Does obesity affect Fibroscan (Fibrosis Scan) and SWE (Shear Wave Elastography) results, potentially causing falsely low values and varices on endoscopy?

Does Obesity Affect Fibroscan and SWE Accuracy?

Yes, obesity significantly impairs Fibroscan (transient elastography) reliability and can produce falsely elevated—not falsely low—liver stiffness values, though modern XL probes and alternative techniques like 2D-SWE substantially mitigate these limitations. 1

Impact of Obesity on Fibroscan Measurements

Technical Failure Rates

• Standard M probe failure rates increase dramatically in obese patients, reaching 16% compared to 1.1% with the XL probe 1, 2
• Unreliable results occur in 50% of obese patients using the M probe versus 25% with the XL probe 1
• The XL probe (2.5 MHz transducer) measuring at 35-75 mm depth reduces failure rates to <5% in patients with BMI >28 kg/m² 1
• Skin-to-capsule distance >22.5 mm significantly reduces M probe reliability 3

Direction of Measurement Error

Contrary to producing falsely low values, obesity actually causes falsely elevated liver stiffness measurements 1, 4:

• Obese patients demonstrate higher LSM results even at the same fibrosis stage 5
• BMI independently correlates with increased LSM values (p<0.02) 4
• Steatosis itself may increase liver stiffness measurements independent of fibrosis 1, 4

Probe-Specific Considerations

• XL probe measurements are systematically lower than M probe by approximately 1.2-1.4 kPa 1, 5, 2
• Recent evidence suggests using the same LSM cutoffs for M probe in non-obese and XL probe in obese patients 1
• The optimal cutoff for significant fibrosis (≥F2) in morbidly obese patients (BMI ≥40) is 12.8 kPa with XL probe, yielding 71.3% accuracy 6

Impact of Obesity on Shear Wave Elastography (SWE)

Superior Performance in Obesity

Point SWE (pSWE/ARFI) demonstrates significantly better feasibility in obese patients compared to transient elastography 1:

• Failure rate of pSWE is 2.9% versus 6.4% for TE (p<0.001) 1
• In meta-analysis, failed exams occurred in 1% with pSWE versus 11% with TE (M probe) among obese patients 1
• 2D-SWE maintains high reliability regardless of skin-to-capsule distance, using a single probe 3

Diagnostic Accuracy

• 2D-SWE and MRE demonstrate the highest AUROCs for significant and advanced fibrosis in NAFLD, outperforming standard TE 1
• Point SWE shows similar diagnostic performance to TE for fibrosis staging (AUROC >0.8) 1
• MRE maintains excellent applicability in obese patients where ultrasound elastography frequently fails 1, 7

Relationship to Varices on Endoscopy

Liver Stiffness and Portal Hypertension

Elevated—not falsely low—liver stiffness measurements in obese patients would overestimate, not underestimate, the risk of varices 1:

• TE is useful for predicting portal hypertension development 5
• Higher LSM values correlate with increased risk of complications including varices
• The concern should be false-positive predictions of advanced disease, not missed varices

Clinical Implications

• Obesity-related falsely elevated LSM could lead to unnecessary endoscopic screening 6, 4
• In morbidly obese patients, a predictive model incorporating LSM, HbA1c, and alkaline phosphatase improves accuracy to 79.6% with 98.3% negative predictive value 6

Practical Algorithm for Obese Patients

Step 1: Initial Assessment

• Use XL probe for all patients with BMI >28 kg/m² or skin-to-capsule distance >22.5 mm 1, 3
• Ensure fasting status (≥2 hours, preferably 4 hours) to avoid artificially elevated measurements 1, 7
• Verify ALT levels are not acutely elevated (>5× ULN), which causes falsely high readings 1

Step 2: Quality Criteria

• Require ≥10 valid measurements with success rate ≥60% 8
• Interquartile range must be <30% of median LSM 8
• If unreliable with XL probe, proceed directly to 2D-SWE or MRE 1, 7

Step 3: Interpretation Adjustments

• Apply obesity-specific cutoffs: for XL probe, use ~1.2-1.4 kPa lower thresholds than M probe 1, 5, 2
• For significant fibrosis (≥F2) in morbid obesity: use 12.8 kPa cutoff 6
• Consider that steatosis, BMI, and insulin resistance independently elevate LSM beyond fibrosis stage 4

Step 4: Alternative Modalities

• MRE is first-line when ultrasound elastography is unreliable or indeterminate 1, 7
• MRE maintains accuracy regardless of obesity or ascites with <5% failure rate 1
• 2D-SWE provides high reliability across all body habitus with single probe 3

Critical Pitfalls to Avoid

• Do not assume low LSM values in obese patients represent true absence of fibrosis—technical failure may yield spuriously low readings 1
• Never interpret isolated LSM without considering ALT levels, steatosis grade, and metabolic parameters 1, 4
• Avoid using standard M probe cutoffs when XL probe is employed 5, 2
• Do not rely solely on TE in morbidly obese patients (BMI ≥40)—accuracy is only 71.3% 6
• Remember that obesity causes overestimation, not underestimation, of fibrosis severity 5, 4