CT Attenuation Criteria for Diagnosing Hepatic Steatosis

Diagnostic Thresholds on Unenhanced CT (American College of Radiology 2023)

• A liver‑to‑spleen attenuation difference > 10 HU (liver lower) diagnoses moderate‑to‑severe steatosis with 100 % specificity and 82 % sensitivity. 1
• An attenuation ratio < 1.0 (liver ÷ spleen) confirms moderate‑to‑severe steatosis. 2
• An absolute liver attenuation < 40 HU correlates with ≥30 % fat content (moderate steatosis). 3

Quantitative Attenuation Values Corresponding to Fat Fraction (American College of Radiology)

*Values derived from ACR guideline tables. 1

Clinical Significance of a 30 HU Liver‑Spleen Difference (American College of Radiology)

• A 30 HU lower liver attenuation compared with the spleen on unenhanced CT far exceeds the 10 HU diagnostic threshold, making the diagnosis of moderate‑to‑severe hepatic steatosis essentially certain. 2

Applicability of the Thresholds

Unenhanced (Non‑Contrast) CT Only (American College of Radiology)

• The above thresholds are valid exclusively on unenhanced CT scans; contrast administration alters HU values and invalidates the criteria. 3

Contrast‑Enhanced CT (American College of Radiology)

• HU measurements are unreliable for steatosis quantification on contrast‑enhanced studies due to variability in timing, injection site, protocol, and patient hemodynamics. 1

Recommended Clinical Work‑up When Moderate‑to‑Severe Steatosis Is Identified on Unenhanced CT (Praxis Medical Insights)

• Obtain comprehensive liver biochemistry (ALT, AST, GGT, alkaline phosphatase, bilirubin, albumin, AST:ALT ratio) to assess hepatic injury and rule out alternative etiologies. 4
• Screen for metabolic‑syndrome components (BMI, type 2 diabetes, hypertension, dyslipidemia, alcohol use) as these are major drivers of NAFLD. 4
• Calculate non‑invasive fibrosis scores (FIB‑4 index or NAFLD Fibrosis Score) to stratify risk of advanced fibrosis. 4
• For patients with intermediate or high fibrosis risk, obtain transient elastography with CAP to simultaneously evaluate steatosis severity and liver stiffness. 4
• Aggressively manage metabolic risk factors (optimize glycemic control, blood pressure, lipid profile) to halt disease progression. 4
• Refer to hepatology when fibrosis scores indicate intermediate/high risk or when ≥ 2 metabolic risk factors are present. 4

Alternative Evaluation When Only Contrast‑Enhanced CT Is Available (Praxis Medical Insights)

• Assess for acute hepatic injury patterns (heterogeneous arterial enhancement, peri‑portal hypoattenuation, diffuse hypo‑enhancement) to exclude ischemic hepatitis, acute hepatitis, or cholestatic disease. 5
• If steatosis remains clinically suspected, perform an unenhanced CT or non‑contrast MRI for accurate fat quantification. 1