One test the is sometimes employed in the field in order to determine sobriety is the Horizontal Gazy Nystagmus Test (HGN).  The idea of this test is that when an intoxicated person moves their eyes to the side, jerking of the eyes will occur before 45 degrees and the person will not be able to follow a slowly moving object smoothly with their eyes.  While this test is not used to determine an exact BAC, it can be used for “probable cause” and general evidence.  It is important that you consult with a Baltimore DUI Lawyer to understand the applicability and admissibility of the HGN test in your case.  Following are guidelines that are often given to officers in order to teach them how to administer the test correctly and also to teach them how to score the test (decide if you’re drunk or not):

How to Administer the Horizontal Gaze Nystagmus Test:

Give the DUI suspect the following instructions from a position of interrogation (that is, with your weapon away from the suspect):
1.    I AM GOING TO CHECK YOUR EYES. (Request that the DUI suspect remove glasses or hard contact lens at this time if they are being worn. Nystagmus is not influenced by how clearly the suspect can see the object he is to follow.)
2.    NOW KEEP YOUR HEAD STILL AND FOLLOW THIS (indicate what he is to follow) WITH YOUR EYES. DO NOT MOVE YOUR EYES BACK TO THE CENTER UNTIL I TELL YOU. (If the DUI suspect moves his head, use a flashlight or your free hand as a chinrest.)
3.    Check the suspect's right eye by moving the object to the suspect's right. Have the suspect follow the object until the eyes cannot move further to the side. Make this movement in about two seconds, and observe:
1.    whether the DUI suspect was able to follow the object smoothly or whether the motion was quite jerky; and
2.    how distinct the nystagmus is at the maximum deviation.
4.    Move the object a second time to the 45-degree angle of gaze, taking about four seconds. As the subject's eye follows the object, watch for it to start jerking. If you think you see nystagmus, stop the movement to see if the jerking continues. If it does, this point is the angle of onset. If it does not, keep moving the object until the jerking does occur or until you reach the imaginary 45-degree angle. Note whether or not the onset occurs before the 45-degree angle of gaze. (The onset point at a BAC of 0.10 percent is about 40 degrees).

If the DUI suspect's eyes start jerking before they reach 45 degrees, check to see that some white of the eye is still showing on the side closest to the ear, as in the photograph. If not white of the eye is showing, you either have taken the eye too far to the side (that is, more than 45 degrees) or the person has unusual eyes that will not deviate very far to the side. Use the criteria of onset before 45 degrees only if you can see some white at the outside of the eye.

Repeat this entire procedure for the DUI suspect's left eye. When observing the left eye at 45 degrees of gaze, some white of the eye again should be visible at the outside (closest to the ear) of the eye.

NOTE: Nystagmus may be due to causes other than alcohol in three of four percent of the population. These other causes include seizure medications, phencyclidine (PCP), barbiturates and other depressants. A large disparity between the performance of the right and left eye may indicate brain damage.

Scoring
Watch for three signs on intoxication in each eye. Give one point for each item checked for a maximum of six points.

1.    Onset of alcohol gaze nystagmus in the right eye occurs before 45 degrees. Do not score this item unless some white is visible on the outside of the right eye (closest to the ear) at the point of onset.

2.    Nystagmus in the right eye when moved as far as possible to the right is moderate or distinct. Do not score this item if you only see the faint jerking that occurs at the onset point.

3.    The right eye cannot follow a moving object smoothly. If you score this item, be sure that the jerkiness was not due to your moving the object in a jerky manner.

4.    Onset of alcohol gaze nystagmus in the left eye occurs before 45 degrees. If you score this item, be certain that some white is visible on the outside of the left eye (closest to the ear) at the point of onset.

5.    Nystagmus in the left eye when the eye moves as far as possible to the left is moderate or distinct.

6.    The left eye cannot follow a moving object smoothly.
If the DUI suspect scores four or more points out of the six possible points on this test, classify his BAC as above 0.10 percent. (NOTE: Yes, the limit for Kansas is .08, but when the tests were studied, they were used to establish .10). Using this criterion you will be able to correctly classify about 77 percent of you suspects with respect to whether they are drunk driving. That probability was determined during limited laboratory and field-testing and is given simply to help you weigh the various sobriety tests in this battery as you make your arrest decision.
Horizontal Gaze Nystagmus Test Scoring Sheet
•    Nystagmus in right eye is moderate or distinct when eye is moved as far as possible to the right.
•    Right eye cannot follow moving object smoothly.
•    Onset of gaze nystagmus in right eye occurs before 45 degrees (some white is visible).
•    Nystagmus in left eye is moderate or distinct when eye is moved as far as possible to left.
•    Left eye cannot follow moving object smoothly.
•    Onset of gaze nystagmus in left eye occurs before 45 degrees (some white is visible)(Decision Point: 4 Clues)