The two point discrimination test evaluates the presence of a lesion affecting the peripheral nerve, posterior column, thalamus or sensory cortex.
Use a device such as a blunted compass that can create two adjacent points of contact. Vary the application between single or double stimulations, asking the patient to indicate whether they experience one or two points of contact. Gradually reduce the separation between the points to determine the minimum distance that can be distinguished.
A positive finding involves the inability to distinguish two local but separate stimulations. The significant distance of separation that indicates abnormality depends on location: on the index finger, a distance of about 5mm should still be distinguishable; on the hallux, the critical distance should be about 10mm. The relevant peripheral nerve and/or nerve root should both be considered.
Factors affecting discrimination include skin thickness, and force and consistency of applied pressure. Consider other neurological test results including proprioception and joint position sense, which both involve the posterior column.