How can I be sure I’m looking at the median nerve and not a tendon on
ultrasound?

At the wrist, nerves and tendons can look similar, especially in short axis. The median nerve has a honeycomb appearance made of dark fascicles surrounded by bright connective tissue.

Tendons, by contrast, look more like uniformly bright fibers, like looking straight at the bristles of a brush. Appearance helps, but it’s not always enough on its own.

The fastest way to confirm you’re seeing the median nerve is with a dynamic check. Ask the patient to wiggle their fingers. The flexor tendons will glide and change position, but the median nerve stays still and remains superficial to the tendons.

Perform this check in both short and long axis. If it doesn’t move with finger motion, you’re very likely looking at the nerve.