Feasibility and performance of the FibroScan XL probe
Authors: William Kemp, Stuart Roberts
Journal: Hepatology (2011)

We read with interest the article by Myers et al.,1 who examined the feasibility and performance of the M and XL FibroScan probes in a cohort of patients with chronic liver disease. Although there are substantial data validating the use of the M probe as a noninvasive assessment of hepatic fibrosis,2 there are few data regarding the use of the XL probe.3, 4 This represents a particularly important issue considering the high prevalence of obesity in western communities and the relationship between body mass index (BMI) and frequency of liver stiffness measurement (LSM) failure and/or unreliability.5 Our analysis of over 2000 FibroScan referrals indicates that 23% of subjects have a BMI >30 kg/m2. In a recent single-center prospective study, we used both the M and XL probes on 98 consecutive patients attending our liver clinic (53% men; mean age, 54 years [interquartile range (IQR), 48-62]; BMI, 26 kg/m2 [IQR, 23.5-30.1]). Two experienced operators (>1,000 FibroScan examinations) performed all the procedures using sonographic images to optimize positioning of the probes. The etiology of liver disease was distributed as follows: hepatitis C, 38%; hepatitis B, 21%; nonalcoholic fatty liver disease, 14%; and other, 27%. Overall, we found there was no difference in the LSM success rate between the M probe (78%) and the XL probe (82%) using the accepted criteria6 of ≥10 valid readings, ≥60% success rate, and an IQR/median LSM ratio of ≤0.30. In 65 subjects, liver stiffness was successfully determined by both the M probe and the XL probe. A further 16 subjects had successful LSMs with the XL probe despite unsuccessful LSMs with the M probe. Twelve subjects had successful LSMs with the M probe only. Therefore, overall, a successful valid FibroScan examination was obtained in 94% of individuals using a combination of the M and/or XL probes. The success rate for the M probe (P > 0.001) was influenced by the BMI, with a substantial decrease in the rate of successful examination in those with a BMI ≥35 kg/m2. This pattern was not observed for the XL probe, with similar success rates observed across the full spectrum of BMI categories (Fig. 1). Probability of a successful FibroScan examination using the M probe (dark gray bars) or XL probe (light gray bars) according to BMI. Success was defined as ≥10 valid readings, ≥60% success rate, and IQR/median ratio ≤0.30. Our findings support the data presented by Myers et al.1 in that the use of the M probe alone is associated with a substantial failure rate that is BMI-dependent. The XL probe is therefore useful in those subjects in whom a valid LSM cannot be obtained with the M probe. However, similar to Myers et al.1 and the data previously published by de Ledinghen et al.,3 we observed that the LSMs were lower with the XL probe (6.1 [range, 4.9-7.9]) compared with the M probe (6.5 [range, 5.1-9.5]) (P > 0.005). This reinforces the concept that the currently published cutoffs for various fibrosis stages may be probe-specific, and revalidation of these cutoffs is required for use of the XL probe. William Kemp Ph.D.* , Stuart Roberts M.D.* , * Gastroenterology Department, Alfred Hospital, Melbourne, Australia, Department of Medicine, Monash University, Melbourne, Australia.