Abdominal Ultrasound for Fatty Liver Disease
Abdominal ultrasonography is the primary and most appropriate initial diagnostic test for suspected fatty liver disease. 1
Why Ultrasound is First-Line
The Korean Association for the Study of the Liver (KASL) 2021 guidelines explicitly designate abdominal ultrasonography as the primary screening modality for NAFLD (Grade B1 recommendation). 1 This recommendation is based on several practical advantages:
- Robust diagnostic performance for moderate to severe steatosis (sensitivity 83.3%, specificity 81.6% when steatosis exceeds 30%) 1
- Comprehensive evaluation of the entire hepatobiliary system beyond just detecting steatosis 1
- Wide availability and cost-effectiveness compared to CT or MRI 1
- Non-invasive with no radiation exposure 1
Important Limitations to Recognize
Ultrasound has critical weaknesses you must understand:
- Low sensitivity for mild steatosis (less than 30% fat infiltration) - this is a major pitfall 1
- Subjective interpretation with operator-dependent variability 1
- Technical difficulties in obese patients - ironically, the population most likely to have NAFLD 1
- Cannot distinguish NASH from simple steatosis - this is crucial since prognosis differs dramatically 1
- Cannot assess fibrosis stage - the single most important prognostic factor 1
When to Move Directly to Risk Stratification
In patients with high pretest probability (diabetes, metabolic syndrome, obesity), the 2021 Gastroenterology clinical care pathway suggests moving directly to noninvasive fibrosis testing without requiring ultrasound confirmation of steatosis. 1 This approach recognizes that:
- The pretest probability is already high enough to warrant evaluation 1
- Ultrasound may miss mild steatosis anyway 1
- Fibrosis stage, not steatosis presence, determines mortality risk 2
Complete Initial Workup Algorithm
When ultrasound detects fatty liver or clinical suspicion is high:
Exclude other liver diseases through detailed alcohol history (>21 drinks/week men, >14 drinks/week women excludes NAFLD diagnosis) 3
Obtain comprehensive laboratory panel: 2, 3
- Liver chemistry panel (ALT, AST, alkaline phosphatase, bilirubin, albumin, INR)
- Complete blood count with platelets
- Hepatitis B surface antigen and hepatitis C antibody
- Fasting glucose or HbA1c
- Fasting lipid profile
- Iron studies (ferritin, transferrin saturation)
Calculate FIB-4 score immediately as first-line fibrosis assessment (age, platelets, AST, ALT) 1, 2
- FIB-4 <1.3 (<2.0 if age >65): Low risk, recheck in 2-3 years
- FIB-4 >2.67: High risk for advanced fibrosis, proceed to elastography or hepatology referral
Critical Pitfall: Normal Liver Enzymes
Do not rely on normal ALT/AST to exclude NAFLD or NASH - this is the most common diagnostic error. 2, 3 Up to 50% of NAFLD patients have completely normal transaminases, and you can still miss 80% of NASH cases if you screen by elevated enzymes alone. 2 The American Association for the Study of Liver Diseases emphasizes that normal liver chemistries do not exclude significant disease. 2, 3
Alternative and Adjunctive Imaging
When ultrasound is inadequate or more precision is needed:
- Controlled Attenuation Parameter (CAP) via transient elastography quantifies steatosis degree (cutoff 276 dB/m for moderate-severe steatosis in Korean studies) 1
- MRI/MR spectroscopy provides superior accuracy for mild steatosis but is expensive and not needed for routine screening 1
- CT has high specificity but low sensitivity and involves radiation exposure 1
Who Should Be Screened
Target these high-risk populations for ultrasound screening: 1, 3
- Persistent liver enzyme elevation (Grade A1 recommendation)
- All patients with diabetes regardless of liver enzyme levels (Grade A1 recommendation) - cost-effectiveness confirmed
- Metabolic syndrome (Grade B1 recommendation)
- Obesity (Grade B1 recommendation)