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American Association for Hand Surgery

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Ultrasound Measurement Error and its Implications for Carpal Tunnel Syndrome Diagnosis
Cory J Demino, MD, University of Pittsburgh School of Medicine, Pittsburgh, PA and John R Fowler, MD, UPMC, Pittsburgh, PA

Ultrasound (US) measurement of the cross-sectional area (CSA) of peripheral nerves has been validated as a method for diagnosing peripheral nerve compression, however, the accuracy of US measurements and variation among examiners has not been well established. The purpose of this study is to determine the percent error from a known measurement among multiple US examiners using a gel phantom model.

Materials & Methods
An US phantom gel, with two embedded, pre-measured cylindrical objects, was constructed and 9 residents, fellows, and attending physicians with upper-extremity US experience were recruited to use US to measure the CSA of each object. Percent error was calculated based on the pre-measured value of each object, which was measured using digital calipers, and calculated using the formula for CSA of a cylinder (A=pi*r2) prior to insertion into the gel phantom.

The mean percent error for examiners' CSA measurements was 9.0 4.7% (range 1.6% - 17.0%). There was a greater mean percent error for the smaller measured object compared to the larger one. There was a very minimal decline in percent error with increase in years of training experience (slope = -0.029, R2 = 0.0002).

This study demonstrates that US examiners were able to measure CSA's of pre-measured objects with approximately 10% error. When considering use of US for diagnosis of carpal tunnel syndrome, a cut-off value of 10 mm2 is often used, and therefore in cases where measurements are within 10% of the diagnostic threshold, caution should be exercised in interpreting the results.

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