Jay Cohen, a Houston-based defense attorney specializing in DUIs, recently had a client with a strange case. When he got out to do the field sobriety test, the guy mostly seemed fine. But then he blew into a breathalyzer and ended up with a well-above-the-limit test.
What happened? “He was in great shape,” Cohen says. One of those guys who never misses a day at the gym. “So of course, he was on the keto diet.” Being in ketosis may have caused the breathalyzer to give a false positive. Armed with this info, Cohen was able to get the DUI dropped.
Here’s how that can happen: In ketosis, your liver breaks down fat for fuel, creating acetone as a byproduct. Some of that acetone is then released through your breath as isopropyl alcohol. The question is, can breathalyzers tell the difference between ethanol alcohol and isopropyl alcohol? It depends.
The inexpensive models that people buy for self-checking their BAC are probably not accurate if they’re on the keto diet, says Alan Wayne Jones, PhD, a professor in forensic technology at Sweden’s Linköping University. These devices rely on semiconductor technology. Inside the device there’s a metal film that measures the change in resistance depending on the number of molecules that hit that film, explains Keith Nothacker, the CEO of BACtrack, a California-based breathalyzer manufacturer.
Basically, regular air expelled from your lungs has a different quantity of molecules than air containing alcohol. From there, the device estimates your BAC, Nothacker says. But Dr. Jones has never seen any evidence that the devices can detect whether those molecules are coming from ethanol or isopropyl.
Most police, however, carry fuel cell-based breathalyzers in their cars—and here’s where things get complicated. Fuel cell breathalyzer manufacturers—including Nothacker—say their devices can tell the difference between ethanol and isopropyl. However, Cohen says he’s never seen peer-reviewed data showing that when either one substance or the other is present, this is indeed the case. Furthermore, “When you mix them together, which is what would happen if you were drinking and in ketosis, we don’t know that the fuel cell devices can differentiate,” he says.
One of the few papers that does exist on this topic was done by Dr. Jones. In 2006, he published a paper examining why a man on a low-carb diet was struggling to start a company vehicle, which was fitted with a breathalyzer ignition lock. “Under some circumstances, acetone can be converted in the body to isopropanol, and that is oxidized like ethanol by fuel cell breath analyzers,” he says. “If a person drank isopropanol, the fuel cell instruments probably could not distinguish it from ethanol, especially if it was combined with grain alcohol.”
Nothacker, however, disagrees. He says his company’s fuel cell breathalyzers are ethanol-specific and won’t create a current with isopropyl, which is what flags a positive result. “In 17 years of business,” he says, “it’s never been a problem.”
And it may not matter: In many states, the preliminary breath test results can’t be used in court. Generally, if you’re arrested for a DUI, they bring you into the station, where you’ll re-test on a machine that uses infrared spectroscopy. These machines can tell the difference between isopropyl and ethanol alcohol, Cohen says. In some states, you can also request a blood test (generally at your expense), which, again, can tell the difference between the two alcohols.
Also, let’s be clear: Claiming you’re on the keto diet while stumbling around during a field sobriety test is not going to be a get out of jail free card. Officers rely on instinct when sniffing out whether someone is drunk or not, says Ranolph Rice, a Baltimore-based DUI attorney. “First they start with the observation phase. Are there signs that lead us to the next point? Odor is the biggest sign, obviously,” he says. If you smell like booze, officers aren’t going to have any sympathy for whatever diet you may be on.
Furthermore, it’s probably rare for a keto diet to push you up and over the legal alcohol limit. If you’ve had a drink or two, maybe. But acetone alone probably can’t make you blow a .08, Rice says. He’s more concerned about clients who have had past DUIs, who now have breathalyzer ignition locks on their cars. In Maryland, those devices keep your car shut off at a much lower level than the DUI limit. “If you blow .02 or higher BAC, you’ve violated your interlock, and they’re going to add another month to your program,” Rice says. He theorizes that a .02 could, in theory, be achievable for a sober person on the keto diet.
Other medical conditions can cause breathalyzers to malfunction, too. Diabetes can cause false positives, for the same reason as the keto diet. GERD or acid reflux can be problematic too. “If you belch or vomit, you’ve got stomach alcohol. If that’s present in your mouth, it may read that,” Cohen says.
So what should you do if you’re pulled over? Cohen and Rice both have the same super-simple advice: Don’t drink and drive, ever. If you know you’re sober, ask to be taken back to the station to use the infrared spectroscopy machines. The blood test, however, is more complicated, Cohen says. While it won’t mark a false positive for being on a low-carb diet, it may catch something else—like medicines you take for ADHD or anxiety. “That’s not going to look good mixed with alcohol,” he says, even if you end up under the limit.
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