Test-retest reliability of combined sensory index: implications for diagnosing carpal tunnel syndrome

When compared to single nerve conduction tests, the combined sensory index (CSI) has been shown previously to have superior sensitivity and specificity for diagnosing carpal tunnel syndrome. The CSI is the sum of (1) median-ulnar ring finger antidromic latency difference at 14 cm (ring-diff); (2) median-radial thumb antidromic latency difference at 10 cm (thumb-diff); and (3) median-ulnar midpalmar orthodromic latency difference at 8 cm (palm-diff). To evaluate the reliability of single nerve conduction tests (ring-diff, thumb-diff, and palm-diff) vs. a combined summary variable (CSI), we conducted a prospective study during which the same investigator performed test and retest sessions on one hand of 32 subjects. Results showed that the CSI had the highest test-retest reliability (Spearman rho = 0.95), when compared with ring-diff, thumb-diff, and palm-diff (Spearman rho = 0.67, 0.75, and 0.74, respectively). Therefore, we propose the use of the CSI as not only an accurate but also a reliable method for diagnosing carpal tunnel syndrome.