The calculation of the distoproximal ratio allows comparison of a sensory nerve action velocity (SNCV) in the distal median nerve segment (SNCV digit III – wrist) with that of the proximal segment (SNCV palm – wrist) of the same nerve. The distoproximal ratio is calculated comparing digit-palm and palm-wrist sensory nerve conduction velocities (SNCV) as follows:
Distoproximal ratio = digit-palm (m/s) / palm-wrist (m/s)
In normal hands, the SNCV in the proximal segment (palm – wrist) was higher than the SNCV of the distal segment (digit – palm), and it is always <1.0. This ratio is reversed (>1) in a patient with carpal tunnel syndrome (CTS).