|Year : 2021 | Volume
| Issue : 4 | Page : 270-274
Noninvasive assessment of nonalcoholic fatty liver disease
Gajanan A Rodge, Mahesh Kumar Goenka
Institute of Gastrosciences and Liver, Apollo Multispeciality Hospitals, Kolkata, West Bengal, India
|Date of Submission||23-Oct-2021|
|Date of Acceptance||11-Nov-2021|
|Date of Web Publication||23-Dec-2021|
Mahesh Kumar Goenka
Director & Head of Department, Institute of Gastrosciences and Liver, Apollo Multispeciality Hospitals, Kolkata, West Bengal
Source of Support: None, Conflict of Interest: None
Nonalcoholic fatty liver disease (NAFLD) is ongoing pandemic of the 21st century. The noninvasive assessment includes serum biomarkers, predictive models, and imaging modalities. The “Steato test,” “NAFLD liver fat score,” and “Fatty liver index” are models developed for noninvasive assessment of hepatic steatosis. Out of the imaging modalities, magnetic resonance imaging proton density fat fraction is the most sensitive test to detect hepatic steatosis. Out of the various serum biomarkers to detect nonalcoholic steatohepatitis (NASH), cytokeratin-18 has been the most widely investigated. Recent approach for the diagnosis of NASH has targeted research toward genetic biomarkers such as PNPLA3 and microRNAs. However, none of the presently available biomarkers or imaging modalities are able to differentiate simple hepatic steatosis from NASH with a high sensitivity and specificity. Different models have been developed to predict fibrosis which are aspartate transaminase (AST)/platelet ratio index (APRI), fibrosis-4 (Fib-4) index, nonalcoholic fatty liver disease fibrosis (NFS) score and body mass index, AST: Alanine transaminase Ratio, diabetes (BARD) score. The accuracy of BARD, APRI, FIB-4, and NFS to detect advanced liver fibrosis was found to be of 0.76, 0.77, 0.84, and 0.84, respectively, in a large meta-analysis. Transient elastography, acoustic radiation force impulse, and magnetic resonance elastography (MRE) are imaging techniques available to detect liver fibrosis. MRE has shown to have a pooled accuracy 0.96 to detect advanced fibrosis in NAFLD patients. Noninvasive tests may not completely replace liver biopsy, but it may help to avoid it where probability of fibrosis is low.
Keywords: NAFLD, hepatic steatosis, liver fibrosis, noninvasive assessment
|How to cite this article:|
Rodge GA, Goenka MK. Noninvasive assessment of nonalcoholic fatty liver disease. Apollo Med 2021;18:270-4
| Introduction|| |
Nonalcoholic fatty liver disease (NAFLD) is the ongoing pandemic of the 21st century. Every one in three individuals is affected by NAFLD. The global prevalence of NAFLD varies from 21% in North America to 31.7% in Middle East. Indian studies have shown the prevalence of NAFLD from 16.6% to 32% in different parts of the country.,, NAFLD includes a wide disease spectrum of disease [Table 1]. It ranges from isolated hepatic steatosis to nonalcoholic steatohepatitis (NASH) involving active liver inflammation, liver cirrhosis, and finally, hepatocellular carcinoma (HCC).
The key diagnostic challenges in diagnosing NAFLD are to accurately detect NASH, quantify the degree of fibrosis, and identify the individuals who are at increased risk of liver-related morbidity and mortality. The stage of fibrosis is very important to prognosticate NAFLD patients as the mortality increases exponentially from stage 0 of liver fibrosis to stage 4 of liver fibrosis. Early prediction of the stage of fibrosis and targeted pharmacotherapy can help to reverse fibrosis and significantly reduce the mortality in individuals with advanced stage of fibrosis. The historical gold standard for confirming the diagnosis of NAFLD and particularly NASH has been liver biopsy. However, liver biopsy also has many limitations as sampling error, reproducibility of assessment among different pathologists, and procedure-related complications like bleeding. This has led to the need and development for noninvasive assessment of liver fibrosis.
Several noninvasive tools have been developed or are being investigated for stratifying patients at risk for NAFLD. This article reviews the available noninvasive methods for assessment of fatty liver disease. The noninvasive assessment includes serum biomarkers, predictive models, and scores which include combination of various tests or different imaging modalities.
Noninvasive assessment of steatosis
All patients who have hepatic steatosis are not NAFL, as there are many other diseases which lead to deposition of fat in liver. Alcohol is one of the common causes of hepatic steatosis, and significant history of alcohol intake needs to be excluded before diagnosis of NAFLD. The noninvasive assessment of hepatic steatosis includes clinical, biochemical, and imaging modalities. Identifying patients with the features of metabolic syndromes such as obesity, hypertension, diabetes mellitus, and dyslipidemia is the key to diagnose patients at risk for NAFLD. However, many patients with NAFLD are asymptomatic and these patients are diagnosed as an incidental finding of elevated liver enzymes or hepatic steatosis on routine ultrasound (US) imaging. The abnormal liver enzymes are usually mildly elevated and around 80% of the patients have normal alanine transaminase (ALT) levels.
Different scores have been developed to detect hepatic steatosis. The components used for calculation of the scores include clinical and biochemical parameters as shown in Table 2. The “Steato test” has a high cut-off of >0.72 for the detection of steatosis with a sensitivity and specificity of 90% each. The other test “NAFLD liver fat score” has a high cut off of >1.257 with a higher sensitivity and specificity (95%). The drawback of these tests are complicated formula's and many of the laboratory tests included are not routinely available. The “Fatty liver index” is easier to calculate, however, the sensitivity and specificity are much lower compared to the “Steato test” and “NAFLD liver fat score.“
The imaging tests are better for the diagnosis of steatosis. US abdomen is the commonly used test in clinical setting as it is inexpensive and widely available. The test is operator dependent and difficult to perform in obese individuals where the suspicion of fatty liver disease is high. The sensitivity is between 60% and 94% and specificity ranges from 66% to 97% for detection of hepatic steatosis.,, Computed tomography (CT) has also been used for detection of hepatic steatosis and is only better that US in the detection of focal hepatic steatosis. The measures used to determine hepatic steatosis of CT scan included the attenuation of liver parenchyma, liver to spleen attenuation difference, and liver to spleen attenuation ratio. It is best detected on a noncontrast CT.
Controlled attenuation parameter (CAP) is a novel method that uses the transient elastography (TE) probe (FibroScan) to detect signal attenuation in the liver. In a recent meta-analysis of 19 studies, CAP detected hepatic steatosis of ≥11%, ≥33%, and ≥66% with a sensitivity of 0.69, 0.77, and 0.88 and specificity of 0.82, 0.81, and 0.78, respectively. The optimal cut-offs to detect steatosis of ≥11%, ≥33%, and ≥66% were found to be of 248 dB/m (range of 237–261 dB/m), 268 dB/m (range of 257–284 dB/m), and 280 dB/m (range of 268–294 dB/m), respectively. The CAP values are influenced by the presence of diabetes and body mass index (BMI) of patient.
Magnetic resonance imaging (MRI) uses the water and fat proton signals from the liver and calculates the signal fat fraction, i.e., the mobile protons which are attributable to the liver fat. MRI is superior to US in determining as well as quantifying minor hepatic steatosis and can detect even 3% of steatosis. The four MR based diagnostic methods for liver fat quantification are as follows:
- Dixon MRI technique
- Modified Dixon type (mDixon)
- Single proton - MR spectroscopy (MRS)
- Proton density fat fraction (PDFF).
The MRS is very sensitive in detecting small amounts of triglycerides which may also not be seen on histological examination. The MRS is, however, time consuming to perform and interpret and samples only a small portion of the liver. MRI PDFF is a rapid MRI technique which can estimate hepatic PDFF across the any segment of liver and provides more accurate quantification of hepatic fat content compared with MRS. Patients with advanced steatosis (grade II and grade III) at MRI-PDFF are found to have a greater chance of abnormal liver function tests as compared to those with Grades 0 and I hepatic steatosis. It was also seen that there was a low level of agreement between MRI-PDFF and USG for grading of hepatic steatosis. [Figure 1] shows the hepatic content in superior and inferior aspect of the liver. Nine values corresponding to nine segments of the liver are measured and an average value is calculated. Although MRI PDFF is not widely available at present, it seems to be a promising option to detect hepatic steatosis in the near future.
|Figure 1: (a) Hepatic steatosis in superior part of liver corresponding to different segments of liver (b) Hepatic steatosis in inferior part of liver corresponding to different segments of liver|
Click here to view
Noninvasive assessment of steatohepatitis
Diagnosis of NASH without any invasive test has remained a challenge for years. A number of serum biomarkers have been investigated for noninvasive diagnosis of NASH. Out of the various serum biomarkers, cytokeratin (CK)-18 has been the most widely investigated. CK-18 is produced from the necrosis of hepatocytes and can be measured in the blood by different immunoassays. CK-18 has shown to have an accuracy of 0.82 to predict NASH while the sensitivity and specificity were 66–78% and 82–87%, respectively., However, CK-18 is not commercially available, shown to have considerable variability in the cut-off values in different studies, and has limited sensitivity at individual level. These limitations have reduced the use of CK 18 in clinical practice. CK-18 has also been used in combination with other biomarkers to develop various predictive models. Some of the models which have been developed for predicting NASH and the components are listed below:
- HAIR model - Hypertension, ALT increased, and insulin resistance
- NashTest - age, sex, height, weight, triglycerides, cholesterol, alpha 2 macroglobulin, apolipoprotein A1, haptoglobin, gamma-glutamyltransferase, ALT, aspartate transaminase (AST), and total bilirubin
- Nash diagnostics - CK-18, adiponectin, resistin
- NICE model - Metabolic syndrome, ALT, CK-18.
Recent approach for the diagnosis of NASH has targeted research toward genetic biomarkers like single nucleotide polymorphisms located in PNPLA3 and its inclusion in NASH score (PNPLA3 genotype, AST, and fasting insulin levels). Expression of noncoding RNAs, particularly microRNAs, such as miR-122 have also been found to be useful as a biomarker for NASH. However, none of the presently available biomarkers are able to differentiate simple hepatic steatosis from NASH with a high sensitivity and specificity. The available imaging modalities cannot reliably differentiate NASH from simple steatosis. The MR-based modalities have assessed accumulation of iron in the liver and found to have an accuracy of 0.91 for diagnosing NASH, with a sensitivity and specificity of 83% and 80%, respectively. The MR modalities show a promising option but need independent confirmation from larger trials.
Noninvasive assessment of fibrosis
The presence and severity of fibrosis are one of the most crucial factors in a particular patient for prognostication. Furthermore, detecting cirrhosis at an early stage before decompensation helps to include the patient in screening programs for HCC and portal hypertension. According to Brunt's classification, F2 is considered as significant fibrosis while F3 or F4 as advanced fibrosis. The serum biomarkers like aminotransferase levels may not always be helpful to detect fibrosis since they are known to show a decreasing trend with histological improvement of steatosis and inflammation, even if the stages of fibrosis advances. The AST/ALT ratio is of more significance, and a ratio >1 is suggestive of advanced fibrosis. Hyaluronic acid (HA) levels are elevated when collagen synthesis is increased and, in advanced liver fibrosis, the dysfunction of sinusoidal endothelium reduces its clearance. It has been shown to predict advanced liver fibrosis with accuracy of 0.75–0.9 in studies with small sample size.
Different models have been developed to predict fibrosis with good accuracy for advanced fibrosis. The AST/platelet ratio index (APRI), fibrosis-4 (Fib-4) index, Nonalcoholic fatty liver disease fibrosis (NFS) score and BMI, AST:ALT ratio, diabetes (BARD) score are some of the scores used to predict fibrosis. A recent large meta-analysis including 64 studies and 13,046 patients with NAFLD compared the different models to detect advanced liver fibrosis. The accuracy of BARD, APRI, FIB-4, and NFS was found to be of 0.76, 0.77, 0.84, and 0.84, respectively, in the meta-analysis. The Hepamet Fibrosis Score (HFS) is another recently developed tool which includes variables such as age, diabetes mellitus, AST, ALT, platelet, albumin, and sex to detect liver fibrosis. The HFS showed to identify patients with advanced fibrosis accuracy of 0.85 which was greater than the FIB-4 and NFS systems.
TE (FibroScan) evaluates the liver stiffness using US-based pulse-echo system. It has shown excellent result in studies which were mainly performed in patient with viral hepatitis with a reported accuracy of 88%–89% and 93%–96% to detect fibrosis and cirrhosis, respectively. Two studies on patients with NAFLD have also shown similar results., Acoustic radiation force impulse (ARFI) is a similar type of US-based modality which uses point shear wave elastography to detect liver stiffness. The reported accuracy for ARFI was of 0.90 to detect significant fibrosis in a systematic review of seven studies with a total of 723 diagnosed NAFLD patients.
Magnetic resonance elastography (MRE) is a novel method to detect advanced liver fibrosis and it allows evaluation of the complete liver. MRE has now been the preferred modality in clinical trial to detect improvement in liver fibrosis where invasive tests like liver biopsy are to be avoided. It has shown to be highly accurate to detect advanced liver fibrosis. The accuracy was 0.84 to detect any amount of liver fibrosis and 0.92 to detect advanced fibrosis in a prospective study. In a recent meta-analysis, the pooled accuracy of MRE to detect advanced fibrosis in patient with NAFLD was found to be 0.96. The different imaging tests to detect hepatic steatosis and fibrosis have been summarized in [Figure 2].
|Figure 2: Imaging kit for NAFLD: NAFLD: Nonalcoholic fatty liver disease; CT: Computed tomography; CAP: Controlled attenuation parameter; MRI: Magnetic resonance imaging proton density fat fraction; TE: Transient elastography; ARFI: Acoustic radiation force impulse; MRE: Magnetic resonance elastography|
Click here to view
| Conclusion|| |
Patients who have risk factors such as obesity, diabetes mellitus, abnormal liver enzymes, or steatosis on US should be suspected to have NAFLD. The noninvasive tests are helpful in these patients to stratify them into high risk and low risk. The higher liver fat on imaging is associated with increased probability of progression to fibrosis. Serum biomarkers or imaging modality, both are primarily used to exclude advanced liver fibrosis. Noninvasive tests may not completely replace liver biopsy, but it may help to avoid it in a large proportion of individuals where the probability of liver fibrosis is low. Patients with significant or advanced fibrosis on noninvasive assessment may still require further invasive tests such as liver biopsy for confirmation.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2]
[Table 1], [Table 2]