From the AGA Journals

FibroScan: M probe underestimates hepatic fat content



When performing transient elastography (FibroScan) to evaluate patients for hepatic steatosis, using an M probe instead of an XL probe may significantly underestimate hepatic fat content, according to investigators.

The findings, which were independent of body weight, suggest that probe-specific controlled attenuation parameter (CAP) thresholds are needed to accurately interpret FibroScan results, reported lead author Cyrielle Caussy, MD, PhD, of the University of California, San Diego, and colleagues.

“We have previously determined the optimal threshold of CAP using either [an] M or XL probe for the detection of ... nonalcoholic fatty liver disease (NAFLD),” the investigators wrote in Clinical Gastroenterology and Hepatology. “However, head-to-head comparison of consecutive measurements of CAP with both the M and XL probes versus MRI-PDFF [proton density fat fraction] ... has not been reported yet.”

Dr. Caussy and colleagues set out to do just that. They enrolled 105 individuals with and without NAFLD who had a mean body mass index (BMI) of 30.6 kg/m2, as this represented a typical population screened for NAFLD. After evaluation for other causes of hepatic steatosis and liver disease, participants underwent MRI-PDFF, which served as a gold standard, followed by FibroScan using both M and XL probes on the same day.

The primary outcome was hepatic steatosis (MRI-PDFF of at least 5%), while the secondary outcome was MRI-PDFF–detected hepatic fat content of at least 10%, the latter of which has been “used in several therapeutic trials as inclusion criteria,” the investigators noted.

A total of 100 participants were included in the final analysis, of whom two-thirds (66%) underwent MRI and FibroScan on the same day, with a mean interval between test types of 11 days. Most participants (68%) had an MRI-PDFF of at least 5%, while almost half (48%) exceeded an MRI-PDFF of 10%.

The mean CAP measurement with the M probe was 310 dB/m, which was significantly lower than the mean value detected by the XL probe, which was 317 dB/m (P = .007). In participants with hepatic steatosis, when the M probe was used for those with a BMI of less than 30, and the XL probe was used for those with a BMI of 30 or more, the M probe still provided a significantly lower measure of hepatic fat content (312 vs. 345 dB/m; P = .0035).

“[T]hese results have direct application in routine clinical practice,” the investigators wrote, “as [they] will help clinicians interpreting CAP measurements depending on the type of probe used.”

Dr. Caussy and colleagues went on to offer a diagnostic algorithm involving optimal probe-specific thresholds for CAP based on hepatic fat content. Individuals screened with an M probe who have a CAP of 294 dB/m or more should be considered positive for NAFLD, while patients screened with an XL probe need to have a CAP of at least 307 dB/m to be NAFLD positive.

For the XL probe, but not the M probe, diagnostic accuracy depended upon an interquartile range of less than 30 dB/m. The investigators noted that this finding should alter the interpretation of a 2019 study by Eddowes and colleagues, which concluded that interquartile range was unrelated to diagnostic accuracy.

“As Eddowes et al. did not perform head-to-head comparison of CAP measurement with both the M and XL probes, this important difference could not have been observed,” the investigators wrote, noting that “an interquartile range of CAP below 30 dB/m should be considered as a quality indicator that significantly improves the diagnostic performance of CAP using the XL probe for the detection of hepatic steatosis in NAFLD.”

The investigators concluded by suggesting that their findings will drive research forward.

“The use of these new thresholds will help to further assess the clinical utility of CAP for the detection of hepatic steatosis and its cost-effectiveness, compared with other modalities, to develop optimal strategies for the screening of NAFLD,” they wrote.

The study was funded by Atlantic Philanthropies, the John A. Hartford Foundation, the American Gastroenterological Association, and others. The investigators disclosed no conflicts of interest.

SOURCE: Caussy C et al. Clin Gastro Hepatol. 2019 Dec 13. doi: 10.1016/j.cgh.2019.11.060.

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