Background. Law enforcement officers routinely conduct psychophysical tests to determine if an impaired driver may be intoxicated or in need of medical assistance. Testing includes assessments of eye movements, using the Horizontal Gaze Nystagmus (HGN), and Vertical Gaze Nystagmus (VGN) tests, which are conducted at roadside by patrol officers. Law enforcement officers trained as Drug Recognition Experts (DREY's) use the HGN and VGN test along with an additional test to assess drug impairment known as the Lack of Convergence (LOC) test. LOC will be present with intoxication due to certain drugs other than, or in addition to, alcohol. The HGN and VGN tests previously also have only been validated when the subject is placed in a standing posture with head upright. The LOC test previously has been validated in the same posture with head upright, but with a high number of false positives. However, certain conditions require that the subject be tested while seated or supine. The goals of the current study are to confirm the validity and reliability of HGN and VGN in the standing posture and to establish their validity and reliability in the seated and supine postures. It is also a goal to determine a criterion distance that reduces the number of false positives, and to establish the validity and reliability of the LOC test for standing, seated and supine postures.
Methods. The study was conducted at alcohol workshops in the Pacific Northwest. Ninety-six volunteer drinkers were tested when sober and three times after drinking alcohol by 40 volunteer officers experienced in administering the tests. Blood alcohol concentration (BAC) was measured objectively with a calibrated breath analysis instrument each time a subject was tested.
Results. The number of eye movement clues observed during the HGN test at any posture increase with increasing BAC. The presence of VGN at any test posture occurs only at high levels of intoxication, as defined for the individual subject. The presence of LOC at any test posture increases with increasing BAC. A criterion distance of 3 in (8 cm) from the bridge of the nose reduces the number of false positives observed.
Conclusions. The HGN test administered in t'he standing, seated, and supine postures is able to discriminate intoxication at criterion BAC's of 0.08 and 0.10%. The HGN test also is able to discriminate intoxication at BAC's below 0.08%. The VGN test is able to identify high levels of intoxication at any test posture. Therefore, these tests can be used by an officer to determine if a driver is intoxicated regardless of whether the driver is standing, seated, or supine. With the new criterion distance, the LOC test administered in the standing, seated, and supine postures aids in the discrimination of intoxication at criterion BAC9s of 0.08 and 0.10%, as well as at BAC's below 0.08%. The LOC test should, therefore, be able to aid in the detection of intoxication due to drugs other than alcohol that have similar effects on convergence. This test can be used by a DRE to determine if a driver is intoxicated regardless of whether the driver is standing, seated, or supine.
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