Background
Liver fibrosis is a wound-healing response to damage caused by chronic liver disease (CLD) [
1]. Liver fibrosis can progress to cirrhosis over years or decades [
2], and results in liver function decline and increased risk of hepatocellular carcinoma (HCC). Liver biopsy has been the gold standard for evaluating the presence and degree of liver fibrosis, but its clinical application is limited by inherent limitations such as invasiveness, sampling errors, and intra- and inter-observer variability [
3]. Recent studies indicated that liver fibrosis could be reversed [
1], creating the need for less invasive clinical tools to monitor and assess the responses of CLD patients to treatments. A number of scoring systems, such as the FibroTest [
4], the aspartate transaminase/alanine transaminase (AST/ALT) ratio [
5], the AST/Platelet Ratio Index (APRI) [
6], FIB-4 (patient age, AST, ALT, and platelet) [
7],
Wisteria floribunda agglutinin-positive Mac-2 binding protein (WFA
+-M2BP) [
8], and machine learning-based clinical predictive models [
9], have recently been used to stage CLD and predict the development of liver fibrosis and cirrhosis. Imaging techniques, such as computed tomography, magnetic resonance imaging [
10], and two recently approved ultrasound-based systems, shear wave elastography and transient elastography (FibroScan) [
11], have also been used clinically to assess the degree of liver fibrosis. However, these imaging modalities have limited accuracy in some patients, such as those with ascites, elevated central venous pressure, and obesity [
12].
Developing noninvasive, accurate, and reliable markers to assess the severity and progression of liver fibrosis in CLD patients has become increasingly important for treatment decisions, for continuous monitoring of patients who have mild liver disease and are not under treatment [
13], and for risk stratification and longitudinal follow-up in clinical trials.
Alterations of bile acids (BAs) [
13‐
15], free fatty acids (FFAs) [
16], and amino acids (AAs) [
17] are closely associated with CLD regardless of etiology. However, the relationship between serum AAs, BAs, and FFAs and the stages of liver fibrosis have not been thoroughly investigated. The aim of this study was to identify serum metabolite markers that reliably predict the stage of fibrosis in CLD patients with chronic hepatitis B virus (HBV) infection, a leading cause of CLD worldwide. We used a targeted metabolomics approach to quantify serum BAs, AAs, and FFAs in 1006 participants in cohort 1 (504 biopsy-proven fibrosis and cirrhosis CLD patients with chronic HBV infection and 502 normal controls, NC), and selected four predictive metabolite markers to construct three machine learning models using random forest (RF). Model 1 diagnosed CLD patients from NC, model 2 differentiated cirrhosis patients from fibrosis patients, and model 3 differentiated advanced fibrosis and early fibrosis patients. The diagnostic accuracy of the three models was further validated in an independent cohort consisting of 300 HBV-CLD patients and 90 NC.
Methods
Study design and participants
Two datasets were enrolled in this study. Cohort 1 was recruited between April 2013 and June 2015 at Shuguang Hospital Affiliated to Shanghai University of Traditional Chinese Medicine, consisted of 1006 participants, including 504 CLD patients with chronic HBV infection and 502 NC as our training cohort to identify serum metabolite markers and establish predictive models (Table
1). All the patients were tested positive for HBV-DNA or positive for hepatitis B surface antigen (HBsAg). Infection with chronic HBV was diagnosed according to the “Guideline on prevention and treatment of chronic hepatitis B in China” [
18]
. More detailed inclusion and exclusion criteria can be found in Additional file
1.
Table 1
Demographic and clinical data of patients with CLD and NC in cohort 1 (training set) and cohort 2 (validation set)
n | 502 | 504 | 349 | 155 | 400 | 104 | 90 | 300 | 134 | 166 | 141 | 159 |
Sex (M/F) | 365/137 | 361/143 | 257/92 | 104/51 | 299/101 | 62/42 | 59/31 | 202/98 | 81/53 | 121/45* | 86/55 | 116/43* |
Age (year) | 36.65 ± 11.73 | 36.58 ± 11.88 | 33.23 ± 9.95 | 44.88 ± 12.22*** | 34.05 ± 10.31 | 48.65 ± 11.51*** | 47.13 ± 9.95 | 47.96 ± 13.28 | 41.55 ± 12.83 | 53.14 ± 11.26*** | 41.21 ± 12.72 | 53.95 ± 10.67*** |
BMI (kg/m2) | 23.08 ± 3.16 | 22.28 ± 3.18*** | 22.02 ± 3.26 | 22.91 ± 2.91** | 22.15 ± 3.22 | 22.87 ± 2.97* | 22.35 ± 1.83 | 23.18 ± 3.13* | 23.28 ± 2.48 | 23.1 ± 3.61 | 23.21 ± 2.49 | 23.15 ± 3.65 |
APRI | 0.09 ± 0.04 | 0.79 ± 1.33*** | 0.6 ± 0.81 | 1.27 ± 2.08*** | 0.63 ± 0.92 | 1.55 ± 2.4** | | 0.65 ± 0.7 | 0.61 ± 0.78 | 0.69 ± 0.57** | 0.6 ± 0.76 | 0.72 ± 0.58*** |
AST/ALT | 0.82 ± 0.32 | 0.69 ± 0.44*** | 0.6 ± 0.31 | 0.94 ± 0.59*** | 0.61 ± 0.33 | 1.1 ± 0.63*** | | 1.09 ± 0.6 | 0.98 ± 0.57 | 1.24 ± 0.61*** | 0.97 ± 0.57 | 1.27 ± 0.61*** |
FIB-4 | 0.62 ± 0.33 | 2.86 ± 7.21*** | 1.42 ± 1.48 | 6.57 ± 12.71*** | 1.56 ± 1.7 | 9.42 ± 15.8*** | | 3.64 ± 3.64 | 2.49 ± 2.58 | 5.26 ± 4.27*** | 2.45 ± 2.53 | 5.54 ± 4.3*** |
ALT (IU/L) | 30.97 ± 15.63 | 176.49 ± 199.81*** | 195.3 ± 213.41 | 128.51 ± 150.32*** | 193.08 ± 208.27 | 94.18 ± 121.97*** | 17.92 ± 7.79 | 81.57 ± 117.77*** | 111.4 ± 141.39 | 57.2 ± 87.35*** | 108.45 ± 138.58 | 57.43 ± 89.06*** |
AST (IU/L) | 21.81 ± 6.84 | 93.21 ± 99.71*** | 95.65 ± 100.23 | 86.98 ± 98.47 | 96.32 ± 102.01 | 77.75 ± 86.32 | 20.25 ± 4.37 | 68.73 ± 73.78*** | 85.6 ± 91.48 | 54.95 ± 51.62* | 83.2 ± 89.82 | 55.74 ± 52.57 |
TBIL (μmol/L) | 15.5 ± 4.84 | 27.73 ± 32.98*** | 21.75 ± 13.27 | 42.87 ± 55.62*** | 23.06 ± 20.64 | 50.58 ± 61.18*** | 13.98 ± 3.62 | 33.67 ± 47.77*** | 23.77 ± 38.4 | 41.77 ± 52.99*** | 23.54 ± 37.45 | 42.77 ± 53.94*** |
ALP (IU/L) | 85.57 ± 19.18 | 89.72 ± 76.26 | 81.3 ± 65.24 | 111.05 ± 95.86** | 82.37 ± 61.46 | 125.77 ± 119.98** | 77.82 ± 19.21 | 93.68 ± 70.86* | 70.57 ± 55.75 | 112.56 ± 76.25*** | 70.97 ± 54.47 | 114.08 ± 77.53*** |
GGT (IU/L) | 17.12 ± 10.72 | 69.18 ± 97.06*** | 61.13 ± 69.75 | 89.73 ± 143.43* | 66.72 ± 74.55 | 81.42 ± 169.46 | 26.26 ± 19.07 | 76.87 ± 82.16*** | 77.22 ± 83.58 | 76.59 ± 81.23 | 76.14 ± 81.77 | 77.53 ± 82.76 |
TP (g/L) | 74.41 ± 4.76 | 73.47 ± 8.55* | 75.68 ± 5.31 | 67.87 ± 12.03*** | 75.42 ± 5.24 | 63.82 ± 13.76*** | 73.65 ± 3.19 | 71.31 ± 17.94*** | 72.89 ± 5.84 | 69.97 ± 23.74*** | 73.07 ± 5.82 | 69.66 ± 24.23*** |
ALB (g/L) | 49.23 ± 2.77 | 40.19 ± 5.91*** | 42.14 ± 3.37 | 35.23 ± 7.81*** | 41.75 ± 3.54 | 32.47 ± 8.67*** | 44.92 ± 2.19 | 37.86 ± 7.18*** | 41.44 ± 4.79 | 34.93 ± 7.48*** | 41.39 ± 4.71 | 34.69 ± 7.54*** |
TBA (μmol/L) | 4.67 ± 3.18 | 28.47 ± 45.11*** | 20.33 ± 37.28 | 49.68 ± 55.81*** | 21.3 ± 36.44 | 65.41 ± 63.96*** | 3.6 ± 2.63 | 43.41 ± 55.16*** | 25.05 ± 37.86 | 58.89 ± 62.37*** | 24.33 ± 37.04 | 61.11 ± 62.91*** |
PLT (109/L) | 261.02 ± 65.25 | 164.52 ± 61.7*** | 184.2 ± 47.74 | 114.68 ± 65.05*** | 179.06 ± 49.86 | 93.23 ± 64.82*** | | 132.44 ± 62.97 | 163.58 ± 51 | 106.03 ± 60.14*** | 161.54 ± 51.18 | 105.27 ± 60.9*** |
Collagen proportionate area | | 7.46 ± 4.01 | 1.96 ± 1.43 | 9.95 ± 6.03*** | 2.71 ± 2.45 | 15.17 ± 7.11*** | | | | | | |
HBV-DNA (log10) | | 6.25 ± 2.42 | 6.32 ± 2.44 | 5.99 ± 2.34 | 6.33 ± 2.38 | 5.39 ± 2.66 | | | | | | |
Negative HbeAg, n | | 191 | 115 | 76 | 142 | 49 | | | | | | |
Negative HbeAb, n | | 223 | 153 | 70 | 175 | 48 | | | | | | |
Negative HbsAg, n | | 29 | 24 | 5 | 26 | 3 | | | | | | |
Cohort 2, recruited between December 2016 and December 2017 at Xiamen Hospital of Traditional Chinese Medicine, consisted of 300 CLD patients with chronic HBV infection and 90 NC. Data obtained from cohort 2 were used as a validation set to further verify the performance of the models established from the cohort 1. Detailed information about this cohort can be found in Additional file
1. Sample size was not determined by statistical methods and was comparable to other studies in the field [
4‐
8,
17,
19].
In this study, the diagnosis and the sample collection were performed using exactly the same protocols to avoid “external” influences. The samples were provided to lab staffs blind samples with respect to patient identity and other clinical information.
The study was organized and led by Shuguang Hospital Affiliated to Shanghai University of Traditional Chinese Medicine, and participated by Xiamen Hospital of Traditional Chinese Medicine. The study was approved by the institutional review board of Shuguang Hospital first (approval no. 2012-206-22-01) and endorsed by the ethics committee of Xiamen Hospital. All participants provided written informed consent.
Liver biopsy
All patients, except those diagnosed with decompensated cirrhosis (presence of any of the following complications in cirrhosis: variceal hemorrhage, ascites, encephalopathy, and jaundice), received a liver biopsy directed by ultrasonography within 1 week after enrollment. The biopsy specimens were fixed with 10% formalin, embedded in paraffin, and stained with hematoxylin/eosin and Masson’s trichrome stain. Examination of a minimum length of 1.5 cm of the liver biopsy and at least six portal tracts were required for diagnosis. Histological grading of necro-inflammation (G0 to G4) and staging of liver fibrosis (S0 to S4) were carried out according to Scheuer’s classification [
20]. All samples were independently assessed by three pathologists from Shanghai Medical College of Fudan University, Shanghai, China, who were blinded to the sample ID. Specimens with discrepant assessments were re-examined until a consensus was reached. The final assessments of the three pathologists were further processed using the kappa concordance test.
Histological assessment of liver injury
The obtained liver tissues via liver biopsy were fixed in 10% formalin (Sigma), processed using established protocols, and embedded in paraffin. Sections (5 μm) of each sample were cut and stained with hematoxylin and eosin (H&E) for histopathological analysis. All sections were examined using a light microscope. Based on the H&E staining results, the necro-inflammation activity of chronic hepatitis was determined as G0 to G4 according to Scheuer’s classification as G0 (absent), G1 (portal inflammation only), G2 (mild interface hepatitis), G3 (moderate interface hepatitis), and G4 (severe interface hepatitis) (Additional file
2: Figure S1).
Collagen proportionate area using digital image analysis
The obtained liver tissues via liver biopsy were fixed in 10% formalin (Sigma). Tissue samples were embedded in paraffin blocks and then sliced into 5-μm-thick sections. Sections were processed and stained with Masson’s trichrome as reported [
21]. Masson staining kits were from Abcam Co., Ltd. (Trichrome Stain, ab150686). Collagen stained blue (Additional file
2: Figure S2). In order to characterize collagen area, Masson’s trichrome-stained slides were scanned with a Leica SCN400 scanner (Leica Microsystems) at × 40 magnification and measured using Aperio ImageScope (v12.3.2.5030, Aperio Technologies). The images were saved as “.scn” format files. The Color Deconvolution algorithm (v9, Aperio Technologies) was used to isolate individual stains for semi-quantification. The percent total positive, total stained area (mm
2), and total analysis area (mm
2) in each visual field were measured and recorded. The analytical data were saved as “.xls” format files. CPA = percent total positive × total stained area/total analysis area.
Serum sample collection
Overnight fasting (12 h) blood samples were collected from all subjects, and sera were delivered to our laboratories on ice within 2 h of collection. Samples were aliquoted and stored at − 80 °C until analysis.
Blood clinical marker measurement
Hematological and standard biochemical tests were performed using an LH750 Hematology Analyzer and a Synchron DXC800 Clinical System (Beckman Coulter, USA) according to the manufacturer’s protocol. The coagulation function was measured using an automatic coagulation analyzer (STAGO Compact, Diagnostica Stago, France). The serum HBV-DNA level was quantified using a real-time polymerase chain reaction (PCR) system (LightCycler 480, Roche, USA).
Samples in cohort 1 were analyzed at the Center for Translational Medicine, Shanghai Jiao Tong University Affiliated Sixth People’s Hospital. Samples in cohort 2 were analyzed at the Metabo-Profile Biotechnology (Shanghai) Co., Ltd. BAs and AAs were quantified using ultra-performance liquid chromatography (UPLC)-triple quadrupole mass spectrometry (Waters XEVO TQ-S, Milford, MA), and FFAs were quantified using UPLC quadrupole time-of-flight mass spectrometry (Waters XEVO G2S, Milford, MA), according to our previously reported protocol [
22‐
25].
The detailed procedure and analysis were performed as described in Additional file
1.
ROC curve is a plot of the true positive rate (sensitivity/recall) against the false positive rate (1 − specificity) at different cutoffs of a binary classifier. AUROC measures the area under the ROC curves, and a higher value of AUROC suggests better classification performances while an AUROC of 0.5 represents the random guess. The PR curve demonstrates the relationship between positive predictive values (precision) and true positive rate (sensitivity/recall), and a higher value of AUPR indicates better diagnostic capacity of the model. PR curves are usually preferable for evaluating unbalanced data compared to ROC curves. NRI and IDI were also used for the evaluation of prediction improvement. We compared RF models to existing clinical indices by splitting the continuous risk scores into ten equal risk intervals (default). We used the R software version 3.2.3 for data analysis and the “PRROC” R package for binary ROC and PR curves [
26], the “pROC” package for calculating the specificities and sensitivities of classifiers [
27], and the “PredictABEL” package for NRI and IDI calculation [
28].
Feature selection and method comparison
Quantitative variables were expressed as mean ± SD for clinical parameters and median (25% quantile, 75% quantile) of log10 transformed concentration for metabolites. Categorical variables were expressed as percentages. The univariate analysis (Wilcoxon’s rank-sum test) was carried out to identify the variables that were significantly different between CLD patients and NC, between fibrosis and cirrhosis (S0–3 vs. S4), and among CLD patients at different fibrotic stages (early stage fibrosis (S0–2) vs. advanced stage fibrosis (S3–4)).
For differential metabolites with
p < 0.001 across, all univariate analyses were used in two machine learning methods, LASSO [
29] and RF [
30], to further select markers for the three classifications listed above. Data were log and
z-score transformed before being fed into LASSO to ensure that the coefficients were comparable with each other. The regularization parameter lambda of LASSO was determined using 10-fold cross-validation (CV). The RF model used 500 decision trees. We ranked the metabolites according to their LASSO non-zero coefficients and RF mean decrease of accuracy, and kept the intersection of top 5 LASSO and RF metabolites in the three classifications. Considering the overlaps of the second and the third classification tasks, we further selected the intersecting variables of these two situations and then merged with variables selected from the first situation to construct our final metabolite markers (Fig.
2).
To identify an appropriate classification method, we introduced two linear models, i.e., logistic regression (LR) and linear discriminant analysis (LDA), and one decision tree-based ensemble model, i.e., RF, for the classifier construction for the markers we selected. For RF, we used 500 decision trees and two candidate variables at each split. For LDA, the tolerance parameter was set to 1.0E−4 (default). We applied 10-fold CV on the training set (cohort 1) to compare the classification performances of these four models and three established fibrosis markers, i.e., APRI, AST/ALT ratio, and FIB-4. AUROC and AUPR were recorded at each internal validation set in CV. We used R packages “randomForest,” “glmnet,” and “MASS” for RF, LASSO, and LDA constructions, respectively [
31,
32].
Predictive model construction and validation
We trained the final RF models for different classification objectives using the training data (cohort 1), with model 1 differentiating CLD and NC, model 2 differentiating fibrosis and cirrhosis, and model 3 differentiating early and advanced stages of liver fibrosis. A total of 500 decision trees were included in a single RF model with two variables randomly sampled as candidates at each split. We re-balanced the sample size for different groups at each bootstrap resampling step for models 2 and 3 considering the unbalanced samples [
33].
In RF, each decision tree was fitted on the bootstrap samples and tested on the untouched OOB samples. Thus, the OOB predictions provided unbiased estimates of how the RF model performed on the training data and were used for the evaluation on cohort 1. We further validated our mark panel-based RF models in the independent validation datasets from cohort 2, and compared results with the established fibrosis markers, AST/ALT ratio, APRI, and FIB-4. ROC and PR curves were drawn, and AUROC and AUPR values, respectively, were calculated to evaluate their diagnostic performances. Optimal cutoffs were selected to maximize the sum of sensitivity and specificity for the RF model. For APRI, FIB-4, and AST/ALT, predefined cutoffs were used (1.0 and 2.0 for APRI to distinguish fibrosis and cirrhosis [
6], 1.45 and 3.25 for FIB-4 to distinguish S0–2 and S3–4 [
7], and 0.8 and 1.0 for AST/ALT to distinguish S0–2 and S3–4 [
5,
34]). Bootstrap resampling (1000 times) was conducted to calculate 95% confidence intervals (CIs) of AUCs for all binary classifiers. A comparison of the AUROC of our biomarker panel vs. FIB-4, AST/ALT, or APRI was performed using DeLong’s test. The significance level was adjusted for multiple testing according to the Benjamini and Hochberg procedure [
35]. Log and
z-score transformed data were also used for constructing heatmaps. The R packages “ggplot2” and “cowplot” were used for data visualization and multiple plots arrangement.
We further derived an RF risk score for each participant based on the marker panel and logit function of the predicted probability (Prob.) of the RF model for corresponding classification objective:
$$ \mathrm{RF}\ \mathrm{score}=\mathrm{logit}\left(\mathrm{Prob}.\right)=\log \left(\frac{\mathrm{Prob}.}{1-\mathrm{Prob}.}\right) $$
F1 scores were then calculated at the predefined cutoffs using following formula:
$$ \mathrm{F}1=\frac{2}{{\mathrm{Precision}}^{-1}+{\mathrm{Recall}}^{-1}} $$
To determine whether the RF score could independently predict the fibrosis staging in the presence of other potential confounding factors, we applied logistic regression on the RF score to differentiate cirrhosis from fibrosis as well as discriminate early and advance fibrosis while adjusting for HBV-DNA levels, the degree of necro-inflammation, HBeAb status, HBeAg status, liver function tests (i.e., PT, ALB, DBIL, IBIL), platelets, BMI, and medication (entecavir) use.
Multi-group classification of S0–2 vs. S3 vs. S4
We built a new RF model based on our metabolite marker panel and applied multinomial regressions to APRI, AST/ALT, and FIB-4 separately to differentiate S0–2 vs. S3 vs. S4 in cohort 1. Then, we compared and validated these multi-group classifiers on both cohort 1 and cohort 2 datasets using micro-average ROC and PR curves. Micro-average ROC and PR curves were calculated by stacking binary classification results from each group together to generate a concatenated binary classification result [
36]. We then calculated AUROC and AUPR with 95% CIs using 100 times bootstrap resampling. We used the “multiROC” R package for calculating the micro-average AUROC and AUPR as well as for plotting [
37].
Discussion
As the prevalence of CLD rises worldwide, accurate and reliable assessments for the severity of this disease are increasingly important for treatment selection and longitudinal monitoring [
13]. Attempts to develop noninvasive tools for staging CLD have yielded multiple scores, indices, and imaging modalities [
4‐
7,
10] that might be used in lieu of liver biopsy, with the AST/ALT ratio, APRI, and FIB-4 as examples [
5‐
7]. Current noninvasive assessments have the advantage of allowing repeated applications and are well-received by the patients. In this study, we identified a panel of metabolite markers that consisted of C18:2 n6t, TCA, Tyr, and a Tyr/Val ratio that was highly correlated with discrete stages of CLD progression in patients with HBV infection.
Histologic staging of CLD by liver biopsy provided a reference standard for our study. In the Scheuer system, one of the most clinically validated systems for staging liver fibrosis, S0 is defined as no fibrosis, S1 as portal fibrosis, S2 as periportal fibrosis, S3 as septal fibrosis, and S4 as cirrhosis [
20]. The clinically overt stage of cirrhosis includes compensated cirrhosis with/without portal hypertension and decompensated cirrhosis [
38]. In this study, we first identified candidate markers that significantly differed between NC and patients with CLD that correlated well with fibrotic stage and necro-inflammation based on univariate, LASSO, and RF analyses. We then constructed diagnostic models to discriminate CLD patients from NC, and to discriminate CLD patients at different fibrosis stages, i.e., early vs. advanced fibrosis (S0–2 vs. S3–4) and fibrosis vs. cirrhosis (S0–3 vs. S4). This resulted in three optimized marker panel-based RF predictive models for staging liver fibrosis that, upon validation, showed acceptable performance across independent cohort. Although the AUROCs of our models in the validation set were not as high as in the training set, we still achieved relatively good AUROC (all > 0.8) considering a rough guide for classifying the accuracy of a diagnostic test in the traditional academic point system [
39]. A decreased external validation/testing accuracy was a common fact when applied machine learning in biomedical studies [
40]. The AUROC and AUPR of our biomarker panel were significantly greater than those of the AST/ALT ratio, APRI, and FIB-4, suggesting superior predictive value for this metabolite marker panel.
Altered BA profile and BA synthesis are associated with various hepatic diseases, such as chronic hepatitis B, primary biliary cirrhosis, chronic hepatitis C, and NAFLD. Circulating BAs are commonly used in clinical practice to assist evaluation of the severity of CLD [
41]. Several studies, including our previous work, on cirrhosis and HCC have shown dramatically increased levels of GCA, GCDCA, TCA, and TCDCA in the circulation of patients with NAFLD [
42], NASH [
42], HBV [
43], cirrhosis [
44], and HCC [
44]. The liver also plays a major role in lipid metabolism by taking up FFAs and manufacturing, storing, and transporting lipid metabolites [
45]. A characteristic pattern of plasma amino acids has been described in cirrhotic subjects [
42,
46,
47], and in samples collected in England and the USA, metabolic and biochemical differences have been shown between stable and unstable cirrhotics [
42,
46]. Advanced liver fibrosis, especially cirrhosis, was also associated with altered plasma AA patterns, including decreased levels of branched chain amino acids (leucine, isoleucine, valine) and increased concentrations of the aromatic amino acids phenylalanine and tyrosine [
19]. An index based on AA concentration has already been proposed for diagnosing liver fibrosis [
17]. In patients admitted to either the Veterans Administration Hospital or the Yale-New Haven Medical Center between 1 January 1965 and 1 May 1966, fasting tyrosine levels tended to be slightly increased in patients with hepatitis and markedly increased in patients with cirrhosis [
48]. The present study showed that a combined panel of FFA, BA, and AA was a strong predictor for CLD progress.
Linoelaidic acid is an isomer of linoleic acid. It has been reported that linoelaidic acid may inhibit the development of tumors through its antioxidant effects, has a role in the prevention of atherosclerosis, and modulates certain aspects of immune system [
49]. The significantly decreased levels of linoelaidic acid may thus be an indication of a disease state. Further research on these findings and human epidemiological data is warranted to confirm this.
The major strengths of our study were the use of large sample sizes to construct and verify all models, and the quantification of the metabolite markers (BA, FFA, and AA) using standardized protocols. Furthermore, participants in the validation set (cohort 2) were recruited independently from those in cohort 1, and this new set of patients confirmed the robustness of our marker panel and predictive models.
The limitations of our study included the following: (1) Use of medications was a confounding factor for our model, but key findings were not altered after correcting for medication use. Larger studies are needed to further evaluate the effect of these medications; (2) HBV infection was the only or major cause of CLD in this study, and the participants were all Chinese. Therefore, the results may not be extrapolated to CLD with other etiologies outside these diseases, or to other racial/ethnic groups. Future large-scale validation studies should include CLD with other etiologies and participants of other race/ethnicity, before implementing this 4-marker panel in clinical practice. (3) In addition to cross-sectional studies, longitudinal studies are needed to further validate the reproducibility of the current findings and the predictive values of the models, especially those used to differentiate early from advanced liver fibrosis, and (4) the cost of full spectrum metabolomic analysis is high. However, if the robustness of this 4-marker panel is proven in future validation studies, specific tests may be developed for only C18:2 n6t, TCA, Tyr, and Val to decrease the cost and to translate this marker panel to clinical practice.
Conclusions
In summary, using targeted metabolomics analyses, we identified four metabolite markers from serum that accurately differentiated CLD patients from NC, and differentiated varied stages of liver fibrosis, including S0–2 vs. S3–4, and S0–3 vs. S4. The diagnostic performance of this novel, noninvasive 4-marker panel was superior to FIB-4, AST/ALT ratio, and APRI. If validated in future studies, this 4-marker panel will be useful in reducing the need for liver biopsies in identifying patients with non-significant fibrosis, as well as aiding in the continued assessment of CLD in patients previously diagnosed with CLD.
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