Inter-vendor Variability of Liver SWE: Why kPa from Different Devices Are Incomparable
Why kPa from different devices cannot be directly compared
According to WFUMB (Ferraioli et al., 2015), results in kilopascals are not comparable between 2D-SWE, pSWE, and TE. The reason is the frequency dependence of the elastic properties of tissue, which requires special caution when comparing quantitative results between techniques. Not only the methods differ, but also the device models within a single manufacturer.
Magnitude of inter-system differences
According to WFUMB (Ferraioli et al., 2018), when comparing systems, agreement was >0.80 for all pairs. Specifically:
| Parameter | Value |
|---|---|
| Average difference between 2D-SWE systems | 1.54 kPa |
| Max. average difference between three out of four pSWE systems | 0.79 kPa |
| Intra-patient concordance (all systems) | 0.89 (95% CI: 0.83–0.94) |
Variability between measurements by different systems is higher with stiffer livers. The range of values from two 2D-SWE systems was close to FibroScan at very high stiffness (>15 kPa), whereas four pSWE systems provided lower values in the upper stiffness range.
Dependence on method and manufacturer
According to the EFSUMB Course Book (Sporea et al., 2018), for 2D-SWE by General Electric in a cohort of healthy individuals, values were 5.1 ± 1.3 kPa, higher in men and patients over 40 years old. This illustrates that even normal values are tied to specific technology and demographics. Samsung predominantly uses pSWE (Virtual Touch / S-Shearwave) and 2D-SWE under different trade names in various generations (RS80A, HERA, V8) — proprietary reference ranges should be checked in the operator's manual or the 'Clinical Reference' section.
How to reduce methodological variability
EFSUMB (Dietrich et al., 2017, Recommendation 15): the least variability when comparing different pSWE or 2D-SWE systems was achieved at a depth of 4–5 cm from the probe (for convex probes) (LoE 4, GoR C).
According to WFUMB (Ferraioli et al., 2018), the accuracy of B-mode techniques (pSWE, 2D-SWE) is similar to TE; some comparative studies show similar accuracy of different systems, but larger prospective studies are needed to identify inter-system differences in accuracy.
Practical conclusions for protocol
Mechanical conversion of values between technologies and manufacturers is not possible. Thresholds are tied to the method and etiology: most studies have been conducted in chronic hepatitis C, so cut-offs may not be applicable to other viral etiologies or NAFLD (WFUMB, 2015). The protocol must specify the device, probe, and method — for example, 'point SWE (ElastPQ), Philips Affiniti 70, C5-1', provide the median of 10 measurements and IQR/Med ≤ 30%.
Frequently asked questions
Can kPa be converted between 2D-SWE, pSWE, and TE?
No. According to WFUMB (2015), results in kilopascals are not comparable between 2D-SWE, pSWE, and TE due to the frequency dependence of tissue elastic properties; mechanical conversion is not permissible.
How significant is the difference between systems?
According to WFUMB (Ferraioli et al., 2018): the average difference between 2D-SWE systems is 1.54 kPa, the maximum between three out of four pSWE systems is 0.79 kPa. Variability is higher with stiffer livers.
At what depth should measurements be taken for minimal inter-system variability?
According to EFSUMB (2017, Recommendation 15), the least variability when comparing pSWE/2D-SWE systems was achieved at a depth of 4–5 cm from the probe (for convex probes; LoE 4, GoR C).
Do systems behave the same at high stiffness?
No. At very high stiffness (>15 kPa), the range of 2D-SWE was close to FibroScan, while four pSWE systems provided lower values in the upper range (WFUMB, 2018).
Are thresholds for hepatitis C applicable to NAFLD?
With caution. According to WFUMB (2015), most studies have been conducted in chronic hepatitis C, so cut-offs may not be applicable to other viral etiologies and NAFLD; data on NAFLD are limited.