Normal IQR/Median Ratio for Elastography Quality Control
For liver elastography measurements to be considered reliable and of adequate quality, the interquartile range to median ratio (IQR/M) should be ≤30% (or ≤0.30). This is the established quality criterion for vibration-controlled transient elastography (VCTE) and has been validated for other elastography techniques.
Quality Threshold and Clinical Significance
The IQR/M ≤30% threshold is explicitly stated in the 2017 AGA guidelines 1, 2 as a fundamental quality criterion for VCTE measurements. This means that when obtaining at least 10 valid liver stiffness measurements, the measurement set is only considered adequate if the IQR/M ratio does not exceed 30%.
Why This Matters for Accuracy
Measurements with IQR/M >30% are associated with significantly higher rates of misclassification:
- When IQR/M is <0.21 (21%), discordance rates with histology are only 7.4%
- When IQR/M is ≥0.21, discordance rates increase to 15.1% 3
- The most discriminant cutoff for identifying unreliable measurements is IQR/M = 0.21 (21%), though the standard clinical threshold remains 30%
The IQR/M ratio directly impacts diagnostic performance. Studies show that when comparing measurements with IQR/M ≤30% versus >30%, the lower reliability group had significantly higher discordance rates with histologic fibrosis staging (31.7% vs 14.7% for significant fibrosis) 4.
Application Across Different Elastography Platforms
This quality criterion is not limited to VCTE:
- Point shear wave elastography (pSWE): The IQR/M ≤30% criterion significantly improves diagnostic performance 4
- ElastPQ shear wave elastography: Applying IQR/M ≤30% improves correlation with FibroScan and enhances diagnostic accuracy 5
Minimum Number of Measurements Required
To calculate a reliable IQR/M ratio:
- VCTE requires minimum 10 valid measurements 1, 2
- Point SWE requires minimum 6 measurements (ICC >0.95 compared to 10 measurements) 4
- ElastPQ can use 5 measurements with comparable reliability to 10 measurements when IQR/M ≤30% is applied 5
Common Pitfalls and Confounding Factors
Factors that increase IQR/M and reduce measurement reliability:
- Low liver stiffness values (<9 kPa): These inherently have poorer reproducibility and higher IQR/M ratios 6
- Early fibrosis stages (F0-F2): Associated with higher discordance rates compared to advanced fibrosis 3
- Obesity (BMI ≥25): Independently decreases measurement agreement 6
- Non-recommended measurement sites: Measurements should be obtained at the mid-axillary line, 9th-11th intercostal space 2
- Measurement depth >5 cm: Associated with higher variability 7
Clinical Action When IQR/M >30%
When measurements fail to meet the IQR/M ≤30% criterion:
- Repeat the examination with attention to technical factors
- Consider using the XL probe in obese patients
- Ensure measurements are obtained from the recommended anatomic location
- Consider alternative modalities (MR elastography) if repeated attempts fail quality criteria
- Do not use measurements with IQR/M >30% for clinical decision-making, as they have unacceptably high false positive and false negative rates
The IQR/M ratio serves as an essential internal quality control metric that must be checked before interpreting any elastography result for clinical management decisions.