Participants
Patients were recruited at the vascular, cardiology, and nephrology departments of 12 hospitals in the Netherlands of which two were tertiary centers. Patients were eligible if they were at least 18 years old, used three AHDs including a diuretic or four AHDs of which at least two could be measured with a validated ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) method, and had an office blood pressure or ambulatory office blood pressure measurement (AOBP) of at least 140 and/or 90 mmHg. The UHPLC-MS/MS method included AHDs that were most prescribed in the recruiting hospitals and included 12 AHDs and four of their [active metabolites]: enalapril and [enalaprilate], perindopril and [perindoprilate], irbesartan, valsartan, losartan and [losartan-carboxylic acid (losartan-CA)], hydrochlorothiazide, bumetanide, spironolactone and [canrenone], amlodipine, barnidipine, nifedipine, metoprolol, and doxazosin [Exclusion criteria were an estimated GFR of 15 ml/min per mBefore participation patients had to provide written informed consent. This study was approved by the local medical ethical committee of the Erasmus Medical Center, Rotterdam, the Netherlands (MEC-2018–027).