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Address correspondence to: Theresa Hydes, Third Floor Clinical Sciences Centre, Liverpool University Hospitals National Health Service Foundation Trust, Longmoor Lane, Liverpool, L9 7AL United Kingdom.
Department of Cardiovascular and Metabolic Medicine, Institute of Life Course and Medical Sciences, University of Liverpool, Liverpool, United KingdomLiverpool University Hospitals National Health Service Foundation Trust, Liverpool, United Kingdom
Department of Cardiovascular and Metabolic Medicine, Institute of Life Course and Medical Sciences, University of Liverpool, Liverpool, United KingdomLiverpool University Hospitals National Health Service Foundation Trust, Liverpool, United Kingdom
Department of Cardiovascular and Metabolic Medicine, Institute of Life Course and Medical Sciences, University of Liverpool, Liverpool, United KingdomLiverpool University Hospitals National Health Service Foundation Trust, Liverpool, United Kingdom
Department of Cardiovascular and Metabolic Medicine, Institute of Life Course and Medical Sciences, University of Liverpool, Liverpool, United KingdomLiverpool University Hospitals National Health Service Foundation Trust, Liverpool, United Kingdom
Nonalcoholic fatty liver disease (NAFLD) is a metabolic disorder that frequently coexists with obesity, metabolic syndrome, and type 2 diabetes. The NAFLD spectrum, ranging from hepatic steatosis to nonalcoholic steatohepatitis, fibrosis, and cirrhosis, can be associated with long-term hepatic (hepatic decompensation and hepatocellular carcinoma) and extrahepatic complications. Diagnosis of NAFLD requires detection of liver steatosis with exclusion of other causes of chronic liver disease. Screening for NAFLD and identification of individuals at risk of end-stage liver disease represent substantial challenges that have yet to be met. NAFLD affects up to 25% of adults, yet only a small proportion will progress beyond steatosis to develop advanced disease (steatohepatitis and fibrosis) associated with increased morbidity and mortality. Identification of this cohort has required the gold standard liver biopsy, which is both invasive and expensive. The use of serum biomarkers and noninvasive imaging techniques is an area of significant clinical relevance. This narrative review outlines current and emerging technologies for the diagnosis of NAFLD, nonalcoholic steatohepatitis, and hepatic fibrosis.
Methods
We reviewed the literature using PubMed and reviewed national and international guidelines and conference proceedings to provide a comprehensive overview of the evidence.
Findings
Significant advances have been made during the past 2 decades that have enhanced noninvasive assessment of NAFLD without the need for liver biopsy. For the detection of steatosis, abdominal ultrasonography remains the first-line investigation, although a controlled attenuation parameter using transient elastography is more sensitive. For detecting fibrosis, noninvasive serum markers of fibrosis and algorithms based on routine biochemistry are available, in addition to transient elastography. These techniques are well validated and have been incorporated into national and international screening guidelines. These approaches have facilitated more judicious use of liver biopsy but are yet to entirely replace it. Although serum biomarkers present a pragmatic and widely available screening approach for NAFLD in large population-based studies, magnetic resonance imaging techniques offer the benefit of achieving high degrees of accuracy in disease grading, tumor staging, and assessing therapeutic response.
Implications
This diagnostic clinical and research field is rapidly evolving; increasingly combined applications of biomarkers and transient elastography or imaging of selective (intermediate or high risk) cases are being used for clinical and research purposes. Liver biopsy remains the gold standard investigation, particularly in the context of clinical trials, but noninvasive options are emerging, using multimodality assessment, that are quicker, more tolerable, more widely available and have greater patient acceptability.
Nonalcoholic fatty liver disease (NAFLD) represents the leading cause of chronic liver disease globally, with an ever-increasing prevalence in many regions, including in the United States, Europe, the Middle East, and Asia, where it occurs in parallel with the epidemics of obesity, metabolic syndrome, and type 2 diabetes (T2D).
Prevalence rates of NAFLD and nonalcoholic steatohepatitis (NASH) specifically are estimated to be 10% to 30% and 3% to 5%, respectively, in the general population, significantly higher in people with obesity (ranging from 50%–90% and 10%–50%, respectively),
and 56% and 37%, respectively, in people with T2D.
NAFLD represents a disease spectrum that ranges from steatosis to NASH, fibrosis, and cirrhosis. NAFLD/NASH is strongly associated with the metabolic syndrome, a cluster of conditions that includes central obesity, hypertriglyceridemia, reduced HDL-C, hypertension, and impaired fasting glucose.
Definition of metabolic syndrome: report of the National Heart, Lung, and Blood Institute/American Heart Association Conference on Scientific Issues Related to Definition.
Indeed, a position statement has recently proposed changing the name NAFLD to MAFLD (metabolic dysfunction associated fatty liver disease) to highlight the metabolic aspect of the disease.
NAFLD develops without signs or symptoms, detected through medical investigations or after a decompensating clinical episode associated with end-stage liver disease. Investigation of NAFLD may be prompted by detection of abnormal liver biochemistry
European Association for the Study of the Liver (EASL) European Association for the Study of Diabetes (EASD) & European Association for the Study of Obesity (EASO). EASL–EASD–EASO Clinical Practice Guidelines for the management of non-alcoholic fatty liver disease.
Further investigation is then required to characterize the position on the NAFLD spectrum to ascertain the current and potential future risk of liver-related morbidity and mortality, specifically cirrhosis or hepatocellular carcinoma (HCC). Percutaneous liver biopsy for histopathologic examination remains the gold standard, but a variety of serum biomarkers and imaging modalities, including ultrasonography, computed tomography (CT), and magnetic resonance imaging (MRI), in addition to technologies that use shear waves, may be harnessed to reduce the need for the invasive procedure (Figure 1). These techniques are increasingly being incorporated into patient pathways and routine clinical practice.
European Association for the Study of the Liver (EASL) European Association for the Study of Diabetes (EASD) & European Association for the Study of Obesity (EASO). EASL–EASD–EASO Clinical Practice Guidelines for the management of non-alcoholic fatty liver disease.
Despite advances in the understanding of the pathophysiology, no specific licensed treatments for NAFLD/NASH have been approved by regulatory agencies for NAFLD, and weight loss remains the cornerstone of management. Significant investment from the pharmaceutical industry has led to a plethora of clinical trials with potential agents for NAFLD, and in parallel there is a need for robust noninvasive assessment tools that may eventually replace histology in clinical and research settings. In this narrative review, we discuss current and emerging tools available for identification and risk stratification of patients with NAFLD and their most appropriate application in routine clinical practice and research studies.
Figure 1Summary of noninvasive markers for the diagnosis and staging of nonalcoholic fatty liver disease (NAFLD). AST = aspartate aminotransferase; ALT = alanine aminotransferase; BARD = BMI, AST:ALT ratio, Diabetes; CK-18 = cytokeratin 18; 1H-MRS = proton magnetic resonance spectroscopy; HAIR = hypertension, ALT, and insulin resistance; MRI = magnetic resonance imaging; NASH = nonalcoholic steatohepatitis; NICE = National Institute for Health and Care Excellence; NIS4 = ; PAI-1 = plasminogen activated inhibitor 1; PDFF = proton density fat fraction; PRO-C3 = N-terminal type III collagen propeptide.
NAFLD is defined by the presence of steatosis affecting more than 5% of hepatocytes quantified by imaging or histology in the absence of secondary causes (eg, hepatitis C, parental nutrition, and in-born errors of metabolism) and in the absence of excessive alcohol intake, defined as ≥20 g/d for women and ≥30 g/d for men according to the European and American guidelines,
European Association for the Study of the Liver (EASL) European Association for the Study of Diabetes (EASD) & European Association for the Study of Obesity (EASO). EASL–EASD–EASO Clinical Practice Guidelines for the management of non-alcoholic fatty liver disease.
In contrast, MAFLD is defined as a diagnosis of hepatic steatosis (via histology, imaging, or serum biomarkers) in addition to (1) overweight/obesity, (2) T2D, or (3) evidence of metabolic dysfunction, defined as having ≥2 of 7 potential metabolic risk factors.
Additional drivers of chronic liver disease, including viral hepatitis and alcohol excess, are not considered criteria for exclusion, but instead it has been suggested that the term dual-etiology liver disease be adopted.
NAFLD Disease Spectrum: Pathophysiology and Clinical Implications
NAFLD is the overarching term that comprises a spectrum of disease that ranges from hepatic steatosis to necroinflammatory changes within the liver (NASH) through to hepatic fibrosis. Movement between these stages in terms of disease progression and resolution is dynamic and can be bidirectional.
The natural history of NAFLD is hugely heterogenous as a result of comorbidities (insulin resistance, visceral obesity, dyslipidemia, and hypertension), lifestyle factors (alcohol, fructose and coffee consumption, and physical activity or inactivity), genetics (patatin-like phospholipase domain–containing protein 3 and transmembrane 6 superfamily member 2),
The prognostic implications of accurate staging are of utmost importance because surveillance for HCC and complications of portal hypertension, in addition to rationale for treatment, depend on accurate knowledge of the position on the disease spectrum.
Hepatic Steatosis
Hepatic steatosis refers to excess accumulation of triglycerides in hepatocytes and is known as a nonalcoholic fatty liver. The presence of hepatic steatosis has metabolic implications and carries the same cardiovascular risk as NASH, mainly as a result of the coexistence of metabolic syndrome and incident T2D.
NASH is a histologic diagnosis characterized by steatosis and hepatocellular injury, which manifests as hepatocyte ballooning and lobular inflammation.
Utility and appropriateness of the fatty liver inhibition of progression (FLIP) algorithm and steatosis, activity, and fibrosis (SAF) score in the evaluation of biopsies of nonalcoholic fatty liver disease.
Fibrosis is the only independent histologic predictor of end-stage events, including hepatic decompensation, need for liver transplantation, HCC, and liver-related and overall mortality.
Fibrosis stage, classified as early fibrosis (F0/1), significant fibrosis (F2), advanced fibrosis (F3), and cirrhosis (F4), strongly correlates with these outcomes.
This disease stage is potentially reversible, however, as demonstrated by a study of 108 patients with NAFLD who underwent paired liver biopsies a mean of 7 years apart in which 42% displayed progression of fibrosis and 18% had regression.
The initial step in the diagnosis of NAFLD is traditionally the detection of steatosis, which is reflected in national and international guidelines (Table I). European,
European Association for the Study of the Liver (EASL) European Association for the Study of Diabetes (EASD) & European Association for the Study of Obesity (EASO). EASL–EASD–EASO Clinical Practice Guidelines for the management of non-alcoholic fatty liver disease.
guidelines advocate the use of ultrasonography as a reasonable first-line investigation, whereas the US guidelines are less prescriptive and highlight that although liver ultrasonography and transient elastography (TE) are more sensitive than liver biochemistry for detection of NAFLD, their utility as screening tools is as yet unproven.
European Association for the Study of the Liver (EASL) European Association for the Study of Diabetes (EASD) & European Association for the Study of Obesity (EASO). EASL–EASD–EASO Clinical Practice Guidelines for the management of non-alcoholic fatty liver disease.
People with abnormal liver enzyme levels in the presence of the metabolic syndrome or BMI >25 kg/m2
People with obesity and T2D or the incidental finding of increased liver enzyme levels in patients with metabolic risk factors
High index of suspicion of NAFLD in people with T2D; do not advise routine screening
Screening of NAFLD may be considered in at-risk groups, such as patients with T2D and obesity
Diagnostic pathway for NAFLD
Ultrasonography for steatosis; if ELF score is ≥10.51, perform TE and ARFI; if ELF score is <10.51, retest every 3 years
Ultrasonography for steatosis and abnormal liver enzyme levels;FIB-4 (≤1.30, 1.3-3.25, ≥3.25) orNFS (≤−1.455, −1.455 to 0.675, >0.675); higher cutoffs if >65 years of age;ELF, TE, and ARFI if indeterminate rangeTE >7.8 kPa possible advanced fibrosis
Ultrasonography for steatosis biomarkers and abnormal liver enzyme levels;NFS and FIB-4, TE, and ELF; specialist referral if medium to high risk
MetS should help target need for biopsy; NFS and FIB-4 useful foridentifying higher likelihood F3/F4 fibrosis; TE and MRE useful foridentifying advanced fibrosis
Ultrasonography and TE may be used as screening tools; noninvasive serum and physical tests afford modest but possibly acceptable accuracies when used to measure the fibrotic burden in patients with NAFLD
AASLD = American Association for the Study of Liver Disease; ARFI = acoustic radiation force impulse; BMI = body mass index; BSG = British Society of Gastroenterology; EASD = European Association for the Study of Diabetes; EASL = European Association for the Study of the Liver; EASO = European Association for the Study of Obesity; ELF = enhanced liver fibrosis; FIB-4 = fibrosis 4; MetS = metabolic syndrome; MRE = magnetic resonance elastography; NAFLD = nonalcoholic fatty liver disease; NFS = NAFLD fibrosis score; NICE = National Institute for Health and Care Excellence; T2D, type 2 diabetes; TE, transient elastography.
There is further disagreement between guidelines regarding routine screening of at-risk populations because the cost-effectiveness is unknown. This disagreement would present a potential insurmountable challenge for health care systems given that approximately 64% of individuals are estimated to be overweight or obese and nearly 10% have a diagnosis of diabetes in a general population setting (Health Survey for England data).
European Association for the Study of the Liver (EASL) European Association for the Study of Diabetes (EASD) & European Association for the Study of Obesity (EASO). EASL–EASD–EASO Clinical Practice Guidelines for the management of non-alcoholic fatty liver disease.
guidelines advise that clinicians be aware that individuals with these conditions are at greater risk of NAFLD but do not recommend widespread screening. It is therefore up to the discretion of the clinician as to whether a patient in a high-risk group is screened for NAFLD. This decision may be triggered by factors such as metabolic risk factor clustering or abnormal liver enzyme levels.
Imaging
Ultrasonography
Liver ultrasonography detects hepatic fat through increased echogenicity of the liver parenchyma. It is widely available, is fairly inexpensive, and offers high levels of sensitivity and specificity (85% and 94%, respectively).
Several methods have been trialed in an attempt to overcome these limitations, including computer-assisted quantitative techniques that examine the hepatorenal echo-intensity ratio and ultrasound hepatic echo-intensity attenuation rate, which can detect mild steatosis with a sensitivity of 81% and specificity of 100%.
Controlled attenuation parameter (CAP) is also superior to ultrasonography for the detection of mild steatosis. This technology uses vibration-controlled elastography to measure the degree of ultrasound attenuation due to hepatic fat. A recent meta-analysis identified that CAP had a pooled AUC of 0.96 for the detection of steatosis grade 1
Diagnostic accuracy of controlled attenuation parameter (CAP) as a non-invasive test for steatosis in suspected non-alcoholic fatty liver disease: a systematic review and meta-analysis.
It does, however, suffer from low levels of specificity and a lack of consensus surrounding the cutoff values, which may be higher for more obese Western populations.
Accuracy of FibroScan controlled attenuation parameter and liver stiffness measurement in assessing steatosis and fibrosis in patients with nonalcoholic fatty liver disease.
CT can also accurately detect moderate and severe hepatic steatosis but similar to ultrasonography is not effective at detecting mild steatosis. It also carries the substantial risk of radiation exposure and has a greater cost.
Several MRI techniques can be used to detect hepatic steatosis and achieve exceptionally high levels of accuracy. MRI-PDFF is a measure of the fraction of MRI-visible excitable protons bound to fat divided by all MRI-visible protons in the liver. It carries the benefit of sampling the whole liver and is not affected by patient (age, sex, body mass index) or liver factors (iron overload and necroinflammation). It can also be used with any clinical MRI platform. In a recent meta-analysis, MRI-PDFF was found to identify steatosis with an AUC of 0.98 and was also able to accurately distinguish different grades of steatosis.
Magnetic resonance elastography vs transient elastography in detection of fibrosis and noninvasive measurement of steatosis in patients with biopsy-proven nonalcoholic fatty liver disease.
but its availability is limited because it cannot be used on a clinical MRI machine and it carries long acquisition times. Both techniques are associated with significant costs, and thus although they are superior over measures such as CAP in terms of performance and a lower failure rate,
it is not feasible to use them for screening. Of note, detection of steatosis by conventional MRI is limited as a result of T1 bias, T2* decay, and signal interference.
Although liver enzymes (alanine transaminase, aspartate transaminase, and γ-glutamyl transferase) are commonly used as screening tests for liver disease, particularly in primary care, they frequently have normal results in individuals with NAFLD, including those with advanced disease. In a population-based study of 2287 individuals who underwent assessment of steatosis under 1H-MRS, 79% were found to have normal alanine transaminase test results.
There has been considerable interest in noninvasive scores to predict the presence of steatosis (Table II). These tests are widely used in epidemiologic studies and to some degree in clinical practice. They are positioned to predict the presence, rather than the severity, of steatosis and, with the exception of the Steatotest, provide a fairly simple and inexpensive way to identify patients at risk of NAFLD.
Table IINoninvasive serum markers of hepatic steatosis.
Sensitivity, 87%; specificity, 86%; AUC = 0.84; cutoffs: <30 for excluding, >60 for ruling in; externally validated against MRS (including small group T2D)
Hepatic Steatosis Index
8 × (ALT/AST) + BMI (+2 if T2D, + 2 if female)
•
Sensitivity, 93.1%; specificity, 92.4%; AUC = 0.81; cutoffs: <30 for excluding, >36 for ruling in
European Association for the Study of the Liver (EASL) European Association for the Study of Diabetes (EASD) & European Association for the Study of Obesity (EASO). EASL–EASD–EASO Clinical Practice Guidelines for the management of non-alcoholic fatty liver disease.
guidelines set out a clear pathway for risk stratification after diagnosis (Table I). The National Institute for Health and Care Excellence in the UK advises performing an enhanced liver fibrosis (ELF) test in all patients with NAFLD; those with a score of ≥10.51 should be investigated further with TE or acoustic radiation force impulse.
The European Association for the Study of the Liver advise using any 1 of a number of scores first line, including the fibrosis 4 (FIB-4) score, NAFLD fibrosis score (NFS), TE, or ELF, followed by specialist referral in the case of a high result for assessment and transient elastography.
European Association for the Study of the Liver (EASL) European Association for the Study of Diabetes (EASD) & European Association for the Study of Obesity (EASO). EASL–EASD–EASO Clinical Practice Guidelines for the management of non-alcoholic fatty liver disease.
ELF is not widely used in the US, but the American Association for the Study of Liver Disease 2018 guidelines support the use of NFS, FIB-4, TE, and magnetic resonance elastography for identifying a higher likelihood of F3/F4 fibrosis.
The Asia-Pacific guidelines advise that “non-invasive serum and physical tests afford modest but possibly acceptable accuracies when used to measure the fibrotic burden in patients with NAFLD.”
Iron-corrected T1 (cT1) is a multiparametric MRI biomarker for liver fibroinflammation. cT1 predicts clinical outcomes in patients with suspected liver disease,
With low measurement failure rates and high repeatability and reproducibility, this tool is increasingly being used in NASH clinical trials. In a pooled analysis of 722 participants from 4 NASH clinical trials, the high diagnostic accuracy of combined cT1 and fat (AUC 0.75) indicate that multiparametric MRI can accurately distinguish patients with high-risk NASH and moderate fibrosis.
When combining the imaging biomarkers cT1 and fat with the commonly tested blood analytes, aspartate transaminase and glucose (n = 682), the diagnostic accuracy improved further (AUC = 0.84). Further research exploring the use of these tools in lower disease prevalence settings (NCT03289897) and in combination with other biomarkers is warranted.
Elastography
TE
Liver fibrosis can be staged using TE (FibroScan, Echosens, Paris, France), which assesses liver stiffness by measuring the velocity of low-frequency (50 Hz) elastic shear waves propagated through underlying tissue; the stiffer the tissue, the faster the wave propagates.
Compared with liver biopsy, it is able to stage fibrosis with AUC values of 0.82, 0.85, 0.94, and 0.96 for stages F1, F2, F3, and F4 fibrosis, respectively.
Cutoffs vary slightly according to each study; in a large prospective cohort of 450 individuals from the United Kingdom, optimal cutoff values for F2, F3, and F4 fibrosis were 8.2, 9.7, and 13.6 kPa, respectively.
Accuracy of FibroScan controlled attenuation parameter and liver stiffness measurement in assessing steatosis and fibrosis in patients with nonalcoholic fatty liver disease.
In common with noninvasive serum markers of fibrosis, TE results can generate high negative predictive values, but it is not as effective at ruling in advanced fibrosis and cirrhosis, particularly in low-prevalence settings.
Accuracy of FibroScan controlled attenuation parameter and liver stiffness measurement in assessing steatosis and fibrosis in patients with nonalcoholic fatty liver disease.
Recently, there has been interest in combining the TE–derived liver stiffness measurement and CAP results with serum transaminases to create a TE-aspartate transaminase score to identify those individuals with NASH, an elevated NAFLD activity score, and significant fibrosis.
FibroScan-AST (FAST) score for the non-invasive identification of patients with non-alcoholic steatohepatitis with significant activity and fibrosis: a prospective derivation and global validation study.
Acoustic radiation force impulse uses a standard ultrasound probe to assess the speed of wave propagation and provide an indirect measurement of liver stiffness. This technique is moderately accurate at assessing severe fibrosis in NAFLD, but the results are influenced by the presence of steatosis and inflammation.
Acoustic radiation force impulse elastography for the non-invasive evaluation of hepatic fibrosis in non-alcoholic fatty liver disease patients: a systematic review & meta-analysis.
This MRI-based method for quantitatively imaging liver tissue has demonstrated high levels of accuracy at detecting liver fibrosis. AUC results reported from a recent meta-analysis of 12 studies involving 910 patients were 0.93, 0.93, and 0.95 for the detection of F2, F3, and F4 fibrosis, respectively.
Magnetic resonance elastography vs transient elastography in detection of fibrosis and noninvasive measurement of steatosis in patients with biopsy-proven nonalcoholic fatty liver disease.
NASH is a histologic diagnosis and signals progression of disease beyond simple steatosis and a step toward fibrosis. Investigators have targeted a variety of mechanistic pathways, including gene expression,
oxidative stress, lipidomic profiles, metabolic factors, and biomarkers of hepatocyte injury. Few of the biomarkers designed to detect steatohepatitis have been validated, however, particularly in prospective studies (Table III), and as a result none are in routine clinical use.
They mainly suffer from poor levels of sensitivity and large proportions of participants falling within an intermediate range. Their suboptimal performance is thought to be attributable to the dynamic nature of NASH, in that disease activity can vary over short periods of time and significant phenotypic variations may exist between individuals in terms of metabolic drivers, genetic influences, and fibrosis stage. These findings have led to considerable investment in this field in the form of the NIMBLE (Non-Invasive Biomarkers of Metabolic Liver Disease) and LITMUS (Liver Investigation: Testing Marker Utility in Steatohepatitis) consortia in the United States and Europe.
Table IIINoninvasive serum markers of NASH.
Biomarker/Components
Mechanism
Accuracy
ALT
Hepatocyte injury
•
Sensitivity, 64%; specificity, 75%; AUC = 0.62; suggested upper limit normal: 21 IU/L for men and 17 IU/L for women;
•
19% individuals with NASH have normal ALT
AST
Hepatocyte injury
•
Sensitivity, 77%; specificity, 62%
CK-18 fragments
M30: ELISA detects caspase-cleaved K18 fragments, marker of hepatocyte apoptosis; M65: ELISA detects total cell death
•
Original study: CK-18 > 250 U/L: sensitivity, 75%; specificity, 81%; AUC = 0.83; metanalysis: sensitivity, 66%; specificity, 82% (increase with optimal cutoffs; varies between studies);
levels correlate with weight loss post bariatric surgery; not available for clinical use
Activated PAI-1
Associated with reduced fibrinolysis (liver fibrosis and atherothrombosis)
•
Association with definite NASH vs non/borderline NASH (odds ratio = 1.20; 95% CI, 1.08–1.34); mean (SD) PAI-1 levels for no NAFLD 11.3 (5), steatosis 16.5 (10), NASH 25 (16) AU/mL (P < 0.001)
OxNASH <55: sensitivity, 81% to exclude NASH; OxNASH >73: specificity, 97% to detect NASH
FGF-21
Regulates energy balance, insulin sensitivity, and lipid metabolism and increases adiponectin levels104
•
FGF-21: sensitivity, 0.62; specificity, 0.78; FGF-21 and CK-18: sensitivity, 0.92; specificity, 0.85; AUC = 0.94 (0.92–0.96); not available for clinical use
Haptoglobin: fibrosis biomarkerα2-macroglobulin: inhibit matrix protein catabolism, regulate cholesterolApolipoprotein A1: main component HDLs (low levels in NASH and fibrosis)
•
Sensitivity, 33%; specificity, 94%; AUC = 0.79
HAIR
Metabolic factors, hepatocyte injury
•
2 or 3 of these predictors: sensitivity, 80%; specificity, 89%
The field of serum markers of fibrosis has made significant progress during the last 2 decades. Some of these scores combine clinical features with simple biochemical tests, whereas others use biological biomarkers of fibrosis. Scores have been well validated in prospective studies for the detection of advanced fibrosis in the setting of NAFLD (Table IV) and are widely used in clinical practice for risk stratification. In a meta-analysis of 64 studies, the FIB-4 score and NFS estimated the presence of advanced fibrosis with an AUC of 0.84 and had high negative predictive values (85%–93%) but relatively low positive predictive values (40%–73%).
Comparison of laboratory tests, ultrasound, or magnetic resonance elastography to detect fibrosis in patients with nonalcoholic fatty liver disease: a meta-analysis.
Similar to the ELF score, high positive predictive values can only be achieved in high prevalence settings, and meta-analysis data indicate that to achieve a specificity of 90% for advanced fibrosis a cut-off value of 10.18 is required.
Another major issue with these scores is that they categorize approximately one-third of people within an indeterminate range. Their performance is also less optimal in the setting of diabetes
Non-invasive risk scores do not reliably identify future cirrhosis or hepatocellular carcinoma in type 2 diabetes: the Edinburgh Type 2 Diabetes Study.
Liver biopsy is used as the reference standard for the diagnosis of NAFLD and the assessment of disease severity, including NASH and fibrosis (Figure 2). It is, however, an invasive procedure that is costly and associated with morbidity and mortality, with a frequent need for repeat sampling.
Liver biopsy is associated with several limitations; specifically, it is invasive and carries a risk of bleeding and death, there is a sampling error given that only 1 per 50,000 of the whole liver tissue is examined, and there is variability in interpretation and cost.
In clinical practice, the use of liver biopsy is not practical for large numbers of patients; however, it is still relied on in a number of circumstances. It should be used in cases of diagnostic uncertainty in terms of disease etiology because a coexistent condition, such as autoimmune hepatitis, would require a different course of treatment. It may also be used to confirm a diagnosis of advanced fibrosis or cirrhosis, given the relatively low positive predictive values inherent to TE and serum fibrosis markers, and the implications this diagnosis has for clinical trial enrollment, HCC, and varices screening. The US NAFLD guidelines advise that the presence of the metabolic syndrome should help target the need for liver biopsy because these individuals are at high risk of disease progression.
Finally, liver biopsy is used to provide standardized end points in clinical trials as discussed below and in some cases is used to assess trial eligibility.
Figure 2Histologic images showing the pathologic spectrum of nonalcoholic fatty liver disease (NAFLD). (A) Simple steatosis, (B and C) NASH, (D and E) fibrosis, and (F) cirrhosis. (A) Perivenular (zone 3) steatosis. (B) Portal tract (left side) and adjacent parenchyma showing steatosis and hepatocyte ballooning (centrally). A small sinusoidal aggregate of mononuclear inflammatory cells is also present (to the right of the asterisk). (C) Steatosis with ballooning and examples of 2 Mallory-Denk bodies (to the right of the asterisks). (D) Perisinusoidal pattern fibrosis, surrounding individual hepatocytes and cords of hepatocytes (blue). (E) Central vein (centrally) with perivenular fibrosis (blue). (F) Cirrhosis. Hematoxylin and eosin, original magnification × 400 (A–C). Masson's trichrome, original magnification × 400 (D and E), × 100 (F). CV = central vein.
Efforts have been made to standardize the diagnosis of steatosis using MRI-PDFF and NASH via biopsy using the National Institute of Health's NASH Clinical Research Network scoring system, named the NAFLD Activity Score (Table V).
The US Food and Drug Administration and European Medicines Agency independently dictate that clinical approval of new drugs requires trials with inclusion of patients with biopsy-proven NASH (NAFLD Activity Score ≥4) with stage 2 fibrosis or higher (ie, those at significant risk of progression to cirrhosis and decompensation).
Food and Health Administration. Non-cirrhotic non-alcoholic steatohepatitis with liver fibrosis: developing drugs for treatment.https://www.fda.gov/media/119044/download.
European Medicines Agency. Reflection paper on regulatory requirements for the development of medicinal products for chronic non-infectious liver diseases (PBC, PSC, NASH).
However, screen failure rates are high, with 73% of participants screened in the PIVENS (Pioglitazone, Vitamin E, or Placebo for Nonalcoholic Steatohepatitis) clinical trial not meeting the eligibility criteria.
Utility and appropriateness of the fatty liver inhibition of progression (FLIP) algorithm and steatosis, activity, and fibrosis (SAF) score in the evaluation of biopsies of nonalcoholic fatty liver disease.
NAFLD Activity Score (Sum of Steatosis Grade Plus Lobular Inflammation Plus Ballooning)
Steatosis grade
<5%5%–33% 33%–66% >66%
0123
Lobular inflammation
No foci <2 foci per 200 field 2–4 foci per 200 field >4 foci per 200 field
0123
Ballooning
None Few (or borderline) balloon cells Many cells or prominent ballooning
012
Fibrosis staging
None Perisinusoidal or periportalMild, zone 3 perisinusoidalModerate, zone 3 perisinusoidalPortal/periportal Perisinusoidal and portal/periportal Bridging fibrosis Cirrhosis
In terms of study end points, steatosis trials report relative and absolute reductions in liver fat as measured by MRI-PDFF or percentage of participants experiencing a greater than 30% reduction.
Histologic Improvement
Histologic improvement is the required end point for Phase III trials and is measured according to improvement in the NAFLD Activity Score, resolution of NASH with no worsening fibrosis, or an improvement in fibrosis stage with no worsening of NASH.
Food and Health Administration. Non-cirrhotic non-alcoholic steatohepatitis with liver fibrosis: developing drugs for treatment.https://www.fda.gov/media/119044/download.
European Medicines Agency. Reflection paper on regulatory requirements for the development of medicinal products for chronic non-infectious liver diseases (PBC, PSC, NASH).
These end points have been chosen with the belief that they will translate into reduced progression to cirrhosis and lower rates of decompensation, lesser requirement for liver transplantation, and improved survival, which would take much longer to measure. Full regulatory approval will be contingent on these benefits being found. Studies increasingly are mandated to report their extrahepatic effects, including serum lipid profiles and renal profile, considering cardiovascular disease is the leading cause of death for individuals with NAFLD. This mandate is particularly relevant given that therapies (including farsenoid X receptor agonists and acetyl-coenzyme A carboxylase inhibitors) increase serum LDL-C and triglyceride levels, respectively.
It is also essential to report all-cause mortality data.
Existing diagnostic scores currently used to risk stratify for fibrosis have not been successful in enhancing screen failure rates in clinical trials. Noninvasive diagnostic biomarkers that can be used to enrich the population or even supplant liver biopsy are appealing.
Conclusions
NAFLD affects a quarter of the population and progresses insidiously toward cirrhosis in a selected cohort. Considerable advances have been made in developing diagnostic techniques and algorithms, which can identify and risk stratify this population without the need for a liver biopsy. The balance of cost, availability, accuracy, and patient acceptability strongly influence their position in clinical practice and in research study protocols, and histology continues to play an important role in accurate initial staging and assessment of therapeutic response. This clinical field is rapidly evolving, however, driven by more sophisticated tiered approaches, advancements in technology, and our deeper understanding of the pathophysiology of this complex disease.
Acknowledgments
All authors made substantial contributions to the conception and development of this article and agreed on the final version.
DISCLOSURES
Dr Cuthbertson reports receiving grants and personal fees from investigator-initiated studies funded by Novo Nordisk and AstraZeneca in the field of glucagon-like peptide 1 receptor agonist and sodium-glucose transport protein 2 inhibitors. Dr Cuthbertson has acted as a consultant and given lectures on behalf of pharmaceutical companies developing or marketing medicines used for the treatment of diabetes and obesity, specifically Astra Zeneca, Novo Nordisk, Boerhinger, Pfizer, Lilly, Ipsen, and Sanofi. The authors have indicated that they have no other conflicts of interest regarding the content of this article.
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European Association for the Study of the Liver (EASL)
European Association for the Study of Diabetes (EASD) & European Association for the Study of Obesity (EASO). EASL–EASD–EASO Clinical Practice Guidelines for the management of non-alcoholic fatty liver disease.
Utility and appropriateness of the fatty liver inhibition of progression (FLIP) algorithm and steatosis, activity, and fibrosis (SAF) score in the evaluation of biopsies of nonalcoholic fatty liver disease.
Diagnostic accuracy of controlled attenuation parameter (CAP) as a non-invasive test for steatosis in suspected non-alcoholic fatty liver disease: a systematic review and meta-analysis.
Accuracy of FibroScan controlled attenuation parameter and liver stiffness measurement in assessing steatosis and fibrosis in patients with nonalcoholic fatty liver disease.
Magnetic resonance elastography vs transient elastography in detection of fibrosis and noninvasive measurement of steatosis in patients with biopsy-proven nonalcoholic fatty liver disease.
FibroScan-AST (FAST) score for the non-invasive identification of patients with non-alcoholic steatohepatitis with significant activity and fibrosis: a prospective derivation and global validation study.
Acoustic radiation force impulse elastography for the non-invasive evaluation of hepatic fibrosis in non-alcoholic fatty liver disease patients: a systematic review & meta-analysis.
Comparison of laboratory tests, ultrasound, or magnetic resonance elastography to detect fibrosis in patients with nonalcoholic fatty liver disease: a meta-analysis.
Non-invasive risk scores do not reliably identify future cirrhosis or hepatocellular carcinoma in type 2 diabetes: the Edinburgh Type 2 Diabetes Study.
Food and Health Administration. Non-cirrhotic non-alcoholic steatohepatitis with liver fibrosis: developing drugs for treatment.https://www.fda.gov/media/119044/download.
European Medicines Agency. Reflection paper on regulatory requirements for the development of medicinal products for chronic non-infectious liver diseases (PBC, PSC, NASH).
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