Effect of probe size and energy (M versus XL probe) on the detection of liver stiffness using transient elastography (Fibroscan)

Background: Measurement of liver stiffness (LS) by transient elastography [TE/Fibroscan] is limited in patients with obesity and ascites. We here test and compare the recently developed more powerful XL probe versus the conventional M probe in obese and non-obese patients.

Methods: LS was measured in 129 (BMI range 17.2–72.4) using both the M and XL probe. In addition, BMI, waist to hip ratio, transaminases and basic liver ultrasound parameters were obtained.

Results: LS was not measurable in 6 patients (4.6%) by the XL probe as compared to 32 patients (24.8%) by the M probe. Of note, 46% of the patients not measurable with the M probe showed cirrhotic stiffness values with the XL probe. ROC analysis indicates that skin-liver capsula distance, hip circumference and BMI are the best clinical parameters that predict valid measurements by the M probe. However, even 10 non-obese patients were not measurable with the M probe. This measurement failure was in two cases due to ascites but mainly to a specific type of subcutaneous fat tissue with poor ultrasound propagation. Notably, even patients with ascites up to 31mm distance and a BMI of 71 were measurable with the XL probe. LS by M and XL probe were highly correlated (r=0.98, P<0.01). However, LS measured with XL probe tends to be ca. 20% less as compared to the M probe resulting in a slightly different cut-off calues (F0<4.8 kPa, F1-F2 4.8–6.4 kPa, F3: 6.4–10 kPa, F4: >10 kPa). While the XL probe drastically improves measurability of LS in all patients, interquartile range (IQR) and so called valid shots (IQR<30%, success rate>60%) are significantly lower. Manual reading of elastograms using a suggested expert mode improves LS measurements in selected individuals.

Discussion and conclusion: The LS of significantly more patients with obesity and ascites are measurable with the more powerful XL probe. However, LS values obtained with the XL probe are systematically slightly lower as compared to M probe resulting in different cut-off values for fibrosis stages.