How do I calculate the wrist-to-forearm ratio on ultrasound?

The wrist-to-forearm ratio isn’t about getting a “better” number—it’s about seeing whether the nerve changes size where it passes through the carpal tunnel. To do that, you measure the same nerve in two locations and compare them.

Start at the wrist crease and measure the median nerve in short axis, tracing just inside the perineurium. Then move about 12 cm proximal into the forearm, where the nerve lies between the flexor digitorum superficialis and flexor digitorum profundus muscles, and measure it again using the same probe angle and pressure.

Divide the wrist CSA by the forearm CSA. For example, a wrist measurement of 13 mm² and a forearm measurement of 9 mm² gives a ratio of 1.4. A ratio around 1.4 is often used as an upper limit of normal, with higher values suggesting focal enlargement at the wrist rather than a naturally large nerve.