We read with interest the study by Zhu et al 1 evaluating the reliability and validity of ultrasound-derived angles for radiation-free scoliosis screening. The intra- and interobserver reliability data are a useful contribution, but before the reported minimal detectable change (MDC) and validity estimates are applied to longitudinal curve monitoring, several methodological points deserve comment. Validity was framed largely through simple linear regression coefficients (R), with agreement categorized as poor, moderate, or excellent by R thresholds, even though Bland-Altman plots were also generated. Correlation coefficients are a misleading index of agreement between two measurement methods, because two techniques can correlate strongly while differing by a clinically meaningful, systematic margin. 2 Framing the validity conclusions mainly around R risks overstating how interchangeable ultrasound and radiographic Cobb angle measurements are. Second, the MDC of 4.93 degrees (automatic) and up to 4.43 degrees (manual) was derived from repeat scans obtained by two observers within the same visit. This same-session design captures inter-rater and immediate re-scan variability but not the between-visit sources of error, such as differences in probe angulation, coupling-gel pressure, and patient posture across weeks to months, that longitudinal monitoring, the technique’s stated principal use, would actually encounter. Standard error of measurement and MDC estimates depend heavily on the conditions under which they are derived, and same-day data may understate the error relevant to serial follow-up. 3
Two further points bear on generalizability. Subjects were stratified into seven overlapping subgroups, by apical location, three BMI categories, and scoliosis status, from a cohort of only 94 participants, without correction for multiple comparisons; the low R of 0.425 in the overweight subgroup may reflect small-cell instability as much as a genuine BMI effect, a pattern that subgroup-analysis guidance cautions against overinterpreting. 4 Finally, 96.8% of participants had Cobb angles below 35 degrees, so the reported reliability and validity metrics were generated in a narrow, largely mild-curve spectrum; diagnostic accuracy estimated in such a restricted spectrum can be inflated relative to the moderate-to-severe population in which vertebral rotation and rib-hump deformity are most likely to interfere with ultrasound landmark identification. 5 We would welcome the authors’ perspective on how these considerations might refine the clinical thresholds proposed for ultrasound-based scoliosis surveillance.
Footnotes
Author Contributions: All authors conceived, drafted, critically revised, and approved the final version of this letter.
Funding: The authors received no financial support for the research, authorship, and/or publication of this article.
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
ORCID iD
Sharath Raj https://orcid.org/0009-0002-9439-7901
Ethical Considerations
This is a Letter to the Editor and does not report original human or animal research; no ethical approval was required.
Data Availability Statement
No new data were generated or analyzed for this letter.*
References
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Associated Data
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Data Availability Statement
No new data were generated or analyzed for this letter.*