Category Archives: Library

Nystagmus: “The Eye Test” (part 3)

As indicated in previous posts, there are three parts of the nystagmus test.  But it is the angle of onset segment which is most critical – primarily because the “distinct” nystagmus and “smooth pursuit” in the other two tests are fairly subjective, while an angle has a certain mathematical nicety to it.

However, the officer’s ability to estimate this angle is critical.  The nystagmus test is premised upon a formula that requires the angle of onset to be subtracted from 50 to obtain a very rough estimation of blood-alcohol concentration (BAC).  An angle of 45 degrees from center, for example, may indicate a possible .05% blood-alcohol concentration; anything before that — for example, 43 degrees indicating .07% — results in a “failure”.  Clearly, if the officer is mistaken in his “guesstimate” by only 5 degrees, a true 47 degree (.03% BAC) ”pass” becomes an observed 42 degree (.08% BAC) “fail”.

So how does the officer measure the angle of onset with precision?

He doesn’t.  At best, he is giving a very rough estimate.  Recognizing the importance of the officer’s skill in estimating angles of onset, the National Highway Traffic Safety Administration recommends that officers use an angle-measuring template and practice with four or five subjects: “Check yourself monthly with the device to be sure that your accuracy has been sustained.”

The simple fact is that no officer actually does this; the last time he used a protractor to estimate angles was in the police academy.  The most common method actually used is to assume that the 45-degree angle from the eye intersects the held object at the suspect’s shoulder: if nystagmus is observed before the pencil or finger reaches a line projecting straight out from the edge of the shoulder, the suspect “fails”.  Very simple.  Of course, the fatal flaw to this method (other than now requiring two estimations) is that we all have different shoulder widths.

In an interesting crime lab study reported in 25 Journal of the Forensic Society 476, 12 police officers measured the angle of onset of nystagmus in 129 actual cases where DUI suspects had been arrested but had not yet been tested by blood, breath or urine.  The officers used a special protractor to help them accurately measure the anlge of onset.  Result?  Even with the aid of a protractor, they consistently over-estimated the angle at low BAC levels and underestimated it at high BACs.  The researchers from the police crime lab concluded that nystagmus cannot be used to accurately predict blood alcohol concentration.

Nystagmus: “The Eye Test” (part 2)

I mentioned in the previous post that few officers understand the nystagmus test, administer it correctly, or score it objectively. Further problems with using the nystagmus test in DUI investigations have been summarized by a noted expert in the area, Dr. L. F. Dell’Osso, Professor of Neurology at Case Western Reserve University School of Medicine and Director of the Ocular Motor Neurophysiology Laboratory at the Veteran’s Administration Medical Center in Cleveland:

Using nystagmus as an indicator of alcohol intoxication is an unfortunate choice, since many normal individuals have physiologic end-point nystagmus…Without a neuro-opthalmologist or someone knowledgeable about sophisticated methods of eye movement recordings, it is difficult to determine whether the nystagmus is pathologic. It is unreasonable that such difficult judgments have been placed in the hands of minimally trained officers. Dell’Osso, "Nystagmus, Saccadic Intrusions, Oscillations and Oscillopsia", 147 Current Neuro-Opthalmology 147.

See also an interesting article by Umeda and Sakata entitled "Alcohol and the Oculomotor System", 87 Annals of Otology Rhinology 69, wherein scientists concluded that gaze nystagmus was one of the least sensitive eye measurements of alcohol intoxication. The nystagmus which officers are trained to believe indicates intoxication is naturally present in some individuals without the presence of alcohol. It can also be caused by many other factors, as the Supreme Court of Kansas has noted after a review of the scientific literature:

Nystagmus can be caused by problems in an individual’s inner ear…. Physiological problems such as certain kinds of diseases can result in gaze nystagmus….Furthermore, conditions such as hypertension, motion sickness, sunstroke, eyestrain, eye muscle fatigue, glaucoma, and changes in atmospheric pressure may result in nystagmus. The consumption of common substances such as caffeine, nicotine, or aspirin also lead to nystagmus almost identical to that caused by alcohol consumption. State v. Witte, 836 P.2d 1110.

Obviously, drinking coffee and smoking cigarettes are not unusual. And note that most DUI arrests occur late at night — just when eyestrain and eye muscle fatigue are most expected. More on nystagmus to come….

Nystagmus: “The Eye Test”

You may have heard of the "eye test" in DUI investigations.  This is the nystagmus field sobriety test or, more accurately (there are 47 different kinds of nystagmus),  the horizontal gaze nystagmus test.  It is one of the most commonly used field sobriety tests, as it is one of three which make up the federally-approved "standardized battery" of tests, or SFSTs.

The test is essentially a measurement of the movement of the eye.  Simply stated, "nystagmus" refers to a distinctive involuntary jerking of the eyes; horizontal gaze nystagmus is a pendular (back and forth) movement.  This type of nystagmus is commonly measured by the officer in three three different ways, each time using an object such as a pencil, penlight or finger placed a foot or so in front of the suspect’s nose and moving it slowly to the left and then to the right.

The first part of the test is to determine the angle of onset of nystagmus — that is, the angle at which the moving eye begins the jerking motion.  The suspect looks straight ahead and, without moving his head, moves his eyes slowly to the right or left.  The officer is supposedly able to detect when the nystagmus begins and is supposedly able to estimate the angle from straight ahead at the point where it begins.  If the onset is prior to 45 degrees, in theory, the blood alcohol level will be over .05%.

The second part of the test is to note whether the jerking becomes more "distinct" when the eye is moved to the lateral extreme — that is, when there is no longer any white of the eye visible at the outside of the eye.

The third part is to determine whether there is a lack of smooth pursuit: rather than following a moving object smoothly from the beginning, the eye jumps or "tugs".

Under federal standards, the officer is supposed to use an objective scoring criteria for each of the three tests, and the total score determines whether the supect passed or failed. 

In reality, few officers understand the test, administer it correctly, or use objective scoring.  Many simply report that they "detected the presence of nystagmus", and subjectively count that as a failure.  It is, however, the characteristics of nystagmus, not the simple presence,  which is relevant to determining possible impairment.  And, unfortunately, many things cause nystagmus and some of us have it under normal conditions.

More on nystagmus next week….

How Breathalyzers Work

Did you ever wonder how breathalyzers work? There is a website which will give you a pretty fair idea. But first, let’s clear up some confusion….

There are many different kinds of “breathalyzers” — or, more accurately, there are many kinds of breath testing devices. The first of the modern breath testers, manufactured by Smith and Wesson many years ago (yes, that Smith and Wesson), was called the Breathalyzer. Since then, various manufacturers have recognized the growing market and come out with their own models, bearing such names as Intoxilyzer, Intoximeter, DataMaster, AlcoSensor, Alcotest and so on; most of these products have been produced in different model versions, such as the Intoxilyer 4011, 5000 and 8000.

To deal with the confusion, the term “breathalyzer” came to be used as a generic term for any breath testing instrument. (To confuse things further, a German company — Draeger — bought the rights to the Breathalyzer brand and have sometimes used that name in some of their models.)

Most of these are evidentiary machines — that is, larger machines generally kept at the police station whose test results are used in evidence. Others are smaller, handheld units carried by officers in the field; generally called PBTs (preliminary breath tests) or PAS (preliminary alcohol screedning), these are less accurate and are usually used as a field sobriety test to help determine whether to arrest a suspect.

The original Breathalyzer operated using a wet chemical method of analysis, employing a disposable glass ampoule of chemicals. Although still occasionally found in law enforcement, this relatively primitive technology was replaced in later machines by infrared spectroscopy, gas chromatography or, mainly in handheld units, fuel cell analysis; a couple of the more recent machines use a combination of infrared and fuel cell.

Now that this has been cleared up, you might want to visit the following sites to understand the actual workings of these gizmos:

Chemical (the Breathalyzer)
Infrared spectroscopy (the Intoxilyzer)
Fuel cell (the Alcosensor)

Note: Gas chromatography is rarely encountered anymore, as it was primarily used in the discontinued Intoximeter 3000.

Note #2: To further understand why these machines aren’t nearly as accurate as law enforcement would have you believe, visit a few of my previous posts:

Breathalyzers — and Why They Don’t Work
Breathalyzer Inaccuracy: Testing During the Absorptive Stage
Breathalyzer Inaccuracy: Post-Absorptive
Breathalyzer Inaccuracy….It Gets Worse
“Close Enough for Government Work”
Why Breathalyzers Don’t Measure Alcohol
How to Fool the Breathalyzer
Breathalyzers and Radio Frequency Interference
Breathalyzers: Why Aren’t They Warranted to Measure Alcohol?

DUI Sobriety Checkpoints: The Slippery Slope

In a number of previous posts I’ve discussed the dubious efficacy, statistics and constitutionality of DUI roadblocks, aka "sobriety checkpoints". However, I’ve only briefly mentioned the potential for abusing the right to set up these roadblocks — the potential for government to use them as a pretext to violate citizen’s rights. Consider the following story from yesterday’s Washington Post:


Safety Stops Draw Doubts

D.C. Police Gather Nonviolators’ Data

Lisa Davis had done nothing wrong. She was wearing a seat belt, was obeying the speed limit and produced a valid driver’s license when D.C. police pulled her over one recent night at a traffic safety checkpoint in a crime-plagued neighborhood.

Even so, an officer jotted down some basic information before letting her go, including her name, address and the time and location of the stop for a police database used for crime solving.

"I’ve got some serious constitutional issues with that," Davis said as she sat in her idling Acura at the checkpoint at Kansas Avenue and Shepherd Street NW in the Petworth neighborhood. "I feel like it’s a violation of my rights. It’s a slippery slope to Big Brother."

The details about Davis and the stop will be fed into the database, which is linked to a computer that includes arrest records and mug shots of criminals….

Civil liberties advocates aren’t the only ones questioning the practice. The policy is sparking concern among some officers who conduct the checkpoint stops, most of which are made in areas where crime, not traffic safety, is the primary concern.

"That’s an invasion of privacy, demanding information from a citizen and putting that in a database," said Officer Gregory I. Green of D.C. police, who is assigned to represent the police union….

The officers jot down motorists’ information on a file-card-size form called a PD-76, which is recorded into the database. The forms also are used for routine traffic stops, and the information will also be used for a racial profiling study, (D.C. Police Chief Charles) Ramsey said.

D.C. homicide Detective Paul Regan said the collection of such data has been "a great intelligence tool".


In the 6-3 United States Supreme Court case holding that DUI checkpoints were permissible, Justice Rehnquist admitted that the stops constituted "seizures" within the meaning of the Fourth Amendment. However, he said, this was justified because "No one can seriously dispute the magnitude of the drunken driving problem or the States’ interest in eradicating it."

Would the Court would have ruled as it did had the justices known what these supposed "sobriety checkpoints" would eventually become?


(Thanks to Steve Oberman of Knoxville, TN.)