Poor drug adherence is one of the main reasons for the failure to achieve treatment targets in hypertensive patients. In patients who receive pharmacological treatment, assessment of drug adherence is of the utmost importance. The aim of this review is to present an update of the methods available to reveal and monitor non-adherence in patients with apparent treatment-resistant hypertension. Methods for monitoring adherence are divided into indirect and direct methods. The indirect methods are mainly based on self-reported adherence and can easily be manipulated by the patient. Directly observed therapy and therapeutic drug monitoring are examples of direct methods. There are limitations and advantages to all of the methods, and because of the patient's ability to manipulate the outcome of indirect methods, direct methods should be preferred. Therapeutic drug monitoring and directly observed therapy with subsequent ambulatory blood pressure measurement are considered to be reliable methods and should be used more in the routine assessment of patients with apparent treatment-resistant hypertension.

We believe that physicians should reflect and discuss these potential challenges, and that patient education, information and a solid patient-physician relationship are essential for achieving drug adherence. Methods for monitoring adherence represent, however, a useful and often necessary supplement.

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Despite being a highly selected cohort referred for tertiary work-up of apparent TRH, BP control was achieved or secondary causes were identified in almost two thirds of the patients. Thorough investigation according to guidelines and DOT with subsequent 24ABPM is needed in work-up of apparent TRH.

Lack of adherence to medication may be the explanation for unsatisfactory drug efficacy and is often misinterpreted as resistance to treatment. When encountering patients with persistent high blood pressure despite antihypertensive treatment, it is therefore important to discover whether they are actually taking their medication. This article aims to provide an updated overview of methods of revealing and monitoring medication adherence. The article is based on non-systematic literature searches in PubMed and on the senior authors' own clinical experience.