Predictors of normal electrodiagnostic testing in the evaluation of suspected carpal tunnel syndrome

Electrodiagnostic studies (electromyography and nerve conduction velocity; EMG/NCV) are used to confirm the diagnosis of carpal tunnel syndrome (CTS) and to evaluate its severity. We investigated the hypothesis that normal electrodiagnostic median nerve testing is predicted by 1) Physician pre-test confidence in the diagnosis of CTS, and 2) Puzzling patient factors. One hundred and forty consecutive patients who underwent electrodiagnostic testing to evaluate for possible CTS were reviewed retrospectively. Both physician confidence in the diagnosis of CTS and puzzling patient factors (heightened illness concern, disproportionate complaints, and vague/nonanatomical/noncharacteristic symptoms) were recorded. Electrodiagnostic testing was used as the reference standard for diagnosis of CTS. Electrodiagnostic testing confirmed CTS in 115 patients and was within normal limits in 25 patients. Low physician confidence in the diagnosis of CTS was highly predictive of a normal electrodiagnostic test (p < 0.001), with high sensitivity (97%), moderate specificity (40%), and high overall accuracy (87%). Puzzling patient factors were moderately predictive of normal electrodiagnostic testing (p < 0.001), with low sensitivity (16%), high specificity (96%), and high overall accuracy (81%). The best multivariable model retained younger age, negative Phalen's test, and low physician confidence as the best predictors of normal electrodiagnostic testing and explained 35% of the variation in test results. A model with low confidence alone explained 19% of the variation in test results. Physician intuition as recorded in the medical record in terms of puzzlement and low confidence are very specific and accurate predictors of normal electrodiagnostic testing in the setting of suspected carpal tunnel syndrome.