlier [23], one of the reasons for the low specificity may be that triceps skinfold thickness is an indicator of presence of excess fat and is not a measure of total body fat.

In summary, our study provides smoothed TSFT percentiles for Indian children and adolescents using a multicentric nationwide data. These percentiles would be useful in field studies for screening adiposity and risk of hypertension.

Acknowledgements: Neha Kajale, Lavanya Parthasarathy, Surabhi Kulkarni, Archana Arya, Anjan Bhattacharya, Sanwar Agarwal, Deepa Pillay and Shamim Momin.

Contributors: AVK: conceptualized and designed the study, coordinated and supervised data collection at the sites, carried out the initial analyses, reviewed and revised the manuscript; RM: data analyses and drafted the initial manuscript; SAC: conceptualized and designed the study, carried out the analyses, reviewed and revised the manuscript; VVK: conceptualized and designed the study, critically reviewed the manuscript; VHE: conceptualized and designed the study, coordinated and supervised data collection at the sites; VGP: carried out data management and analyses. All authors approved the final version of manuscript.

Funding: Novo Nordisk India Pvt. Ltd. Ms Rubina Mandlik was funded by a Fellowship Grant from the University Grants Commission (UGC), Government of India.

Competing interests: None stated.

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