Liver Fat Quantification: Agreement and Reproducibility of Ultrasound-Derived Fat Fraction Measurements Using 5C1 and DAX Probes in Subjects Stratified by BMI

This study aimed to assess the agreement between the 5C1 and deep abdominal transducer (DAX) ultrasound probes when measuring ultrasound-derived fat fraction (UDFF) across different body mass index (BMI) categories and to evaluate the intra- and inter-observer reproducibility of liver fat measurements.In this prospective study 63 subjects (32 with BMI < 25 and 31 with BMI ≥ 25) underwent UDFF measurements. Two observers performed five measurements per subject using both probes (5C1 and DAX). Intra- and inter-observer reliability were assessed using intraclass correlation coefficients (ICC). Agreement between probes and observers was evaluated using Spearman correlation and Bland-Altman analysis, stratified by BMI subgroup.The intra- and inter-observer reliability of UDFF measurements were excellent, with no relevant systematic bias between observers or probes. The 5C1 and DAX probes showed strong correlations (r = 0.92 and 0.93). In normal-weight subjects, differences between probes were minimal, with limits of agreement (LoA) of approximately ±2%. In subjects with BMI ≥ 25, the mean differences between probes were low (up to 2.6%), but the limits of agreement were wider (-8% to +13%), comparable to the inter-observer variability.UDFF measurements with both the 5C1 and DAX probes showed consistently high reliability across BMI categories. Although variability increased with a higher BMI and liver fat, no systematic bias was observed, and both probes performed comparably. These findings support their interchangeable use in clinical settings. Validation against a reference standard is warranted to assess accuracy in challenging patient populations.