How is fatty liver graded on Ultrasound (USG)?

Grading of Fatty Liver on Ultrasound (USG)

Fatty liver on ultrasound is graded into three categories: mild, moderate, and severe, based on specific sonographic features related to liver echogenicity and visualization of intrahepatic structures. 1

Ultrasound Grading System

Grade 1 (Mild Steatosis)

• Mild diffuse increase in liver echogenicity
• Clear visualization of diaphragm and intrahepatic vessel walls
• Minimal to no attenuation of ultrasound beam 1

Grade 2 (Moderate Steatosis)

• Moderate increase in liver echogenicity
• Obscuration of the diaphragm
• Partial obscuration of intrahepatic vessel walls
• Moderate attenuation of ultrasound beam 1

Grade 3 (Severe Steatosis)

• Marked increase in liver echogenicity
• Non-visualization of the diaphragm
• Non-visualization of intrahepatic vessel walls
• Marked attenuation of ultrasound beam 1

Diagnostic Accuracy and Limitations

Ultrasound has excellent specificity (93.6%) for detecting moderate to severe steatosis (>30% fat infiltration), but significantly lower sensitivity (53.3-65%) for mild steatosis 1. This creates several important clinical considerations:

• False negatives are common with mild steatosis - 25% of patients with histologically proven fat infiltration may have normal ultrasound findings 2
• False positives occur frequently - echogenic abnormalities may result from fibrosis or inflammation rather than fat 2
• Observer variability is substantial - interobserver agreement for fatty liver presence is approximately 72% (kappa = 0.43) and intraobserver agreement is about 76% (kappa = 0.54) 3

Correlation with Other Diagnostic Methods

When compared to liver biopsy (gold standard), ultrasound grading shows:

• Good overall correlation (Spearman's correlation 0.80) 4
• Limited accuracy for individual grade prediction - many patients with mild steatosis on ultrasound have moderate steatosis histologically 4
• All patients with severe steatosis on histology have at least moderate-to-severe steatosis on ultrasound 4

When compared to MRI:

• Moderate correlation between ultrasound score and MRI-determined hepatic fat fraction (r = 0.68) 4
• High false positive rate with mild steatosis - 72% of patients with mild steatosis on ultrasound had negative MRI findings 4
• Wide range of actual fat content in moderate steatosis - MRI-determined hepatic fat fractions ranging from 1-39% 4

Clinical Implications

• Body Mass Index (BMI) and triglyceride levels show statistically significant associations with ultrasound grading of fatty liver 5
• Ultrasonic elastography can provide quantitative assessment of fatty liver severity with high sensitivity (97.14%) and specificity (91.11%) 6
• Presence of nodular liver surface, decreased right lobe-caudate lobe ratio, or signs of portal hypertension on ultrasound suggests progression to cirrhosis rather than simple steatosis 1

Pitfalls and Caveats

1. Avoid over-reliance on ultrasound alone for diagnosis of mild steatosis due to high false negative rates
2. Be cautious when interpreting echogenic liver in patients with chronic liver disease - abnormalities may represent fibrosis/inflammation rather than fat 2
3. Consider the significant inter- and intra-observer variability when following patients longitudinally 3
4. Use additional diagnostic tools (MRI, elastography, or biopsy) when accurate quantification of hepatic fat is clinically important 4, 1