"The prosecution has a duty to divulge exculpatory evidence pertaining to the defendants. I don't know if withholding such evidence is a felony--more like prosecutorial misconduct."
That depends. If the lab report was deliberately altered to omit the exculpatory evidence, then that goes beyond a failure to disclose, and approaches falsifying evidence, which is a felony, I believe. To prove that, they would probably have to find a draft of the report with the exculpatory evidence included.
On the other hand, the discussion between the lab chief and the DA to exclude the evidence from the report may constitute conspiracy - and even if the underlying crime is a misdemeanor, conspiracy is usually a felony.
Fromm NC Dept of Department of Justice
Attorney General Roy Cooper
(http://www.ncdoj.com/default_contactus_form.jsp?sectionid=ag&subsectionid=general)
District Attorneys:
In North Carolina, district attorneys are Constitutional officers directly elected by the people in the district where they serve and are not a part of the Attorney General's Office. Complaints against a member of a local district attorney's office should be directed to the staff member's immediate supervisor and, if unresolved, to the district attorney.