Per se impairment, reasonable doubt, margins of error, and all that lies between

G.S. 20-138.1(a)(2) prohibits a person from driving a vehicle upon a highway, street or public vehicular area after having consumed sufficient alcohol that the person has, at any relevant time after the driving, an alcohol concentration of 0.08 or more. S.L. 2006-253 amended this subsection to provide, effective for offenses committed on or after December 1, 2006, that “[t]he results of a chemical analysis shall be deemed sufficient evidence to prove a person’s alcohol concentration.” The meaning and constitutionality of this latter provision were considered and upheld by the court of appeals in State v. Narron, 193 N.C. App. 76 (2008) (discussed in this post). Narron held that the “shall be deemed” provision, contrary to the defendant’s argument, did not establish a mandatory presumption.  Instead, it merely set forth the prima facie standard for proof of impairment under the per se prong of G.S. 20-138.1. The court explained that the “[r]esults of a chemical analysis are sufficient evidence to submit the issue of a defendant’s alcohol concentration to the fact-finder” who “may find it adequate proof.” Id. at 81, 84. Let’s suppose you are a defense lawyer (a proposition that is hardly a stretch for many readers) representing a defendant charged with impaired driving based on a reported alcohol concentration of 0.08 resulting from a breath test administered in compliance with all statutory and regulatory procedures. On what basis might you argue to the judge in district court or the jury in superior court that the alcohol concentration result introduced into evidence [...]