Primary aldosteronism (PA) is a severe and often underdiagnosed form of secondary hypertension. Determining the aldosterone to renin ratio (ARR) in hypertensive patients has been shown to be a valuable screening test for identification of patients suffering from PA. Since the introduction of a more widespread ARR screening the number of PA patients significantly increased worldwide. Interpretation of ARR might be challenging: Several factors from posture to interfering drugs affect the ARR and need to be taken into account when collecting samples. In addition, the wide variety of available assay methods and lack of well-established cut-offs present a challenge to the clinician. This review discusses the usefulness and possible difficulties of ARR screening.