Blood Tests vs. Breathalyzer Tests: Pros and Cons

In the realm of DUI and DWI law, the accuracy and reliability of sobriety tests are crucial for both prosecution and defense. Two of the most commonly used methods to establish evidence of impairment are blood tests and breathalyzer tests. In this article, we will examine the merits and drawbacks of each, shedding light on how these tests may influence a DUI/DWI case.

Breathalyzer Tests

When you imagine a driver being pulled over on suspicion of DUI or DWI, the image of a police officer holding a breathalyzer to the driver’s face certainly comes to mind. These machines are often thought of as a means to definitively determine whether a driver is intoxicated or not, but the reality is more complicated than that.

Let’s briefly discuss the upsides and downsides of the use of breathalyzer tests during routine traffic stops.


  1. Quick and Convenient: Police officers can administer breathalyzer tests on the spot, providing immediate results that enable swift decision-making, rather than having to administer a lengthy blood test at a separate location from the initial traffic stop.
  2. Non-Invasive: The test merely requires the individual to blow into a device, making it less invasive compared to blood tests. The non-intrusive nature of a breathalyzer test may result in suspected drivers being more willing to use it, simplifying the process for all interested parties.
  3. Cost-Effective: Breathalyzer tests are relatively inexpensive, making them a practical choice for routine traffic stops. Obviously, blood tests would be impractical to perform on the side of the road.


  1. Calibration Concerns: The machines require meticulous calibration. Improper calibration can yield inaccurate results. Breathalyzers have been proven to yield inaccurate results with a margin of error of up to 40%
  2. Interfering Substances: Mouth alcohol, certain medications, and even some foods can potentially skew results. Something as benign as cough syrup can lead to a blood alcohol content spike, despite the substance doing nothing to affect your ability to drive.
  3. Operator Error: The administering officer must follow a strict procedure. Deviation from this protocol can compromise the test’s reliability, which can lead to an arrest and a charge, despite the improper procedure by the arresting officer.

While cheaper and more convenient, breathalyzer tests have an alarming margin of error, estimated around 40%, which means that a faulty result could result in an arrest, fines, legal representation fees, and other headaches that you will have to contend with, even if your blood alcohol content was not truly above the legal limit.

Now, let’s take a look at blood tests.

Blood Tests

While a blood test is an option that can be asked for at the time of the traffic stop, this requires the refusal of a standard breathalyzer test; doing so will immediately incur a DUI charge, which may lead to an arrest. While later court proceedings may clear you of your charge as a result of the blood test, you will most likely be required to deal with the consequences of a DUI arrest in the immediate future.


  1. Accuracy: Blood tests are often considered the “gold standard” because they directly measure blood alcohol content (BAC) or the presence of drugs. These tests do not suffer from a wide margin of error as breathalyzer tests do.
  2. Comprehensive: These tests can detect a wide array of substances, beyond just alcohol, providing a fuller picture of impairment. After all, alcohol is not the only substance with the capacity to impair a driver’s ability to operate a vehicle.
  3. Storage of Evidence: Blood samples can be stored and retested, offering the opportunity for independent verification by the defense. This means that an unfavorable result is not always the final one.


  1. Intrusive: Blood tests are more invasive, requiring the puncturing of the skin and drawing blood, often in a medical setting. 
  2. Time-Consuming: The need to transport the individual to a medical facility and await results can introduce delays. Testing blood vials is also a slow process, elongating the rest of the DUI/DWI ordeal.
  3. Risk of Contamination: Improper handling or storage can contaminate the sample, potentially leading to a false positive. The alcohol used to sanitize the skin around the puncture wound from the syringe can actually contaminate the results as well, which is always important to keep in mind.
  4. Timing of the Test: A common defense that DUI/DWI attorneys will argue is that a driver’s blood alcohol content will change in the time following the consumption of alcohol. As time passes, the driver’s BAC will spike before decreasing, meaning that a breathalyzer test taken immediately after consuming alcohol may produce results below the legal limit, whereas a blood test taken later will produce results during the BAC spike, leading to a DUI/DWI charge.

While far more thorough than its counterpart, blood tests come with a slew of time-consuming processes that may extend the duration of your legal battle by days or even weeks. To the defendant, an accurate portrait of his or her blood alcohol content at the time of the arrest may be worth the wait.

Legal Implications

The choice between blood and breath tests can have profound implications for DUI/DWI cases. Defense attorneys often scrutinize the administration of these tests to identify any procedural or technical errors that could invalidate the results. Therefore, understanding the pros and cons of each can be crucial for both prosecution and defense.

Ultimately, an attorney will know the right course of action to take should you find yourself on the receiving end of a DUI/DWI charge.

What You’ve Learned

Both blood tests and breathalyzer tests have their strengths and weaknesses, and the choice between the two will often depend on the specific circumstances of a DUI/DWI stop. Understanding these nuances can be invaluable for attorneys who specialize in DUI/DWI cases, as well as for the individuals they represent.

If you’re facing a DUI charge, then fill out the form at the top of this page or call (800) 384-5297 for a free consultation with a qualified DUI attorney.