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Purpose: The strength of thumb abduction and index finger flexion was measured with a load cell mounted on a custom-made device. The resulting ratio (thumb-index ratio, or TIR) was used to diagnose carpal tunnel syndrome (CTS) and was compared with the gold standard (electrodiagnostic studies). The ratio was used as an internal control in each subject., Methods: Sixty-one patients (80 hands) with a clinically and electrodiagnostically confirmed diagnosis of idiopathic CTS and a control group of 51 asymptomatic volunteers (102 hands) were evaluated. The strength of thumb abduction and index finger flexion was measured in standardized fashion in each hand three times, and the mean was taken to calculate TIR., Results: Thumb-index ratio was statistically significant in differentiating between a CTS patient and a normal control. A TIR of 0.51 had a 98% specificity and a 92% positive predictive value for diagnosing CTS. Thumb-index ratio greater than 0.51 was neither sensitive nor specific for diagnosis of CTS. Twenty-four (30%) hands in the investigational group had a TIR < or =0.51 compared with 1 hand (1%) in the control group. There was a significant difference in the TIR between hands with a motor amplitude of < or =4.0 mV and those with an amplitude >4.0 mV., Conclusions: Most patients with CTS do not appear to have notable weakness of thumb abduction strength. A TIR < or =0.51 had high specificity for differentiating between CTS and controls. However, the performance of this device needs to be evaluated in a general population of patients who present with signs and symptoms of CTS before it would be clinically applicable.