Drug level testing as a strategy to determine eligibility for drug resistance testing after failure of ART: a retrospective analysis of South African adult patients on second-line ART.

PI-exposure testing revealed non-adherence in half of patients experiencing failure on second-line ART and accurately predicted the presence or absence of clinically relevant PI resistance. PI-exposure testing constitutes a novel screening strategy in patients with virological failure of ART that can differentiate between different underlying causes of therapy failure and may allow for more effective use of limited resources available for drug resistance testing.