Color desaturation may occur disproportionately to retention of acuity and form perception. No bright light tests are performed before red desaturation testing. Occlude the suspect eye. Using your white clinic coat as the background have the patient compare the color of the red cap with their good eye then with their suspect eye. Record the responses as equal, slightly dimmer, much dimmer, etc.
Positive response requires further neurologic testing, but optic nerve disease is suspect.
The patent is asked to look at the central cap and the patient is asked if either the central or peripheral cap is equal to or brighter than the other. In a normal eye the central cap should be equal to or brighter than the peripheral cap. If the patient reports the central cap as dimmer or duller red, optic nerve disease is suspected. The patient may report one cap as being faded . The same procedure should have already been performed on the non-suspect eye so the patient will have something to compare.