How does Interquartile Range (IQR)/median help validate falsely low Fibroscan values in assessing liver fibrosis?

IQR/Median Ratio as a Quality Control Metric for FibroScan Reliability

The IQR/median ratio serves as a critical quality control parameter to identify unreliable FibroScan measurements, with values ≥0.30 (or ≥30%) indicating poor measurement quality that can lead to falsely elevated or inaccurate liver stiffness values, particularly overestimation of fibrosis severity. 1, 2

Understanding the IQR/Median Quality Metric

The interquartile range (IQR) divided by the median liver stiffness measurement (IQR/M) quantifies the variability between individual measurements during a FibroScan examination 2:

• IQR/M <0.30 (or <30%) indicates consistent, reliable measurements 3, 1
• IQR/M ≥0.30 (or ≥30%) signals excessive variability and unreliable results 3, 1

How IQR/Median Identifies Falsely Low (and High) Values

Mechanism of Detection

The IQR/median ratio functions as a precision indicator rather than specifically detecting "falsely low" values 2. More accurately, it identifies unreliable measurements in either direction:

• High IQR/median ratios (≥0.21-0.30) are associated with significant discordances between FibroScan and actual histological fibrosis stage 2
• Studies show that IQR/M ≥0.21 is specifically associated with overestimation of liver fibrosis, not underestimation 2
• When IQR/M ≥0.21, discordances of at least two fibrosis stages occurred in 15.1% of cases versus only 7.4% when IQR/M <0.21 2

Impact on Diagnostic Accuracy

The diagnostic performance of FibroScan deteriorates significantly with elevated IQR/median ratios 2:

• For cirrhosis (F4) detection: AUROC drops from 0.95 (95% CI 0.92-0.99) when IQR/M <0.21 to 0.86 (95% CI 0.77-0.94) when IQR/M ≥0.21 2
• For advanced fibrosis (≥F3): AUROC decreases from 0.89 (95% CI 0.83-0.95) to 0.80 (95% CI 0.72-0.88) with higher IQR/median ratios 2

Three Essential Validity Criteria for FibroScan

A valid liver stiffness evaluation (LSE) requires ALL three criteria to be met simultaneously 3, 1:

1. ≥10 successful measurements obtained 3, 1
2. Success rate ≥60% 3, 1
3. IQR/median ratio <0.30 (or <30%) 3, 1

Clinical Consequences of Invalid Measurements

When these validity criteria are not met, diagnostic accuracy suffers substantially 1:

• Correlation with biopsy: Valid LSE showed rs=0.577 versus invalid LSE rs=0.259 (p=0.022) 1
• AUROC for significant fibrosis: 0.83 for valid LSE versus 0.66 for invalid LSE (p=0.048) 1
• Negative predictive value at 8 kPa cutoff:
  • For significant fibrosis: 84% (valid) versus 71% (invalid) 1
  • For advanced fibrosis: 100% (valid) versus 93% (invalid) 1

Clinical Algorithm for Interpreting FibroScan Results

Step 1: Verify Measurement Validity

Before interpreting any FibroScan value, confirm all three validity criteria are met 3, 1:

• Check that ≥10 measurements were obtained 3, 1
• Verify success rate is ≥60% 3, 1
• Confirm IQR/median <0.30 3, 1

Step 2: Action Based on Validity Status

If ANY validity criterion fails 3, 1:

• Do not use the FibroScan result for clinical decision-making 1
• Repeat the examination, potentially with an experienced operator 1
• Consider alternative non-invasive tests (APRI, FIB-4, ELF) 3, 4
• Consider referral for MR elastography if available 3
• In high-risk patients, consider hepatology referral for further evaluation 3

If all validity criteria are met 5:

• Proceed with standard interpretation using established cutoffs:
  • <7.0 kPa: Low probability of significant fibrosis 5
  • 7.0-12.5 kPa: Significant fibrosis present 5

  • 12.5 kPa: Cirrhosis likely 5

Common Pitfalls and Technical Considerations

Factors Affecting IQR/Median Ratio

Several technical and patient factors can elevate the IQR/median ratio 3, 1:

• Operator inexperience: Validity rates improve significantly with the first 10-50 examinations performed 1
• Severe obesity: Limits ultrasound penetration and measurement consistency 3
• Narrow intercostal spaces: Reduces acoustic window quality 3
• Ascites: Interferes with measurement reliability 3
• Patient movement or inadequate breath-holding 1

Important Clarification on "Falsely Low" Values

The IQR/median ratio does not specifically identify "falsely low" FibroScan values 2. Research demonstrates that elevated IQR/median ratios are actually associated with overestimation rather than underestimation of fibrosis 2. The ratio serves as a general quality indicator for measurement precision, flagging results that may be inaccurate in either direction, though the predominant error pattern is overestimation 2.

When Non-Invasive Tests Are Discordant

If FibroScan results conflict with serological markers (APRI, FIB-4) 3:

• Verify FibroScan validity criteria were met 3, 1
• Consider additional testing with MR elastography if available 3
• Refer to hepatology for expert evaluation 3
• Consider liver biopsy in selected cases where management would change 3