What is FibroScan, how does transient elastography assess liver fibrosis and controlled attenuation parameter assess steatosis, and what are the typical kilopascal and dB/m cut‑offs for fibrosis stages and steatosis grades?

FibroScan: Technology, Methodology, and Clinical Interpretation

What FibroScan Is and How It Works

FibroScan (vibration-controlled transient elastography, VCTE) is a non-invasive ultrasound-based device that measures liver stiffness to assess fibrosis and simultaneously quantifies hepatic fat content through the Controlled Attenuation Parameter (CAP). 1

Transient Elastography Mechanism for Fibrosis Assessment

• The probe generates low-frequency elastic shear waves that propagate through the liver tissue between the ribs, and the device measures the velocity of these waves as they travel through the hepatic parenchyma. 1

• Liver stiffness is calculated using Hooke's law, where tissue stiffness is proportional to the square of the shear wave propagation speed—faster wave velocity indicates stiffer (more fibrotic) liver tissue. 1

• Results are expressed in kilopascals (kPa), with a measurement range of 1.5–75 kPa and an upper limit of normal approximately 5.0–5.5 kPa. 1

• The device examines a liver volume more than 100 times larger than a standard liver biopsy specimen, reducing sampling error. 1

Controlled Attenuation Parameter (CAP) for Steatosis Assessment

• CAP utilizes the same ultrasound signals from transient elastography to measure ultrasound attenuation caused by hepatic fat, providing simultaneous assessment of steatosis during the same examination. 1

• CAP values are expressed in decibels per meter (dB/m) and correlate with the percentage of hepatic steatosis on histology. 2

Technical Procedure and Validity Requirements

Patient Preparation

• Patients must fast for at least 4 hours before examination, as food intake increases hepatic blood flow and falsely elevates liver stiffness measurements. 1, 3, 4

• The patient lies supine with the right arm raised in maximal abduction to widen the intercostal spaces, and the probe is positioned perpendicular to the skin over the right lobe of the liver through the intercostal space. 1, 3

Measurement Protocol

• At least 10 valid measurements must be obtained, with the device automatically calculating the median value. 1, 3, 4

• A technically valid result requires: ≥10 successful measurements, success rate ≥60%, and interquartile range (IQR) <30% of the median value. 1, 3, 4

• High IQR values (≥30%) indicate unreliable measurements that should not be used for clinical decision-making, commonly caused by patient movement, inadequate fasting, obesity, narrow intercostal spaces, or operator inexperience. 3, 4

Kilopascal Cut-offs for Fibrosis Stages

General Chronic Liver Disease

• Normal liver stiffness: <5.0–5.5 kPa 1

• Significant fibrosis (≥F2): 7.1–8.8 kPa (AUROC 0.79–0.83) 4, 5

• Severe fibrosis (F3): 9.5–12.5 kPa (AUROC 0.90) 5

• Cirrhosis (F4): 12.5–17.6 kPa (AUROC 0.95–0.98, with positive and negative predictive values of 90% at 17.6 kPa) 4, 5

NAFLD/MASLD-Specific Thresholds

• <8.0 kPa: Rules out advanced fibrosis with 93% sensitivity; indicates F0-F1 (no to minimal fibrosis) 3, 4

• 8.0–12.0 kPa: Indeterminate risk; indicates progressive fibrosis requiring hepatology referral 3, 4

• >12.0 kPa: High risk for advanced fibrosis/cirrhosis; requires urgent hepatology referral and consideration of liver biopsy or MR elastography 3, 4

Alcohol-Related Liver Disease

• <8.0 kPa: Reliably rules out advanced fibrosis 3

Portal Hypertension Assessment

• >20–25 kPa: Diagnoses clinically significant portal hypertension (CSPH) with AUROC 0.93, indicating need for variceal screening 3

• ≥21 kPa: Predicts clinical decompensation events with accuracy equivalent to hepatic venous pressure gradient ≥10 mmHg 3

Specific Complications (Cut-offs with NPV >90%)

• Esophageal varices grade 2/3: 27.5 kPa 5
• Child-Pugh B or C cirrhosis: 37.5 kPa 5
• History of ascites: 49.1 kPa 5
• Hepatocellular carcinoma: 53.7 kPa 5
• Esophageal bleeding: 62.7 kPa 5

Controlled Attenuation Parameter (CAP) Cut-offs for Steatosis Grades

Normal and Steatosis Thresholds

• Normal CAP range: 156–287 dB/m 6

• Any steatosis (S≥S1): ≥248–302 dB/m 3

Steatosis Severity Grades

• Mild steatosis (S1, 5–33% hepatic fat): 250 dB/m (AUROC 0.79) 6, 2

• Moderate steatosis (S2, 34–66% hepatic fat): 299 dB/m (AUROC 0.76) 6, 2

• Severe steatosis (S3, >66% hepatic fat): 327 dB/m (AUROC 0.70) 6, 2

• At a cut-off of 283 dB/m for significant steatosis (≥10%), CAP demonstrates 76% sensitivity, 79% specificity, 87% positive predictive value, and 64% negative predictive value. 2

Advantages and Clinical Utility

• FibroScan is painless, non-invasive, rapid (5–10 minutes), provides immediate results, and has excellent reproducibility with minimal learning curve for operators. 1, 7

• The technique has excellent diagnostic performance for liver fibrosis across chronic liver diseases of various etiologies, with particular strength in detecting cirrhosis (AUROC 0.96). 1, 5, 7

• Serial measurements can track disease progression or regression in response to treatment, making it valuable for longitudinal monitoring. 1, 3, 7

Important Limitations and Confounding Factors

Technical Limitations

• FibroScan cannot be reliably performed in patients with ascites (elastic waves cannot reach liver parenchyma), narrow intercostal spaces (probe positioning difficulty), or obesity (BMI >28 kg/m²), though an XL probe is available for obese patients. 1, 3, 4

• Failure rates are 1.1–3.5% in Asian populations and 4.3–10.5% in Western populations, primarily due to differences in body habitus. 1, 3

• The examination is not recommended during pregnancy due to changes in liver position. 1

Clinical Confounders That Falsely Elevate Liver Stiffness

• Acute hepatitis with severe inflammation and necrosis causes falsely high readings despite mild fibrosis. 4, 6

• Elevated ALT levels significantly reduce diagnostic accuracy for cirrhosis. 4

• Extrahepatic cholestasis, passive hepatic congestion from right heart failure, and hepatic edema independently elevate liver stiffness. 3, 4, 6

• Recent alcohol consumption falsely elevates measurements; ideally perform after 2 weeks of abstinence. 1

Diagnostic Limitations

• FibroScan measures stiffness only and cannot diagnose the etiology of liver disease or distinguish between different causes of elevated stiffness. 3, 4

• CAP performance is higher in patients with mild (F0-F1) fibrosis (AUROC 0.89) compared to advanced fibrosis F2-F4 (AUROC 0.72). 2

• Ultrasound and CAP cannot distinguish simple steatosis from steatohepatitis or identify inflammation. 6

Common Clinical Pitfalls to Avoid

• The most common error is performing FibroScan without adequate fasting, leading to overestimation of fibrosis stage and inappropriate escalation of care, particularly near clinical decision thresholds (8 kPa, 10–15 kPa). 3

• Do not rely on FibroScan alone to rule out other causes of liver disease—it only measures stiffness and cannot diagnose alternative etiologies. 3, 4

• Results failing IQR criteria (<30% of median) should be documented as "technically inadequate" rather than reporting an unreliable numerical value. 3

• In obese patients, use the XL probe to improve success rates and reduce IQR. 3

• Consider alternative assessment methods (MR elastography or liver biopsy) when IQR criteria are not met or when clinical context suggests confounding factors. 3, 4