Rising Blood Alcohol Defense

The rising BAC defense is a legal strategy used in DUI cases to argue that a driver’s blood alcohol concentration was below the legal limit while driving, even if it tested above the limit later. This defense is based on the scientific fact that alcohol takes time to be absorbed into the bloodstream. In other words, a person’s BAC can continue to rise after they stop driving (during the interval before a breath or blood test is administered).

DUI defense attorneys employ this argument in situations where the timing of drinking is critical. For example, if a driver consumed their last drink very shortly before being pulled over, it's possible that their BAC was still increasing at the time of the stop. By the time the official test is conducted (often 20–45 minutes later), the BAC might have climbed above the legal threshold (0.08% in most jurisdictions), even though it was lower while the person was actually driving.

Scientific Basis

After ingestion, alcohol typically takes about 30 to 60 minutes (or longer, depending on individual factors) to fully absorb into the bloodstream and reach peak BAC. During this absorption phase, BAC levels are rising. Key factors influencing how quickly someone absorbs alcohol include:

Time of last drink: Drinks consumed right before driving may not hit peak effect until after driving.
Food in the stomach: A full stomach slows alcohol absorption, delaying the peak BAC.
Alcohol amount & strength: Larger quantities or high-proof drinks can extend the absorption period (and raise the eventual peak).
Biological sex: Differences in body composition and metabolism mean absorption rates can vary between men and women.
Body weight & metabolism: A person’s size and individual metabolic rate will influence how quickly BAC rises and falls.

In a rising BAC scenario, the individual might have been in the tail end of the absorption phase while driving. For instance, imagine a driver finishes several drinks at a bar and drives away immediately. At the moment of driving, their BAC might be, say, 0.06% (below the limit). Twenty minutes later, when tested, their BAC might measure 0.09% due to continued absorption. The rising BAC defense would assert that the legally relevant BAC (at driving time) was under the limit.

Legal Considerations

While scientifically plausible, the rising BAC defense requires convincing evidence of the drinking timeline. Defense attorneys often introduce testimony or records about when and how much the defendant drank, sometimes supported by expert toxicologists who can explain absorption curves. The goal is to introduce reasonable doubt about whether the defendant was actually over 0.08% at the time of driving, even if the later test was over the limit.

It’s worth noting that not all courts readily accept this defense. Because it can be difficult to prove the exact timing of peak BAC, judges and juries may be skeptical without strong corroborating evidence. Additionally, some jurisdictions have enacted "per se within two hours" laws, which essentially presume that a chemical test result above the limit—if taken within two hours of driving—reflects the BAC at the time of driving. These laws are designed to counter the rising BAC argument by limiting its applicability. Nonetheless, in cases where the facts support it, a rising BAC argument can be a potent part of a DUI defense strategy.

Interactive Absorption Illustration

The following is a simplified illustration of how timing can affect BAC. Adjust the slider to select how many minutes before driving the last drink was consumed:

Minutes since last drink when driving: 30 min

Nearing peak: About 30 minutes since last drink. The driver's BAC was likely near its peak around the time of the stop or shortly after, so the test BAC might be similar or only slightly higher than it was while driving.

*If the last drink was just before driving (a very short interval), there's a good chance BAC was still rising at the time of the stop. At longer intervals (well over an hour), the person would likely have already peaked and begun eliminating alcohol by the time they were tested, so a "rising BAC" explanation becomes less applicable.