Weber Two-Point Discrimination Test
Two point discrimination is the ability to discern that two nearby objects touching the skin are truly two distinct points, not one. It is often tested with two sharp points during a neurological examination  and is assumed to reflect how finely innervated an area of skin is. In clinical settings, two-point discrimination is a widely used technique for assessing tactile perception. It relies on the ability and/or willingness of the patient to subjectively report what they are feeling and should be completed with the patient’s eyes closed.
The examiner uses a paper clip, two point discriminator, or calipers to apply pressure on two adjacent points in longitudinal direction or perpendicular to the long axis of the finger. The minimal distance with which the patient can distinguish between two stimuli is found by moving from proximal to distal. This distance is called threshold for discrimination. The area being tested must not be seen by the patient and the patient must concentrate on feeling the points. For accurate results, the hand must be immobile on a hard surface and it must be ensured that the two points are simultaneously touching the skin. The patient is asked to report whether one or two points was felt. The smallest distance between two points that still results in the perception of two distinct stimuli is recorded as the patient's two-point threshold. Performance on the two extremities can be compared for discrepancies. Although the test is still commonly used clinically, it has been roundly criticized by many researchers as providing an invalid measure of tactile spatial acuity, and several highly regarded alternative tests have been proposed to replace it.
There must be no skin blanching as it indicates too much pressure being applied. There may be an increase or decrease in the distance between the points depending on the patient's response. Starting distance between the points can be easily distinguished. If the patient is hesitant to respond or becomes inaccurate, the patient is required to respond accurately on several trials before the test can be repeated.
Normal discrimination distance recognition is less than 6mm, but it varies from person to person. This test is best for hand sensation involving the static holding of an object between the finger and thumb requiring pinch strength.
Normal and impaired performance
Body areas differ both in tactile receptor density and somatosensory cortical representation. Normally, a person should be able to recognize two points separated by 2 to 8 mm on fingertips. On the lips, it is 2 to 4 mm, and on the palms, it is 8 to 12 mm and 30–40 mm on the shins or back. The posterior column-medial lemniscus pathway is responsible for carrying information involving fine, discriminative touch. Therefore, two-point discrimination can be impaired by damage to this pathway or to a peripheral nerve.
Two Point Discrimination Value
- Normal <6mm
- Fair 6-10mm
- Poor 11-15mm
- Protective where only one point is perceived.
- Anesthetic where points are not perceived.
Weber's two-point discrimination test is the most common method used to of assess sensibility of the upper extremity. The amount of force applied between one and two points easily exceeds the resolution or sensitivity threshold for normal sensation. Tremendous variance in pressures applied resulted in poor levels of interrater reliability. This perhaps explains some of the lack of agreement in reporting discriminatory function. The number of correct responses required may vary slightly from examiner to examiner.
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