Attenuation Coefficient (ATI/UGAP) vs. CAP for Quantitative Assessment of Liver Steatosis
Two Different Methods, Different Units of Measurement
Quantitative assessment of liver steatosis is currently implemented using two approaches that should not be mixed. The attenuation coefficient (ATI/UGAP and similar technologies of attenuation imaging) is measured in dB/cm/MHz, whereas the Controlled Attenuation Parameter (CAP) on the FibroScan device is expressed in dB/m. These are different physical parameters; mechanically converting one value into another is not possible.
CAP (FibroScan)
CAP is a method for the quantitative assessment of steatosis associated with transient elastography (TE) and performed on the FibroScan device. In a cohort of 115 patients, using the histological grade of steatosis as a reference, CAP significantly correlated with steatosis (r=0.81, p<0.00001). The AUROC for detecting steatosis >10% and >33% were 0.91 and 0.95, respectively.
A meta-analysis (11 studies) calculated the following AUROC for predicting the presence of S≥1, S≥2, and S≥3: 0.85, 0.88, and 0.87, respectively.
| Steatosis Grade | Cut-off CAP (dB/m) |
|---|---|
| S>1 | 232.5 |
| S>2 | 255 |
| S>3 | 290 |
According to WFUMB (Ferraioli, 2024), a value of 288 dB/m determined in a cohort controlled by MRI-PDFF can serve as the best available reference for detecting steatosis (S>0).
Attenuation Coefficient (ATI/UGAP / attenuation imaging)
Attenuation imaging technologies provide the attenuation coefficient in dB/cm/MHz. Validated, model-specific thresholds are difficult to identify in peer-reviewed literature; manufacturers usually calibrate their algorithms to published ATT/UAP ranges, but exact values for a specific model and software version should be checked with the operator's manual. [clarify specific thresholds for the device in use]
Attenuation Coefficient Measurement Protocol (WFUMB, Ferraioli 2024)
| Parameter | Recommendation |
|---|---|
| Fasting | Not required if the measurement is performed separately |
| Breathing | Breath-hold |
| Imaging | Best B-mode quality; perpendicularity of the probe to the capsule is not mandatory |
| Access | Right intercostal space |
| Measurement box | Perpendicular to the probe, length 3 cm |
| Upper edge of the box | 2 cm below the liver capsule |
| Quality | IQR/M ≤ 15% |
| Final value | Median (or mean) of 3–5 measurements |
Practical Conclusion
CAP and ATT/UAP are different parameters with different units and threshold ranges. When interpreting, specify which unit and algorithm the specific device provides, and check thresholds with the operator's manual. The most unified reference for attenuation methodology today is the WFUMB recommendation (288 dB/m for S>0 in an MRI-PDFF-controlled cohort).
Frequently asked questions
Can CAP be converted to the ATI/UGAP attenuation coefficient?
No. CAP (dB/m) and ATT/UAP (dB/cm/MHz) are different physical parameters with different units. Mechanical conversion from one to the other is not possible.
What are the CAP cut-offs for steatosis stages?
According to the EFSUMB Course Book (2018): S>1 — 232.5 dB/m, S>2 — 255 dB/m, S>3 — 290 dB/m. According to WFUMB (2024), the reference for S>0 is 288 dB/m (cohort controlled by MRI-PDFF).
How should the measurement box be positioned when measuring the attenuation coefficient?
According to WFUMB (Ferraioli, 2024): right intercostal space, box perpendicular to the probe, length 3 cm, upper edge 2 cm below the capsule, breath-hold, IQR/M ≤ 15%, median of 3–5 measurements.
Is fasting necessary before measuring the attenuation coefficient?
According to WFUMB, fasting is not necessary if the attenuation coefficient measurement is performed separately.
Where to find attenuation coefficient thresholds for a specific device?
Validated model-specific thresholds may not be available in peer-reviewed literature; exact values and algorithms should be checked with the operator's manual of the specific device. [clarify]