[bratgpttamer]

This may be true in some situations, but in others, it can result in automatic suspension of your license, and/or they can just get a warrant for a blood test.

I've also seen people convicted because, despite several hours' delay in obtaining a BAC, they were able to extrapolate back to the time of the accident. (There is apparently some dispute among forensic professionals on the validity of this, which implies the extrapolated value could be higher than actual)

[xkcd-sucks]

Originally it was thought that alcohol metabolism had zero order kinetics (alcohol concentration goes down over time linearly, irrespective of dose/concentration) and now there is evidence to suggest it's first order kinetics at least in some regimes (rate of disappearance depends on instantaneous concentration). So the practical implication is they need to do a few more blood draws over time to extrapolate concentration a few hours ago:

    - Lazy (old model): Take one blood sample, assume all people metabolize alcohol at the same rate, add delay time * generic zero order constant
    - Not lazy (old model): Take 2 blood samples to determine that person't zero order constant + add delay time*K to  original sample
    - New model: Take 3 blood draws, do a quadratic curve fit, determine original concentration from that