ACE-I, Angiotensin-converting enzyme inhibitors; ALQ, Above the Limit of Quantitation; Adherence; BD, Bidaily; BMI, Body mass index; CHF, Chronic Heart Failure; CID, Collision-induced dissociation; CV, Co-efficient of variation; Carvedilol; DBS, Dried Blood Spots; Dried blood spots; EMA, European Medicines Agency; ESI, Electrospray ionization; Enalaprilat; HF, Heart Failure; ISTD, Internal standard; ITP, Initial testing procedure; LC-MS/MS; LC-MS/MS, Liquid Chromatography with tandem mass spectrometry; LLOQ, lower limit of quantitation; LOD, Limit of detection; MRM, Multiple reaction monitoring; NYHA FC, New York Heart Association Functional Classification; OD, Once Daily; Perindoprilat; QC DIL, Quality control dilution; QC LLOQ, Quality control lowest level of quantification; QC, Quality Control; QCH, Quality control high; QCL, Quality control low; QCM, Quality control medium; S/N, signal-to-noise ratio; SOP, Standard operating procedure; ULOQ, upper limit of quantification; VAMS, volumetric absorptive micro sampling