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

The reliability of this test depends on the proper administration, proper scoring, and proper training being adhered to. The degree of reliability attached to this test is 77 percent. Based upon the officer's training, a score of four points out of a maximum of six, instructs the officer to reach a decision point to arrest the suspect for a violation of the New Jersey Drinking Driving Law. It should be noted that false positive and negatives can occur. To the best of my knowledge, approximately three or four subject's during State Police controlled drinking exercises have scored zero points on this test, yet had a BAC above a .10% and showed little or no other signs of intoxication. This test is not valid with a subject in a supine position. 

It can be administered to a seated or standing individual as long as they are erect and outside the vehicle. It should not be administered while a person is seated in their vehicle, especially with the window closed. At an accident scene, with a suspect lying down this test would be invalid due to possible positional alcohol nystagmus being the cause of the nystagmus encountered. This is a vestibular type of nystagmus that is evident when the amount of alcohol in the vestibular system is in unequal proportions to the amount of alcohol in the bloodstream and reacts to gravity such as the changing position of the head.