Estimation of the healthy upper limits for serum alanine aminotransferase in Asian populations with normal liver histology

Authors


  • Potential conflict of interest: Nothing to report.

Abstract

A recent study in young Italian subjects suggested that the healthy thresholds for serum alanine aminotransferase (ALT) levels should be adjusted to 30 IU/L for men and 19 IU/L for women when assessing risk factors for nonalcoholic fatty liver disease. Our aim was to assess serum ALT concentrations in healthy Korean individuals and to determine the factors affecting ALT levels in these populations. We included 1,105 potential liver donors (643 men and 462 women) with biopsy-proven normal livers. Median ages were 25 years in men and 30 years in women, with a median body mass index (BMI) of 22.3 kg/m2 in men and 21.4 kg/m2 in women. The calculated thresholds for ALT values in these subjects were 35 IU/L for men and 26 IU/L for women. Age and BMI were independently correlated with ALT levels in both sexes, whereas serum total cholesterol concentration was significant only in men and blood glucose level only in women (P < 0.05). When we chose a subgroup of 665 individuals (346 men and 319 women) using Prati criteria, modified by the BMI cutoff points for Asians (<23 kg/m2), we found that the healthy ALT values were 33 IU/L for men and 25 IU/L for women. The mean ALT concentrations for subjects within the Prati criteria were significantly lower than for those outside the criteria (16.7 versus 19.5 IU/L for men, 12.8 versus 14.9 IU/L for women; P < 0.001). Conclusion: The healthy ALT thresholds in biopsy-proven normal Asians were clearly lower than the previously accepted thresholds, as has also been noted in Europeans. Age, BMI, and/or other metabolic parameters significantly affect ALT levels, even in subjects with normal livers. (HEPATOLOGY 2010.)

The upper limit of normal for serum alanine aminotransferase (ALT) has long been considered to be 40 IU/L regardless of sex or body mass index (BMI).1–3 This concentration was determined principally from studies performed before the introduction of anti–hepatitis C virus (HCV) testing, and prior to development of the concept of nonalcoholic fatty liver disease (NAFLD).4, 5 A recent study in Italian populations suggested that healthy serum ALT values should be adjusted to 30 IU/L for men and 19 IU/L for women, when assessing risk factors for NAFLD.6 In addition, these newer criteria showed superior sensitivities in identifying patients with HCV viremia. Accordingly, an expert panel addressing the management of patients with chronic hepatitis B infection has recommended that decisions to initiate antiviral therapy or to perform liver biopsy should be based on these new ALT standards.7

Ideally, the normal range of serum ALT concentration should be determined in individuals without definite liver disease. Such subjects are difficult to identify in a population-based study, however, because ultrasonographic determination of NAFLD is expensive. Furthermore, current diagnostic modalities (apart from liver biopsy) cannot completely exclude subjects with NAFLD.8–10 Therefore, an alternative approach was used by Prati and colleagues,6 who analyzed ALT ranges in presumed healthy populations after excluding subjects with risk factors for liver diseases, including NAFLD. However, it is not certain whether ALT standards obtained from metabolically normal populations reflect values in healthy individuals with histologically normal livers. In addition, it is not clear whether healthy ALT ranges in Asians may necessarily be derived from values in Caucasian populations.

We therefore determined the thresholds for healthy ALT values, and factors influencing ALT levels, in healthy Korean populations. To this end, we selected populations with biopsy-proven normal livers from living liver donors, and also investigated the effect of application of the Prati criteria on the ALT levels in these populations.

Abbreviations

ALT, alanine aminotransferase; BMI, body mass index; HCV, hepatitis C virus; NAFLD, nonalcoholic fatty liver disease; TG, triglyceride.

Patients and Methods

This study was approved by the Investigation and Ethics Committee for Human Research at the Asan Medical Center, Seoul, Korea.

Study Population.

Between July 2004 and June 2009, 2,054 consecutive voluntary potential living liver donors underwent ultrasonography-guided liver biopsy as part of routine preoperative evaluation for living donor liver transplantation at the Asan Medical Center. Before liver biopsy, subjects were screened for alcohol use of ≥40 g/week and presence of serum hepatitis B surface antigen, anti-HCV, and antibody to human immunodeficiency virus. Demographic parameters, including measurement of height, weight, and blood pressure; laboratory test results, including complete blood cell count; and serum concentrations of aspartate aminotransferase, ALT, alkaline phosphatase, total bilirubin, albumin, fasting glucose, total cholesterol, triglyceride (TG), and uric acid; were performed on all subjects. All potential donors were checked for serum ceruloplasmin levels and serologic markers of autoimmune liver disease, and all were examined by ultrasonography. Subjects with ALT levels exceeding 120 IU/L (three times the earlier upper limit of normal) were excluded from further workup, including liver biopsy. Normal histology on liver biopsy was defined as fatty changes in <5% of hepatocytes, including both macro-and microvesicular steatosis, and absence of significant fibrosis or inflammatory cell infiltration. Of the 2,054 potential liver donors, 758 (36.9%) were classified as having >5% steatosis with or without the presence of significant inflammation and/or fibrosis, and 191 (9.3%) showed significant inflammation and/or fibrosis, despite steatosis in <5% of hepatocytes (Fig. 1). The remaining 1,105 (53.8%) study subjects were classified as having normal liver histology and were eventually included in the current analysis. Among these subjects, we selected 665 (60.2%) as being at clinically low risk for NAFLD using the criteria of Prati and colleagues, modified by the BMI cutoff points for Asian populations.11 These 665 subjects had BMI <23 kg/m2, serum TG concentrations ≤200 mg/dL, serum glucose levels ≤105 mg/dL, and serum total cholesterol concentrations ≤220 mg/dL.

Figure 1.

Summary of potential living liver donors undergoing ultrasound-guided liver biopsy during routine preoperative evaluation for living donor liver transplantation. Of the 2,054 subjects, 1,105 (53.8%) were classified as having normal liver histology, defined as fatty changes in <5% of hepatocytes and the absence of significant inflammation and fibrosis.

Liver Biopsy and Laboratory Tests.

As part of preoperative evaluation, all subjects underwent ultrasonography-guided percutaneous biopsy of the right lobe of the liver using 18-gauge Stericut needles. Biopsy samples were fixed in 10% (vol/vol) formalin and stained with hematoxylin-eosin, and all pathologic specimens were reviewed by expert liver pathologists. All serum ALT measurements were performed by the same method. Analyses of serum biochemistry (fasting serum total cholesterol, TG, and plasma glucose levels), including ALT level assessment, were performed using a TBA 200FR NEO autoanalyzer (Toshiba, Tokyo, Japan). All aspartate aminotransferase/ALT tests employed the alpha-ketoglutarate reaction. In our laboratory, the conventional threshold of 40 IU/L was routinely used both for men and women, as recommended by the Korean Association for the Study of the Liver.12 Viral markers, including hepatitis B surface antigen, anti-HCV, and anti–human immunodeficiency virus, were determined using commercially available enzyme immunoassays (Abbott Laboratories, Chicago, IL). Our laboratory has been certified by the College of American Pathologists.

Statistical Analysis.

All results are presented as the mean ± standard deviation, median (range), or frequency. By convention, we set the healthy ALT thresholds at the 97.5th percentile, a figure employed for the distribution of a continuous variable in a normal population based on the American Gastroenterological Association recommendations.13 In addition, we also calculated the upper reference limits at the 95th percentile, as recently adopted in several studies, including the work of Prati's group.6, 14 Between-group differences were analyzed by way of analysis of variance and Student t test. Univariate and stepwise multivariate linear regression analyses were employed to identify independent variables affecting ALT levels. All statistical analyses were performed using SPSS version 13.0 (SPSS, Chicago, IL) and P < 0.05 were considered statistically significant.

Results

Clinico-Biochemical Features of All Potential Liver Donors with Normal Liver Histology.

Table 1 shows mean values for clinical and laboratory parameters of all 1,105 potential liver donors with normal liver histology, consisting of 643 men and 462 women, all of whom were negative for serum hepatitis B surface antigen and anti-HCV and had no history of alcohol abuse. The mean ages were 27.2 ± 8.4 years (median, 25 years) for men and 31.6 ± 9.3 years (median, 30 years) for women. Mean BMI was 22.7 ± 2.7 kg/m2 for men (median 22.3 kg/m2) and 21.6 ± 2.6 kg/m2 (median, 21.4 kg/m2) for women. The average serum total cholesterol, serum triglyceride, and blood glucose concentrations were 158.5 mg/dL, 85.4 mg/dL, and 94.4 mg/dL, respectively, for men and 164.1 mg/dL, 69.7 mg/dL, and 93.0 mg/dL, respectively, for women. For men, the 50th, 75th, 90th, 95th, and 97.5th ALT percentiles were 16, 22, 28, 31, and 35 IU/L, respectively, whereas in women, the corresponding figures were 13, 15, 20, 24, and 26 IU/L.

Table 1. Clinical and Biochemical Characteristics of All Study Subjects
VariableAll (N = 1,105)Men (n = 643)Women (n = 462)
  1. Values are expressed as the mean ± standard deviation.

Age (years)29.1 ± 9.027.2 ± 8.431.6 ± 9.3
BMI (kg/m2)22.2 ± 2.722.7 ± 2.721.6 ± 2.6
Total cholesterol (mg/dL)160.9 ± 28.3158.5 ± 28.3164.1 ± 28.0
Triglyceride (mg/dL)78.8 ± 40.485.4 ± 43.769.7 ± 33.3
Glucose (mg/dL)93.8 ± 9.194.4 ± 8.893.0 ± 9.4
ALT (IU/L)16.1 ± 6.617.9 ± 6.813.5 ± 5.2

Association Between Serum ALT Levels and Clinico-Biochemical Variables in All Potential Liver Donors with Normal Liver Histology.

By univariate analysis, age and BMI were positively correlated with ALT concentration, as were sex, serum total cholesterol and TG, and blood glucose concentration (P < 0.001 for each comparison; Table 2). Multivariate linear regression showed that age, sex, BMI, and serum total cholesterol concentration were independent variables positively related to ALT levels (P < 0.001 for each comparison; Table 2). When independent variables were analyzed separately by sex, both age and BMI were significant in both sexes, whereas serum total cholesterol concentration was significant only in men and blood glucose level only in women (P < 0.05 for each comparison; Fig. 2A-D and Table 3).

Table 2. Associations Between Serum ALT Levels and Clinical and Biochemical Factors in All Study Subjects
VariableUnivariate AnalysisMultivariate Analysis
Slope (95% CI)P Value*Slope (95% CI)P Value*
  • *

    Linear regression analysis.

  • Abbreviation: CI, confidence interval.

Age (years)0.094 (0.051–0.136)< 0.0010.101 (0.058–0.143)< 0.001
Sex−4.545 (−5.284 to −3.806)< 0.001−4.676 (−5.423 to −3.930)< 0.001
BMI (kg/m2)0.684 (0.546–0.822)< 0.0010.405 (0.270–0.540)< 0.001
Total cholesterol (mg/dL)0.038 (0.024–0.051)< 0.0010.028 (0.015–0.041)< 0.001
TG (mg/dL)0.029 (0.019–0.038)< 0.001
Glucose (mg/dL)0.105 (0.062–0.147)< 0.0010.037(−0.002–0.077)0.063
Figure 2.

Scatter plot showing associations between age, BMI, and serum ALT concentration. Age and BMI were positively correlated with serum ALT levels in both men (A,B) and women (C,D). *Pearson's correlation coefficient.

Table 3. Multivariate Linear Regression Analysis by Sex in All Study Subjects
VariableMenWomen
Slope (95% CI)P Value*Slope (95% CI)P Value*
  • *

    Linear regression analysis.

  • Abbreviation: CI, confidence interval.

Age (years)0.145 (0.082–0.207)< 0.0010.073 (0.020–0.125)0.007
BMI (kg/m2)0.535 (0.346–0.724)< 0.0010.260 (0.077–0.443)0.006
Total cholesterol (mg/dL)0.038 (0.019–0.056)< 0.001
Glucose (mg/dL)0.054 (0.003–0.104)0.037

Clinico-Biochemical Features of Subgroup Populations Based on Modified Prati Criteria.

When we assessed whether individuals met the modified Prati criteria (BMI <23 kg/m2, TG ≤200 mg/dL, glucose ≤105 mg/dL, total cholesterol ≤220 mg/dL), we found that 665 subjects (346 men, 319 women) belonged to this subgroup. The mean age was 26.0 ± 7.6 years for men and 30.0 ± 8.2 years for women (Table 4; P < 0.001 for each comparison). The median serum ALT level and the 75th, 90th, 95th, and 97.5th percentiles were 15, 20, 26, 29, and 33 IU/L, respectively, in men and 12, 14, 18, 22, and 25 IU/L, respectively, in women. The mean ALT concentrations were 16.7 ± 6.4 IU/L in men and 12.8 ± 4.9 IU/L in women who met the modified Prati criteria, which were significantly lower than the values (19.5 ± 7.0 IU/L in men, 14.9 ± 5.5 IU/L in women; P < 0.001, respectively) in those who did not attain these norms, a finding consistent with comparisons of other clinical and metabolic parameters (P < 0.001 for each comparison; Table 4).

Table 4. Clinical and Biochemical Characteristics of Subgroups Within and Outside the Prati Criteria
VariableMenWomen
Within the Prati Criteria (n = 346)Outside the Prati Criteria (n = 297)P Value*Within the Prati Criteria (n = 319)Outside the Prati Criteria (n = 143)P Value*
  • The criteria of Prati et al. were modified by BMI cutoff points for Asian populations. Values are expressed as the mean ± standard deviation.

  • *

    Student t test.

Age (years)26.0 ± 7.628.6 ± 9.1< 0.00130.0 ± 8.235.2 ± 10.5< 0.001
BMI (kg/m2)20.8 ± 1.424.8 ± 2.1< 0.00120.4 ± 1.524.5 ± 2.3< 0.001
Total cholesterol (mg/dL)152.7 ± 24.8165.3 ± 30.7< 0.001159.5 ± 25.2174.4 ± 30.9< 0.001
TG (mg/dL)79.6 ± 37.392.2 ± 49.2< 0.00165.9 ± 28.978.2 ± 40.3< 0.001
Glucose (mg/dL)92.5 ± 7.396.7 ± 9.9< 0.00191.3 ± 8.996.7 ± 9.5< 0.001
ALT(IU/L)16.7 ± 6.419.5 ± 7.0< 0.00112.8 ± 4.914.9 ± 5.5< 0.001

Associations Between Serum ALT Level and Clinico-Biochemical Variables in Subgroup Populations Based on the Modified Prati Criteria.

Univariate analysis showed that serum ALT concentration correlated with all quantitative clinical and biochemical variables, as did sex (P < 0.05 for each comparison; Table 2). Multivariate analysis showed that age, sex, BMI, and serum total cholesterol concentration were significantly and independently associated with ALT concentration (P < 0.05 for each comparison; Table 5), consistent with the results observed before the application of the modified Prati criteria. Sex-specific multivariate analysis showed that only age was significantly associated with ALT levels in both sexes, whereas BMI and serum total cholesterol were associated with serum ALT only in men (P < 0.05 for each comparison; Table 6).

Table 5. Associations Between Serum ALT Levels and Clinical and Biochemical Factors in the Subgroup of Subjects Meeting the Prati Criteria
VariableUnivariate AnalysisMultivariate Analysis
Slope (95% CI)P Value*Slope (95% CI)P Value*
  • The criteria of Prati et al. were modified by BMI cutoff points for Asian populations.

  • *

    Linear regression analysis.

  • Abbreviation: CI, confidence interval.

Age (years)0.102 (0.045–0.158)< 0.0010.137 (0.082–0.193)< 0.001
Sex−3.901 (−4.774 to −3.028)< 0.001−4.346 (−5.231 to −3.462)< 0.001
BMI (kg/m2)0.831 (0.523–1.138)< 0.0010.485 (0.191–0.779)0.001
Total cholesterol (mg/dL)0.024 (0.006–0.042)0.0090.021 (0.004–0.039)0.017
TG (mg/dL)0.016 (0.002–0.029)0.021
Glucose (mg/dL)0.058 (0.002–0.115)0.043
Table 6. Multivariate Linear Regression Analysis by Sex in Subgroups of Subjects Meeting the Prati Criteria
VariableMenWomen
Slope (95% CI)P Value*Slope (95% CI)P Value*
  • The criteria of Prati et al. were modified by BMI cutoff points for Asian populations.

  • *

    Linear regression analysis.

  • Abbreviation: CI, confidence interval.

Age (years)0.172 (0.082–0.263)< 0.0010.117 (0.053–0.181)< 0.001
BMI (kg/m2)0.738 (0.271–1.204)0.002
Total cholesterol (mg/dL)0.036 (0.008–0.064)0.011

Discussion

We found that serum ALT upper limits in healthy Asian populations with normal liver histology were lower, in a sex-specific manner, than were the earlier figures for such limits (45 IU/L for men and 34 IU/L for women),15 consistent with the results of a European study by Prati and colleagues.6 A substantial discrepancy also exists between the thresholds calculated in the present study and the reference limit currently adopted at our hospital (40 IU/L for both sexes). In fact, the ALT thresholds in the study of Prati's group were derived using the 95th percentile cutoff rather than the 97.5th, which might result in somewhat lower limit for ALT levels compared with the American Gastroenterological Association recommendations.13 However, even when using the 95th percentile cutoff, the healthy upper limits for ALT were 29 IU/L for men and 22 IU/L for women in our current study, thus also lower than the older criteria (41 IU/L for men and 30 IU/L for women) calculated at the 95th percentile of the reference distribution.15 Although slight differences between our data and those of Prati's group are apparent, these do not seem to be significant, considering the heterogeneity of serum ALT data measurements in different clinical laboratories.16

Previous studies have consistently found that BMI, and serum total cholesterol and TG levels are important factors influencing serum ALT concentrations, after excluding patients with viral hepatitis or alcohol-or drug-related liver diseases.6, 14 These data have been interpreted in two ways. Piton and colleagues14 suggested that ALT levels should be interpreted with reference to BMI, in agreement with the positive association observed in the present study. In contrast, Prati and colleagues suggested that ALT should be determined in healthy subjects at low risk for NAFLD, to exclude the influence of NAFLD, which can further progress to more advanced liver disease. The cited study, however, did not exclude individuals with NAFLD per se, as neither liver biopsy nor ultrasonographic examination was performed. Thus, ALT ranges in healthy individuals were determined in 3,927 subjects (1,995 men and 1,932 women) with normal BMI and normal serum total cholesterol and TG concentrations.6

In the present study, healthy ALT concentrations in biopsy-proven normal populations were practically identical to those of Prati and co-workers. We found that ALT levels increased with higher BMI or serum cholesterol levels, in subjects without any fatty changes, as determined histologically, and even in patients at lowest risk for NAFLD. In addition, mean ALT levels were significantly lower in subjects who fulfilled the Prati criteria, compared with those who failed testing in biopsy-proven normal populations. Such findings reveal that ALT elevation in subjects with high BMI and/or elevated serum cholesterol does not necessarily imply hepatocyte damage. An earlier study reported that elevated ALT levels not associated with fatty liver were frequently noted in obese Asian women.17 Several recent works have also suggested that ALT elevation associated with obesity or diet does not necessarily imply ALT leakage caused by cellular damage associated with NAFLD and may be related, at least in part, to increased ALT synthesis.18, 19

In the current study, 46% of men and 31% of women with histologically proven normal livers did not fall into the category of patients obeying the Prati criteria, and these populations had significantly (albeit slightly) higher ALT levels than those fulfilling the cited criteria. Although subpopulations outside the Prati criteria did not have current liver disease, the possibility remains that such patients are more susceptible to NAFLD, possibly indicating a propensity toward increased liver-related morbidity and mortality in the future. Therefore, in our opinion, healthy ALT values should be defined in a true normal population reliably identified on the basis of both normal liver histology and normal metabolic parameters.

ALT upper limits of normal are used as reference values for liver disease diagnosis in screening of general populations, including blood donors, and to trigger treatment initiation in patients with liver diseases. However, there is a considerable overlap in ALT levels between healthy subjects and patients with serious liver diseases. Several studies have shown that subjects with elevated ALT levels as defined by the Prati criteria but within normal ranges according to the older criteria, have increased liver-related mortality or may show active hepatic inflammation cases of chronic hepatitis B virus infection.20–22 In addition, redefined healthy upper ALT thresholds in chronic hepatitis C patients who achieved sustained virological responses were significantly lower than the pretreatment threshold for normal (30 IU/L for both sexes).23 However, recent population-based studies suggested that application of the new ALT standards may not be cost-beneficial.24, 25 Using the updated Prati definition, about 40% of the general population of the United States would have abnormal ALT levels, suggesting that application of these new definitions could result in a dramatic increase in the use of medical resources.24 Similarly, although application of the newly calculated healthy limits to patients with chronic viral hepatitis would identify more patients with active inflammation, this approach would doubtless result in the unnecessary use of antiviral agents, in that an elevated ALT level might be a consequence of patient age, BMI, total cholesterol level, or any/all of these combined with NAFLD. Therefore, careful interpretation of clinical parameters or a liver biopsy would be necessary before initiating treatment with antiviral agents, especially in older patients, those with high BMI or serum total cholesterol, and/or those with NAFLD.

The inherent limitations of the present study are the retrospective nature of the work and the skewed age distribution. Both our data and those of Prati's group were obtained from younger subjects, and our results indicate that serum ALT levels are positively age-dependent. However, a large population study based on histologic data may not be practically feasible in a general aged population. Because the difference in healthy ALT thresholds obtained from our true normal population and the subjects of Prati's study do not differ significantly, further investigation based on the Prati criteria may be of assistance in determining healthy ALT standards in older populations.

In conclusion, our estimates based on histologic data indicate that the healthy upper limits of ALT for Asians are apparently lower than the previously accepted values, and are similar to newly calculated values for European populations. Serum ALT levels vary with age and with clinical and metabolic indices, even in populations with normal livers. Therefore, the best interpretation of ALT values should take these parameters into account.

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