Tests for De Quervain's syndrome include both imagery tests and diagnostic tests.
Imagery tests, such as x-rays or MRIs can be used to look for signs of inflammation or a nodule on the tendon in question. They are also helpful in ruling out other conditions you may be suffering from. An X-ray can help identify bone spurs or other bone related malformations that may be causing the pain or contributing to the irritations. An MRI can see softer tissue than an X-ray and may pick up nodules or swollen areas of the tendon, synovial sheath or tensynovium.
The most common diagnostic test for De Quervain's syndrome is Finkelstein's test. In Finkelstein's test, your doctor or physical therapist will direct you to make an up and down fist, like when you start playing "Rock, Paper, Scissors", with your thumb tucked into your palm and your fingers wrapped around it tightly. The wrist is flexed so that the top of the fist is in the same plane as the forearm.
The wrist is then further flexed downward elongating the top of the fist and stretching the tendons that run along the outside of the thumb through your wrist. If you have De Quervain's syndrome you will experience pain on the outside of the thumb or directly below your thumb on the wrist. The pain may be severe.
Additionally a biopsy can be performed to test for scar tissue or inflamed tissue of the tendon or the tenosynium , though this is usually not necessary.