Background The combined sensory index (CSI) is the most sensitive electrodiagnostic criteria for carpal tunnel syndrome (CTS), and the CSI and Bland criteria have been shown to predict surgical treatment outcomes. The proposed ultrasound measurements have not been assessed against the CSI for diagnostic accuracy and grading of CTS severity. Objective To investigate the use of ultrasound evaluations for both diagnosis and assessment of severity grading of CTS in comparison to electrodiagnostic assessment. Design All patients underwent an electrodiagnostic evaluation using the CSI and Bland severity grading. Each patient underwent an ultrasound evaluation including cross‐sectional area (CSA), the change in CSA from the forearm to the tunnel (∆CSA), and the wrist‐forearm ratio (WFR). These measurements were assessed for diagnostic and severity grading accuracy using the CSI as the gold standard. Setting Tertiary academic center. Participants All patients referred for electrodiagnostic evaluation for CTS were eligible for the study. Only those with idiopathic CTS were included and those with prior CTS treatment were also excluded. Ninety‐five patients were included in the study. Interventions Not applicable. Main Outcome Measures The primary study outcome measure was concordance between CSI diagnosis and severity categories and the ultrasound measurements. Both outcomes were also assessed using Bland criteria. Results Optimal cut‐points for diagnosis of CTS were found to be CSA ≥12 mm2, ∆CSA ≥4 mm2, WFR ≥1.4. Using these cut‐points, C‐statistics comparing diagnosis of CTS using ultrasound measurements versus using the CSI ranged from 0.893‐0.966. When looking at CSI severity grading compared to ∆CSA, however, the C‐statistics were 0.640‐0.661 with substantial overlap between severity groups. Conclusions Although ultrasound measurements had high diagnostic accuracy for CTS based on the CSI criteria, ultrasound measurements were unable to adequately distinguish between CSI severity groups among patients with CTS.

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