PBT vs. DRE Evaluations: Approaches to Determining Roadside Impairment

Impaired driving enforcement has evolved dramatically in recent decades, with states adopting different tools and strategies to help law enforcement identify drivers under the influence of alcohol, drugs, or even prescription medications. The most common approaches are Preliminary Breath Tests (PBTs) and Drug Recognition Expert (DRE) evaluations and almost every officer uses Standardized Field Sobriety Tests (SFST) as a baseline. Each of these represent different philosophies in how impairment is detected and documented. Some states embrace the use of some methods, while others restrict officers to one or the other. Understanding these approaches, their strengths, and their limitations is critical for law enforcement, state agencies, and organizations involved in workplace and traffic safety.

What Is a PBT?

A Preliminary Breath Tester (PBT) is a portable, handheld device used by law enforcement to estimate a driver’s (or individuals) blood alcohol concentration (BAC) during a roadside stop. These devices are small, battery-operated, and provide quick results in the field. Unlike evidential breath testing devices (EBTs), which are used in controlled environments like police stations and drug and alcohol collection sites, PBTs are designed for roadside use.

The PBT gives officers an objective measure of alcohol presence and approximate concentration, helping support decisions such as probable cause for arrest. However, results from a PBT are not always admissible as court evidence due to their portable nature and potential environmental variables that may affect accuracy.

Preliminary Breath Test Approval

Approved PBT devices, such as the Intoximeters Alco-Sensor FST, must undergo rigorous testing and approval processes. This includes certification by the National Highway Traffic Safety Administration (NHTSA), followed by state-specific approvals before law enforcement can deploy them in the field. Some states maintain their own lists of approved devices, ensuring that officers only use units that meet performance and reliability standards. We’ve included a short-list at the end of this blog for your reference.

What Is a DRE?

The Drug Recognition Expert (DRE) program takes a very different approach. Instead of relying primarily on an alcohol measuring device, it relies on specially trained officers to recognize and document signs of impairment caused by alcohol, drugs, or other substances.

DREs undergo an extensive training program overseen by the International Association of Chiefs of Police (IACP) with support from NHTSA. The training equips officers to systematically evaluate subjects through a 12-step protocol. This includes:

• Documenting observations in a structured, legally defensible format.
• Checking vital signs (pulse, blood pressure, body temperature).
• Conducting psychophysical and divided-attention tests.
• Evaluating eye signs beyond the Horizontal Gaze Nystagmus (HGN) test.
• Interviewing the subject and considering medical conditions.

DREs fill the gap that SFSTs and PBTs cannot cover: detecting impairment from drugs or prescription medications that don’t show up on an alcohol test. Forming an expert opinion on whether the individual is impaired and by what category of drug. The DRE system is particularly valuable when dealing with drug-impaired drivers, where a PBT may return a negative alcohol reading even though the subject shows clear signs of impairment. DREs are trained to recognize categories of drugs, such as depressants, stimulants, hallucinogens, and narcotics, that affect motor and cognitive function.

Honorable Mentions: What is the Standardized Field Sobriety Test (SFST)?

The Standardized Field Sobriety Test (SFST) is a set of three standardized tests designed to identify signs of impairment and help establish probable cause for arrest. Serving as the foundation of roadside impairment enforcement across the nation, the SFST was developed and validated by the National Highway Traffic Safety Administration (NHTSA).

This standardized series includes three key tests:

1. Horizontal Gaze Nystagmus (HGN) – Officers check for involuntary jerking of the eyes, which is exaggerated under alcohol impairment.
2. Walk-and-Turn (WAT) – Assesses balance, coordination, and the ability to follow instructions by requiring the subject to walk heel-to-toe, turn, and return.
3. One-Leg Stand (OLS) – Evaluates balance and divided attention as the subject stands on one foot while counting aloud.

In Tennessee and other states that do not allow PBTs, SFSTs are critical for establishing probable cause. While they are highly validated for alcohol impairment, SFSTs are less conclusive for drug impairment, which is where DREs play a role.

States Divided: PBTs, DREs, or Both

The adoption of PBTs varies widely across the United States. California is an example of a state that allows the use of both PBTs and DRE evaluations. Officers can use the PBT for alcohol screening, while relying on DREs for cases involving suspected drug use. This combination gives law enforcement flexibility and strengthens roadside enforcement.

Other states, such as Tennessee, have not approved PBTs for roadside use. Instead, Tennessee relies heavily on DRE-trained officers to establish probable cause in impairment cases. States that restrict PBT use often cite concerns about accuracy, legal admissibility, and overreliance on devices instead of officer judgment. For these jurisdictions, the DRE approach provides a human-centered method that accounts for a wider range of impairment scenarios, including prescription medications or illicit substances for which a PBT would provide no data.

Pros and Cons of PBTs

Advantages of PBTs include speed, portability, and objectivity. In alcohol-related stops, a PBT can quickly give officers quantifiable evidence of alcohol in the body. This can save time, support probable cause, and reduce subjectivity in roadside decisions.

Limitations, however, are significant. PBTs do not detect the presence of drugs, they can be influenced by environmental conditions, and their results are often inadmissible in court. Officers must still follow up with evidential testing or a DRE evaluation to build a case.

Pros and Cons of DREs

The strength of the DRE program lies in its comprehensive evaluation process. A trained DRE can identify impairment from a wide variety of substances, including those not detectable by breath alcohol testing. DRE findings are typically well regarded in court due to the rigorous training and standardized methodology officer’s follow.

The downside is that DRE evaluations are time-intensive and resource-heavy. Training officers to become DREs requires significant investment, and not every jurisdiction has the personnel or funding to field a large DRE team. Additionally, DRE assessments, while systematic, remain somewhat subjective and can be challenged in court.

Pros and Cons of SFST

In states, such as Tennessee, where preliminary breath tests (PBTs) are not authorized, law enforcement officers rely heavily on Standardized Field Sobriety Tests (SFSTs) and Drug Recognition Experts (DREs) to assess impairment. The SFST battery, Horizontal Gaze Nystagmus, Walk-and-Turn, and One-Leg Stand, provides a structured and nationally recognized method for evaluating motor coordination, balance, divided attention, and physiological cues linked to alcohol or drug use. These tests are accessible, cost-free beyond training, and give officers a standardized foundation for establishing probable cause.

However, they are inherently subjective, influenced by an officer’s interpretation, environmental conditions, or an individual’s health, age, or nervousness. While effective for detecting alcohol, SFSTs are less reliable in identifying drug impairment, which is why Tennessee supplements them with specially trained DRE officers.

Why Some States Say “Yes” to PBTs and Others Say “No”

States that approve PBTs see them as a valuable roadside screening tool, giving officers objective BAC readings to support probable cause. These states often use a hybrid approach, combining SFSTs, PBTs, and DRE evaluations.

States that restrict PBTs, such as Tennessee, cite concerns over accuracy, calibration, and the limited scope of PBTs (alcohol only, not drugs). Courts in these states prefer stronger evidence from evidential breath or blood tests, supported by SFST and DRE findings.

States that restrict PBT use often point to three main factors:

1. Legal admissibility – Many courts are hesitant to allow PBT results due to concerns about calibration and environmental variability.
2. Accuracy concerns – Portable devices may be less reliable than evidential instruments used in controlled settings.
3. Broader scope of impairment – Since PBTs only detect alcohol, states may prefer a DRE-based system that covers drugs, alcohol, and prescription medication.

Where the Future Is Headed

As impaired driving continues to pose a major public safety risk, many states are reevaluating their approaches. The most effective programs may involve a hybrid strategy, using PBTs for quick alcohol screening, combined with DRE evaluations and SFST for broader drug impairment detection.

Manufacturers like Intoximeters, with products such as the Alco-Sensor FST, continue to work with NHTSA and states to ensure devices meet strict reliability and performance standards. At the same time, DRE programs expand as more officers receive training, ensuring that even in states where PBTs are limited, officers remain equipped to handle complex impairment cases.

Bottom Line

Whether through technology like PBTs or human expertise via DREs, law enforcement agencies face the challenge of balancing objectivity, accuracy, and practicality when addressing impaired driving. Some states lean on the reliability and breadth of the DRE program, while others embrace the efficiency of PBTs alongside DRE evaluations. For agencies, policymakers, and companies providing these tools, understanding both approaches, and their pros, cons, and legal contexts, is essential to ensuring safer roads and effective enforcement.

• Even in states that permit PBT use, PBT results are rarely, if ever, admissible as the sole evidence for conviction. They are primarily used for establishing probable cause or screening.
• States that don’t authorize PBTs often cite legal and scientific concerns: reliability, potential device error, environmental effect, calibration, and the possibility that a PBT result could be misused or challenged.
• Where PBTs are used, there’s nearly always parallel reliance on DRE evaluations, especially when drug impairment is suspected.
• Device manufacturers and agencies interested in deploying PBTs must ensure devices are approved by the relevant state authority (or forensic lab), operators are certified, calibration and maintenance are rigorously maintained, and policies/law allow for their defined use.
• States like Tennessee have doubled down on officer training and observation, positioning SFSTs as the cornerstone of DUI enforcement while relying on DRE expertise to address the limitations of alcohol-centric testing methods. This approach reflects a broader tension between the objectivity that devices like PBTs can offer and the legal scrutiny they attract, versus the flexibility, but subjectivity, of officer-administered roadside assessments.

State-by-State Snapshot: PBT Use vs Reliance on DRE / Evidential Only

Below is a state-by-state breakdown showing which states allow Preliminary Breath Tests (PBTs) for law enforcement, vs. those that do not clearly permit them (or rely more heavily on DREs or evidential tests).

It should be noted that the data is not complete, as laws change, and some states have mixed policies, but this gives a good snapshot.

States with No Clear PBT Authorization / Emphasis on DRE or Evidential Only