The present study evaluated data from the general population of healthy Korean adolescents to estimate the upper normal limit of ALT. The results indicate that the upper limits (95th percentiles) for estimating the prevalence of NAFLD in this population were 24.1 U/L for boys and 17.7 U/L for girls. These values are similar to, albeit lower than, the values among American adolescents (25.8 U/L for boys, 22.1 U/L for girls) .
The present study used the upper normal limit of ALT to estimate the prevalence of NAFLD, which revealed values of 10.8% among boys and 6.6% among girls in 2015. In contrast, use of the previous standard values (30 U/L for boys and 19 U/L for girls) generated estimated NAFLD prevalences of 6.7% among Korean boys and 5.1% among Korean girls in 2015  [5, 11]. Thus, the prevalence of NAFLD in this population appears to be unexpectedly high, and we recommend aggressive management for patients who may have undetected NAFLD based on the previous standard ALT values.
The prevalence of NAFLD (elevated ALT levels (> 30 U/L)) was 3.6% in boys and 2.8% in girls from 1594 adolescents aged 10 to 19 years using the 1998 KNHANES.  The prevalence of NAFLD in Chinese children found out to be 9.03% with ALT thresholds > 25.8 U/L for boys > 22.1 U/L for girls using China Health and Nutrition Surveys.  Although there is a lack of uniformity in the data, but similar results were obtained with studies conducted in China from our study. Emma et al. pooled prevalence of NAFLD as 7.6% (95%CI: 5.5 to 10.3%) using meta-analysis and showed that it did not differ by geographical region among children and adolescents. [14, 15] Even with heterogenicity of defining NALFD across studies, it seems that the prevalence of NAFLD increase with time and has globally similar prevalence.
This study had some important limitations. First, we used elevated ALT plus elevated BMI as criteria to diagnose NAFLD. Some NAFLD patients may have normal ALT levels and it does not parallelly match the histological severity of NAFLD in children.  So, in order to diagnose NALFD, imaging investigations or histology confirmation should be included. However, we have not been able to confirm NAFLD using imaging investigation in this cohort study as it has not been used in every cycle of KNHANES. Therefore, it could not be included in the criteria of NAFLD in this study. However, ALT has been recommended as a screening tool for NAFLD and has previously been used for population-based epidemiological studies. [7, 8, 17] Furthermore, the upper normal limits of ALT (24.1 U/L for boys and 17.7 U/L for girls) should be validated using liver biopsies [7, 18]. Second, the KNHANES data do not include information regarding the use of hepatotoxic medication. Third, although ours is the most recent study to estimate the sex-specific upper normal limits of ALT among Korean adolescents, additional criteria should be considered when using ALT levels to estimate the prevalences of other chronic liver diseases in Korea. As we mentioned at the beginning, NAFLD should be diagnosed using liver biopsy or imaging studies. So, results from present study cannot directly be used in clinic level.
However, even with these limitation, this is first study to define upper normal ALT level of adolescents by using KHANES data. And according to our data, prevalences of adolescents NAFLD in Korea might be more serious than we used to think. As the result of this study shows normal value of ALT can varies by some circumstances, ALT laboratory thresholds used for children should be re-examined. The physicians should be aware not to underdiagnose NAFLD patient when ALT level is in normal value we used to use in clinic. However, the defining NAFLD (BMI ≥ 85th percentile plus elevated ALT) used in the study is only tools to estimate NAFLD in public populations, so this upper normal ALT results cannot directly be used in clinic. We hope these results can give clues that ALT level can be adjusted for each sex and ethnics. And ALT could be more useful tool to determine who may need more detailed medical examinations for NAFLD if ALT is adjusted specific for each ethnics and sex.