Screening for increased ALT levels is conducive to identifying individuals at the risks of some chronic and metabolic diseases, but blood collection of serum ALT is invasive and cannot be widely used for a massive epidemiological survey among children and adolescents because of subject burden and cost. Considered as a simple, easily obtained, inexpensive, and convenient screening measure, anthropometry can be monitored in a large crowd and might be an important surrogate marker for ALT concentrations, if the discriminative accuracy of the anthropometric parameters is very high for diagnosing abnormal ALT. Primary focused on the diagnostic performance, anthropometric data for Shenzhen adolescents aged 9–17 years were used to detect elevated serum ALT in our present survey, and we confirmed the belief that anthropometric measurements were important predictors of elevated ALT among Shenzhen adolescents. An elevated ALT activity was more common in overweight or obese adolescents than those with a normal or relatively low BMI z-score. BMI and weight showed an approximate diagnostic performance for predicting the presence of elevated ALT, followed by the z-score of BMI, height, and age.
Consistent findings that the proportion of elevated ALT greatly increased with increasing degree of obesity (e.g. the percentile or z-score of BMI), and higher proportion among overweight or obese adolescents than normal-weight ones were shown in previous studies and our research, regardless of the use of diagnostic criteria for an elevated ALT activity7, 13, 18, 32. Based on the thresholds of ALT activity (40 U/L for boys and girls), nearly 6.6% of the Mexican youths (9.8% of boys and 3.8% of girls) had elevated ALT levels by using the baseline data from 1262 participants of 8–19 years in the Mexican Health Worker Cohort Study (MHWCS); elevated ALT concentration was also observed in 2.7% of normal-BMI, 14.2% of overweight, and 28.9% of the obese children and adolescents13. According to the same thresholds (i.e. criterion II), the percentage of elevated ALT among Shenzhen adolescents of 9 − 17 years was 0.89% for normal-weight, 4.96% for overweight, and 20.58% for obese participants. Mexican youths had a slightly higher crude proportion of elevated ALT than Shenzhen adolescents, without adjusting for age and gender based on the distribution of the world population. Likewise, using a sample of 1591 youths from the 2008 − 2009 Korea National Health and Nutrition Examination Survey (KNHANES), another study conducted by Seung Park and cooperators also found a close prevalence of elevated ALT (> 33 U/L for boys and > 25 U/L for girls) among Korean youths of 12 − 18 years — 5.9% (95%CI 4.9%−7.2%) for the overall study population, 15.7% (11.3%−21.5%) for overweight adolescents (85th ≤ BM < 95th percentile), and 34.9% (25.6%−45.4%) for subjects with a BMI ≥ 95th percentile32. The odds ratios (ORs) for elevated ALT also sharply increased with the greater levels of obesity, with an OR (95% CI) of 7.23 (4.33–12.10) for overweight and 23.62 (12.98–42.98) for obese adolescents, comparing to normal-weight adolescents (BMI < 85th percentile) in an unadjusted analysis32.
Screening for school-adolescents, it is clearly more convenient to collect anthropometric measures than biochemical measures, but the abilities of several individual characteristics and anthropometric indices to correctly predict elevated serum ALT in Shenzhen children are questionable and need to be assessed. Determined by the univariate ROC curves, our results showed that the anthropometric parameter of BMI had the best superiority of discerning the presence of elevated ALT (AUC = 0.789 for criterion I and 0.850 for II), weight and BMI-z displayed the second and third highest detection accuracy (AUC range from 0.747 to 0.850), and followed by height, age, and gender (from 0.490 to 0.695). As such, we believed that the single variable of height, age, or gender was not reliable surrogate measure of an activity of elevated ALT among Shenzhen adolescents of 9 − 17 years. Further, age, gender, and height were considered as covariates together with each predictor of obesity indices (i.e. weight, BMI, or BMI-z) to predict elevated ALT in the subsequent multivariate model analyses, although the diagnostic accuracy of the combined models did not markedly improve. On the other hand, the current study firstly, as far as we known, investigated the association of height z-score with serum ALT in adolescents and estimated the usefulness of height-z as a predictive index of elevated ALT. Nevertheless, the present result showed a null discriminative power of height z-score for predicting an elevated ALT serum activity.
For BMI, several studies had pointed out that BMI was able to determine the presence of elevated ALT among adults, although the accuracy was not very high in absolute term — the AUC (95% CI) was 0.658 (0.633–0.683) for men and 0.651 (0.616–0.685) for women in the rural areas of China14 and 0.64 (0.60–0.68) for Italian general population19. Focused on adolescents, a representative study with a sample of 454 youths of 11–17 years from 2 northern Italian cities also found a similar diagnostic performance of dichotomized BMI for elevated ALT, with an AUC of 0.64 (0.50–0.77)17. Compared to the dichotomized BMI model, the Italian adolescents study also indicated a more accurate univariable model of BMI-z (AUC = 0.71 [0.59–0.81]), and the predictive ability was increased substantially by considering gender together with BMI-z (AUC = 0.80 [0.71–0.89])17. In addition, based on the data from the National Health and Nutrition Examination Survey (NHANES) during 1999 to 2014, an United States study consisted of 5019 adolescents of 12–19 years suggested a significant correlation between BMI z-score and serum ALT levels (r = 0.29, P < 0.0001)4. Significant positive associations of ALT with obesity indices (e.g. BMI, body fat percentage, truncal fat mass, total fat mass, waist circumference, and WHtR) were further confirmed in Korean male adolescents21.
Several potential limitations of our research should be recognized. First, participants were only from a single city of China, and it might be therefore difficult to generalize these findings to other adolescent populations. Second, our cross-sectional study omitted to measure some important individual anthropometric parameters — hip circumference, waist circumference, neck circumference, and the subsequent indicators of WHpR, WHtR, and A Body Shape Index (ABSI), which might achieve a better diagnostic performance and to be more sensitive predictive indicates for diagnosing an elevated ALT activity among the indigenous adolescents. For predicting the ALT levels, WHtR and to some extent BMI were congruously shown to be the best body indices in some Asians, as compares to waist circumference, hip circumference, WHpR, and the useless index of ABSI12, 14, 16. Another limitation was our inability to consider the potential confounders of ethyl alcohol intake and hepatitis B virus (HBV) and C virus (HCV) infections, which were well-known risk factors for increases in ALT4, 12, 17, 19. However, alcohol consumption and HBV and HCV infections were less prevalent in school-adolescents of China33, 34, and we hypothesized that the obesity indices could be even more important predictors of elevated ALT among the adolescents of Shenzhen, and the above potential confounders were not likely to have a substantial impact on current results.