Assessment by Fibroscan of fibrosis in nonalcoholic fatty liver disease: XL versus M probe?

To the Editor:

The study by Myers et al.,1 which tests the performance of the new Fibroscan XL probe, in comparison to the standard M probe, in the evaluation of liver stiffness in overweight/obese patients with metabolic (nonalcolholic fatty liver disease [NAFLD]) or viral (hepatitis B virus [HBV] or HCV) chronic liver disease, marks an important point, i.e., that the failure of measurement occurs only in 1% of patients with the XL probe as compared with 16% with the M probe.

While this clearly increases the reliability in the setting of patients with a high body mass index (BMI), some issues should be considered. First of all, the XL probe does not improve the accuracy in predicting the stage of fibrosis, as assessed by using liver biopsy as the “gold standard.” In fact, both the M and XL probes showed a comparable area under the curve (AUC) for significant and severe fibrosis and for cirrhosis. Second, 27% of cases using the XL probe were still inadequate. In addition, it would be interesting to know if the BMI, other than affecting reliability of stiffness measurement using both M and XL probes, is also able to interfere with its performance in predicting fibrosis, as we have recently shown in a cohort of biopsy-proven NAFLD patients.2 These considerations somewhat mitigate our enthusiasm for the XL probe, since even in overweight/obese patients the M probe is clearly not inferior, when the test was feasible, to the XL.

Another relevant issue is that with the inclusion of patients with viral liver diseases (HBV and HCV), the diagnostic performance of Fibroscan3 may be vastly different and could lead to potentially misleading results in the setting of obese liver patients, where most if not all have NAFLD.

Finally, interobserver reproducibility was not explicitly assessed in this study, which reports data on a new diagnostic tool, recorded at five different centers by five different operators. Thus, a potential observation bias cannot be excluded.

In our opinion, given the already high cost of the Fibroscan, there is no sufficient evidence yet to suggest that the XL probe should complement the M probe to assess fibrosis in overweight/obese patients. In fact, other techniques such as acoustic radiation force impulse (ARFI), easily implementable on standard ultrasound machines, can give a precise, noninvasive assessment of fibrosis in chronic liver disease while bringing to zero the number of unreliable examinations even in patients with a high BMI.4

Salvatore Petta XX*, Antonio Craxì XX*, * Sezione di Gastroenterologia, Di.Bi.M.I.S., University of Palermo, Palermo, Italy.