August 4, 2014

Predicting blood alcohol concentration is tricky

Rasmus Bååth, who is doing a PhD in Cognitive Science, in Sweden, has written a web app that predicts blood alcohol concentrations using reasonably sophisticated equations from the forensic science literature.

The web page gives a picture of the whole BAC curve over time, but requires a lot of detailed inputs. Some of these are things you could know accurately: your height and weight, exactly when you had each drink and what it was. Some of them you have a reasonable idea about: is your stomach empty or full, and therefore is alcohol absorption fast or slow. You also need to specify an alcohol elimination rate, which he says averages 0.018%/hour but could be half or twice that, and you have no real clue.

If you play around with the interactive controls, you can see why the advice given along with the new legal limits is so approximate (as Campbell Live is demonstrating tonight).  Rasmus has all sorts of disclaimers about how you shouldn’t rely on the app, so he’d probably be happier if you don’t do any more than that with it.


Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »


  • avatar

    I’d be pretty wary about using a breathalyser in a drink-test-drink-test situation because for a given consumption of alcohol the breath concentration is a (lowercase) u shaped function of time.

    Suppose you consume one standard drink. Initially the breath reading is high because alcohol in the mouth triggers the sensor. In fact (and I’ve done this personally) a single mouthful of beer can give a reading over the limit (when I did it the crystals turned green).

    This level fades with time until the alcohol gets into the blood then the lungs whereupon the level rises again.

    Then as the alcohol is metabolised the breath reading drops.

    So that one standard drink can give any number of different breath readings depending on the time the reading is taken especially if the readings are taken within, say, the first half hour.

    The blood concentration isn’t u shaped. It just rises and falls. But remember that it keeps on rising after you have stopped drinking.

    3 years ago