If a police officer suspects that you’ve been driving drunk, he must administer the breath test within a “reasonable time” in order for the results to be readily admissible at trial.
What constitutes a reasonable time depends on the circumstances. Though at least one case, Commonwealth v. Marley, held that a two-and-half hour period between the defendant’s driving and his breath test was reasonable.
If the delay between driving and testing is not reasonable, prosecutors can introduce the breath test results at trial only with the accompanying testimony of an expert witness. The expert must be sufficiently qualified to explain the technicalities of “retrograde extrapolation.”
According to the Supreme Judicial Court:
Retrograde extrapolation is a mathematical calculation used to estimate a person’s blood alcohol level at a particular point in time by working backward from the time the blood alcohol level was tested and factoring in rates of absorption and excretion.
Commonwealth v Colturi, 448 Mass. 809, note 2 (2007).
If you believe the prosecutor is obligated to present such an expert at trial, you will likely need to file a motion in limine during the pretrial stage asking the court to rule on the issue.