Choosing FibroScan XL or M Probe: Skin-to-Capsule Distance and BMI
Principle of Probe Selection by Body Weight
According to the manufacturer's recommendations (EFSUMB Course Book, 2nd Edition, 2018), three types of FibroScan probes are available: the S-probe for pediatric practice, the M-probe for patients of normal weight, and the XL-probe for overweight and obese patients.
The choice between M and XL probes is determined by the skin-to-capsule distance (SCD): the M-probe is used when SCD ≤ 2.5 cm, and the XL-probe is used when SCD > 2.5 cm (M measures at a depth of 2.5–6.5 cm, XL at 3.5–7.5 cm). In modern versions of the device, the Automatic Probe Selection (APS) tool automatically measures SCD and indicates the required probe; in practice, the choice is initially guided by BMI, then confirmed by the APS tool.
Reliability of Measurements and the Role of Obesity
Reliable measurements are defined as the median of 10 valid LSM with an interquartile range (IQR) < 30% and a success rate (SR) = 60% (manufacturer's recommendations, EFSUMB Course Book, 2018). With these quality criteria, reliable measurements can be obtained with the standard M-probe in only 70–85% of patients, with obesity being the most significant factor for unreliable measurements.
| Situation | M-probe | XL-probe |
|---|---|---|
| Proportion of reliable measurements in obese patients | ~45% | ~75% |
Differences in Stiffness Values Between Probes
The XL-probe provides lower liver stiffness values compared to the M-probe (EFSUMB Course Book, 2018). In obese patients with a high degree of steatosis, TE using the M-probe may be less accurate in diagnosing severe fibrosis in NAFLD; additional studies are needed on the impact of steatosis on LSM measured by the XL-probe.
In a study of 838 patients with NAFLD (BMI 33.6 ± 6.5 kg/m²) using both probes, intra- and inter-expert correlations were generally high (r=0.82 and r=0.70, respectively). However, correlation decreased with the XL-probe: intra-expert correlation M vs. XL was r=0.85 vs. 0.75 (p=0.0003) (Ferraioli et al., 2018; Vuppalanchi et al. 2018).
Threshold Values for the M-Probe
For the M-probe, cut-offs of 7.9 kPa and 9.3 kPa are provided with sensitivity and specificity of 90% for ruling out and confirming F3 fibrosis in NAFLD, respectively (EFSUMB Course Book, 2018). There are no specific quality criteria from the manufacturer for the XL-probe; researchers apply the same criteria as for LSM (Ferraioli et al., 2018).
Frequently asked questions
What is the skin-to-capsule distance threshold for choosing XL over M?
The M-probe is used when the skin-to-capsule distance (SCD) is ≤ 2.5 cm, and the XL-probe is used when SCD is > 2.5 cm. In modern devices, SCD is automatically measured by the Automatic Probe Selection (APS) tool.
How much does the XL-probe increase the proportion of reliable measurements in obese patients?
In obese patients, the XL-probe provides reliable measurements in approximately 75% of cases compared to ~45% with the M-probe (EFSUMB Course Book, 2018).
Can M and XL probe values be directly compared?
No: the XL-probe systematically provides lower stiffness values, so the probe values are not directly interchangeable (EFSUMB Course Book, 2018).
What are the reliability criteria for LSM measurement?
The median of 10 valid measurements with IQR < 30% and a success rate ≥ 60% (manufacturer's recommendations, EFSUMB Course Book, 2018).
Are there separate quality criteria for the XL-probe?
There are no specific quality criteria from the manufacturer for the XL-probe; researchers use the same criteria as for LSM (Ferraioli et al., 2018).