This question has been addressed in a number of ways in the literature. Analytical reliability of breath testing instruments has received much attention. Isolating sources of error inherent to the instrumentation, eliminating as many as possible and quantifying the rest within acceptable limits is frequently studied. The scientific community recognizes that testing of human subjects' breath is fraught with many more variables than testing samples which are controlled, like the simulators used to check the analytical validity of the instruments. Science continues to test, re-test and examine the the nature of breath alcohol testing.
Law often requires one answer for the present case.
American courts have tended to accept breath alcohol testing as valid and reliable for purposes of admission of test results into evidence. Another factor which goes to the weight of a scientific technique's reliability as evidence under Daubert is the extent to which it has been evaluated by the scientific community at large. Alcohol pharmacokinetics and the dynamics of alcohol transport in the human body have been extensively studied and subjected to peer review. Articles which establish the physiological bases for testing error complement those which establish acceptable limits of analytical error inherent to the instruments. General acceptance of a technique and methodology within a relevant and recognized scientific community were requisite under the Frye standard which governed prior to Daubert, and is still a relevant consideration under the new paradigm.