What is CAP (Controlled Attenuation Parameter)?
CAP is an ultrasound-based technology integrated into the FibroScan device that measures the degree of ultrasound attenuation caused by hepatic fat, providing a rapid, noninvasive quantification of liver steatosis expressed in dB/m. 1
How CAP Works
CAP uses the same radiofrequency signals acquired during transient elastography (for liver stiffness measurement) but processes them differently to specifically detect fat content in the liver. The technology measures how ultrasound waves are attenuated (weakened) as they pass through fatty liver tissue—the more fat present, the greater the attenuation 1, 2.
Clinical Application
CAP provides simultaneous assessment of both steatosis (via CAP) and fibrosis (via liver stiffness measurement) in a single 5-minute examination, making it an efficient point-of-care tool 3. The measurement is:
- Operator-independent
- Reproducible
- Painless and rapid
- Performed at the bedside
Diagnostic Thresholds
According to EASL guidelines, CAP values above 275 dB/m can be used to diagnose hepatic steatosis, showing over 90% sensitivity for detecting any degree of steatosis 4. Research supports additional cutoffs for grading severity:
- S0 (no steatosis): <220-238 dB/m
- S1 (mild, 5-33%): 238-260 dB/m
- S2 (moderate, 34-66%): 260-290 dB/m
- S3 (severe, >66%): >290 dB/m 2, 3
The most commonly validated cutoff for detecting ≥10% steatosis is 283 dB/m, with 76% sensitivity and 79% specificity 2.
Current Guideline Position
CAP is considered a promising point-of-care technique but is NOT yet recommended as a first-line tool for steatosis detection 4. The EASL 2021 guidelines state that conventional ultrasound remains the first-line recommendation despite its limitations, while CAP's limited availability and lack of head-to-head comparisons with ultrasound prevent it from being a primary recommendation 4.
Performance Characteristics
CAP demonstrates strong diagnostic accuracy:
- AUROC 0.80-0.90 for detecting significant steatosis (≥S1) 2, 3, 5
- Correlation with MRI-PDFF: r=0.62-0.73 5, 6
- Superior to conventional ultrasound in detecting mild steatosis, identifying fatty liver in 40% of patients with normal ultrasound findings 7, 8
Important Caveats
CAP performance is reduced in patients with advanced fibrosis (F2-F4), where diagnostic accuracy drops from AUROC 0.89 in F0-F1 to 0.72 in F2-F4 fibrosis 2. Additionally:
- BMI and skin-to-liver capsule distance affect measurements—patients with BMI ≥28 kg/m² or increased subcutaneous fat may require XL probe 2
- CAP correlates independently with BMI and metabolic syndrome features 8
- Measurement variability does not significantly impact performance 2
Comparison with Other Modalities
MRI-PDFF remains the most accurate non-invasive method for steatosis quantification but is expensive and limited in availability, making it more suitable for clinical trials than routine practice 4. Recent data shows newer attenuation imaging (ATI) technology may outperform CAP (AUROC 0.90 vs 0.85 for ≥S1 steatosis) 5, 6, though CAP remains more widely available.
Clinical Utility
CAP is particularly valuable for:
- Detecting subclinical steatosis not visible on conventional imaging 7, 8
- Monitoring treatment response, where a decline of ≥46 dB/m or ≥15% from baseline suggests clinically meaningful steatosis reduction 9
- Screening living liver donor candidates as an adjunct to biopsy 10
- Risk stratification in patients with metabolic syndrome or NAFLD when combined with fibrosis assessment