Typically, a defendant's breath test results are going to be admissible in a DWI prosecution under Texas Rule of Evidence 403. Texas Rule of Evidence 403 states that all relevant evidence is admissible unless the probative value of the evidence is substantially outweighed by its prejudicial affect.
Intoxilyzer results are probative without retrograde extrapolation testimony. The Texas Court of Criminal Appeals has stated that the trial court abuses its discretion in excluding intoxilyzer results, which are relevant to the issue of intoxication without extrapolation evidence. Also, in the famous Mata case, the Texas Court of Criminal Appeals addressed the necessary factors to establish the reliability of expert testimony on retrograde extrapolation, but not the necessity for retrograde extrapolation testimony as a prerequisite for admission of intoxilyzer results.
What that means without speaking in legalese is the breath and blood results are typically going to find their way into evidence even without the state putting on an expert witness. Based on their training and experience they can tell that even though the blood test was taken an hour or so after the individual was driving they could estimate and tell you what the blood alcohol concentration level was at the time of driving.
A defendant charged with DWI is allowed to call his own expert witness or cross-examine and confront the state's expert witness on the issue of the blood alcohol concentration level at the time of driving. It is not against the law to have a blood alcohol concentration of .08 or higher after you are driving, it is simply against the law to have it while you are driving. Your blood alcohol concentration level stays a low level typically shortly after you have consumed the alcohol and then can continue to rise for up to an hour and fifteen to thirty minutes after the time of consumption and then begins a very, very slow decline in the elimination phase.