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The aim of this study is to assess whether additional median -to-ulnar comparative tests will improve the diagnostic rate of carpal tunnel syndrome (CTS).We recruited 248 hands of 162 CTS patients, and 166 hands of 83 controls. One hundred and sixty-eight (68%) symptomatic hands had abnormal median distal latencies or palm-wrist latencies. We performed three additional comparative tests in the remaining symptomatic hands and the non-CTS hands. The first test compared median distal motor latency (MDL) recorded from the second lumbrical muscle (2L) and ulnar distal latency recorded from interossei muscles (INT) (2L-INT). The second test compared median and ulnar antidromic sensory latencies (MS-US). And the third test compared median and ulnar nerve latencies in the palm-to-wrist segment (PM-PU).In control subjects, upper limits of median-to-ulnar differences were: 2L-INT= 0.4 ms, MS-US= 0.5 ms, PM-PU= 0.4 ms. In CTS patients with normal conventional electrodiagnostic methods, MS-US difference showed the lowest sensitivity (21.3%). The diagnostic sensitivity of 2L-INT was 27.5% and PM-PU 47.5%. With PM-PU test, additional 15.3% diagnostic rate could be got.For CTS patients with normal results from the standard methods, PM-PU is a good additional comparative test to further improve diagnostic rate.