[USFRIENDINVICTORIA]

Breathalyzer tests can be off by +/- 20% or more. Even assuming the best equipment, and proper administration; a breathalyzer is not a perfectly reliable measure of BAC. That's because breathalyzers don't measure blood alcohol content -- they measure the concentration of alcohol vapour in the lungs, and apply a mathematical formula to that measurement, to come up with an estimated BAC.

If you've already submitted to a breathalyzer (and, assuming you haven't abused other drugs); and failed that test by a small margin -- it might actually be in your interests to demand a blood test. The blood test might exonerate you. (Don't do this if you know you've been drinking a lot -- the blood test may also give you a higher reading.)

[2harddrive]

Breathalyzer tests can ALSO be manipulated by the officer, if he or she knows how, to yield a false positive - indicating acute intoxication in someone with a true 0.00 level.