How Police Test for Blood Alcohol Content: A Detailed Guide

When a police officer suspects a driver is under the influence, they initiate a carefully structured investigation that involves a series of tests designed to measure impairment and establish Blood Alcohol Content (BAC). These procedures are standardized to ensure consistency and to meet the legal requirements for evidence in a DUI case. Understanding these methods can clarify the difference between a preliminary field assessment and a legally admissible evidentiary test.

This guide breaks down the primary methods law enforcement uses to test for alcohol impairment, from the initial traffic stop to the final chemical test.

The Investigation Process: A Step-by-Step Approach

A typical DUI investigation follows a three-phase process developed by the National Highway Traffic Safety Administration (NHTSA):

1. Phase One: Vehicle in Motion. The officer observes the driver's operation of the vehicle, looking for signs of impairment such as weaving, swerving, or braking erratically.
2. Phase Two: Personal Contact. After the traffic stop, the officer interacts with the driver, noting indicators like the smell of alcohol, slurred speech, or fumbling with documents.
3. Phase Three: Pre-Arrest Screening. The officer will likely ask the driver to submit to a series of tests to gauge their level of impairment.

This third phase is where the most well-known testing methods come into play.

Method 1: Standardized Field Sobriety Tests (SFSTs)

Before any chemical testing, an officer will typically administer a battery of three standardized tests known as the SFSTs. These are not tests of BAC, but rather tests of a person's ability to perform the divided-attention tasks required for safe driving.

The Three Standardized Tests:

1. Horizontal Gaze Nystagmus (HGN) Test: Nystagmus is an involuntary jerking of the eyes. This jerking can be exaggerated by the presence of alcohol. The officer will ask the driver to follow a stimulus (like a pen or a finger) with their eyes, looking for specific clues that indicate impairment.
2. Walk-and-Turn (WAT) Test: This is a "divided attention" test that requires the driver to listen to and follow instructions while performing a physical task. The driver is asked to take nine heel-to-toe steps along a straight line, turn in a specific manner, and return. The officer looks for eight distinct clues of impairment, such as starting too soon, stepping off the line, or using arms for balance.
3. One-Leg Stand (OLS) Test: This is another divided attention test where the driver is instructed to stand on one foot, with the other foot raised about six inches off the ground, and count aloud for 30 seconds. The officer looks for four clues, including swaying, hopping, or putting the foot down.

The results of the SFSTs, combined with the officer's other observations, are used to establish the "probable cause" needed to arrest the driver for DUI and require a chemical test.

Method 2: Breath Tests (Breathalyzers)

Breath testing is the most common method for estimating BAC. There are two main types of devices used:

Preliminary Alcohol Screening (PAS) Devices

• What they are: These are portable, handheld breathalyzers used by officers in the field during a traffic stop.
• Purpose: The PAS test is part of the pre-arrest screening. Its purpose is to help the officer determine if there is probable cause for a DUI arrest.
• Legal Standing: In most states, the results of a PAS test are not admissible in court to prove that a driver was over the legal limit, due to their lower accuracy compared to evidentiary devices. However, refusing a PAS test can still have consequences in some jurisdictions.

Evidentiary Breath Testing (EBT) Devices

• What they are: These are larger, more sophisticated desktop machines located at the police station, courthouse, or mobile testing facilities.
• Purpose: After an arrest, the driver will be asked to submit to an evidentiary test. The result of this test is admissible in court as evidence of their BAC.
• Procedure and Calibration: These devices are subject to strict regulations regarding maintenance, calibration, and operation. The officer administering the test must be properly trained and certified, and a specific observation period (usually 15-20 minutes) is required before the test to ensure mouth alcohol doesn't skew the results.

Method 3: Blood Tests

A blood test is the most direct and accurate method of measuring BAC.

• When it's used: A blood test may be used if the driver is injured and unable to perform a breath test, if the officer suspects the driver is under the influence of drugs, or in some cases, as an alternative to a breath test.
• Procedure: A qualified medical professional must draw the blood in a controlled environment. The blood sample is then sent to a crime lab for analysis, typically using gas chromatography.
• Legal Standing: A properly conducted blood test is considered highly accurate and is powerful evidence in court. Strict chain-of-custody procedures must be followed to ensure the sample's integrity.

Method 4: Urine Tests

Urine tests are less common for alcohol DUI investigations because they are less accurate for determining a driver's BAC at the specific time they were operating the vehicle. Urine alcohol concentration lags behind blood alcohol concentration. However, they are sometimes used, particularly in cases involving suspected drug use.

The Role of Consumer Tools

It's vital to understand that educational tools like an online BAC calculator or a personal breathalyzer have no standing in a legal context.

• BAC Calculator: This is a planning tool. It helps you estimate your BAC based on averages. It cannot tell you your actual BAC.
• Personal Breathalyzer: These devices are for personal education and awareness. They are not calibrated or maintained to the standards of law enforcement devices and their results cannot be used to challenge a legal test.

Conclusion: A System of Standardized Procedures

The process police use to test for BAC is a methodical one, designed to move from general observations of impairment to scientifically validated, legally admissible evidence. From the initial traffic stop to the final chemical test, each step is governed by laws and procedures that aim to ensure accuracy and protect the rights of the driver. The best way to navigate this system is to avoid it entirely by always planning for a safe and sober ride home.