This description of the Horizontal Gaze Nystagmus test posted by California DUI Lawyer.

Eyeglasses must be removed in order to make a more accurate determination. Eyeglasses may impede the person's peripheral vision and may impede the cop's ability to observed eye carefully. Recent information reveals that Contacts do not have to be removed.. The officer is trained to make note of the presence of contact lenses. A person without vision in one eye, such as a glass eye should not be given this test. 

NOTE:  IF THE SUSPECT HAS AN OBVIOUS EYE DISORDER OR ARTIFICIAL EYE, HGN SHOULD NOT BE ADMINISTERED

It is improper to evaluate one eye and double the score. You cannot assume that the other eye will render the same results. With the lazy eye condition, the officer is trained not to administer this test. Recent information indicates that color blindness does not affect this test. Certain individuals have a pathological nystagmus which is normal, and natural for that person. 

Three to four percent of the general population have a natural or normal nystagmus without having consumed any alcohol at all. In the Final Report of March 1981 entitled "Development And Field Test Of Psychophysical Tests For DWI Arrest" which was prepared for the U.S. D.O.T. National Highway Traffic Safety Administration" (NHTSA), this report advises that nystagmus could be the result of brain damage, of illness {e.g., Korsakoff's syndrome} or of unknown etiology. A large disparity between the eyes may indicate a medical problem.