As an odd mix of law and fact (as well as substance and discovery issues), the attorney-client privilege can involve varying appellate standards.
In United States v. Bisanti, 414 F.3d 168, 170-71 (1st Cir. 2005), the court explained an approach that many (although not all) courts take: “issues of privilege are subject to a three-part standard of review . . . . Factual determinations are reviewed for clear error, legal determinations are reviewed de novo, and evidentiary determinations are reviewed for abuse of discretion.”
Lawyers handling the appeal of privilege issues must become familiar with the appropriate appellate standard, which unfortunately can vary from jurisdiction to jurisdiction and court to court.