Is the M probe really necessary to measure liver stiffness by FibroScan?

We thank Drs. Sharma and Arora for their comments regarding our study describing the utility of the FibroScan XL probe for liver stiffness measurement (LSM) in overweight and obese patients with various liver disorders.1 The findings of their single-center, Indian study suggest that LSM using the FibroScan M probe may be unnecessary in light of the high success rate of the XL probe. In our multicenter study of a heavier cohort (median body mass index [BMI] 30 kg/m2 versus 25 to 26 kg/m2 in their study), FibroScan failure occurred in only 1% of patients with the XL probe versus 16% with the M probe. Corresponding rates of reliability (valid shots ≥10, success rate ≥60%, and IQR/M ≤30%) were 73% and 50% with the XL and M probes, respectively. Had we taken the approach of Sharma and Arora and first measured patients with the XL probe, reserving the M probe for those with unreliable XL probe results, an additional 27% (20/74) of patients (7% of the total) would be reliably measured with the M probe (Fig. 1). Depending on the outcome of interest (i.e., transient elastography [TE] failure versus reliability), the latter data suggest that the M probe still has a role for LSM, even among overweight and obese patients. If one examines the characteristics of patients with unreliable XL probe measurements, those who were reliably measured with the M probe were lighter (median BMI 32 versus 36 kg/m2; P = 0.008) and were less likely to have a skin-capsular distance exceeding 25 mm (the measurement depth of the M probe; 30% versus 55%; P = 0.07) than those in whom the M probe was unreliable. Reliability of liver stiffness measurement (≥10 valid measurements, IQR/M ≤30%, and success rate ≥60%) using the XL and M probes (*). Before discarding the FibroScan M probe as a useful tool for measuring liver stiffness, additional considerations warrant discussion. Importantly, the larger diameter of the tip of the XL probe (12 mm versus 9 mm with the M probe) may lead to a higher rate of LSM failure or LSM overestimation in smaller adults with narrow intercostal spaces due to interference from the ribs with shear wave propagation and measurement. Due to the limited data describing use of the XL probe in nonobese individuals, the magnitude of this potential problem is unclear. Second, as confirmed by our study and others,1-3 LSMs obtained using the M and XL probes differ systematically due to their different regions of interest (25-65 mm from the skin for the M probe and 35-75 mm with the XL probe). In patients who can be successfully measured with both probes, liver stiffness measured using the XL probe is 1 to 2 kPa lower than that of the M probe.1 Because liver stiffness thresholds will therefore differ by probe (and disease etiology), additional research would be necessary to define the optimal thresholds for use with the XL probe in nonobese individuals. In light of the hundreds of publications that have documented the performance of the M probe for LSM in these patients, it is far too early to recommend use of the XL probe in all patients. Robert P. Myers M.D., M.Sc.*, * Liver Unit, Division of Gastroenterology and Hepatology, University of Calgary, Calgary, Alberta, Canada.

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