PHYSICIAN- AND PATIENT REPORTED ADHERENCE TO ANTIHYPERTENSIVE DRUGS VERIFIED BY SERUM DRUG CONCENTRATION

Objective: Non-adherence to drugs remains a challenge in hypertension treatment. Indirect assessments such as questionnaires and physician guessing have been the mainstay of such evaluation. Existing adherence questionnaires suffer from lack of both consensus and validation against objective methods. Recently, direct measurement of antihypertensive drugs in serum has become available. We aimed to compare both physician- and patient-reported adherence to antihypertensive treatment with serum drug concentration measurements. Design and method: We extracted data from 161 consecutive patients in an ongoing, national multicenter study, who were examined at a hypertension clinic. Patients were aged 18 years or older and used 2 or more antihypertensive drugs. All completed a medical history assessment, laboratory analyses and office- and ambulatory blood pressure (BP) measurements. Additionally, they filled out a five-item questionnaire designed to uncover both non-intended and intended non-adherence to antihypertensive drugs. Each item was scored 1–4, with the lowest obtainable total score of 5 indicating perfect adherence, and 20 points indicating absolute non-adherence. Investigators guessed patient adherence status prior to measuring BP and the results of serum drug concentrations became available. Adherence was objectively confirmed by serum drug concentration measurement using liquid chromatography tandem mass spectrometry, of which the patient was not informed. Patients were defined as non-adherent if serum concentration of at least one drug was below a pre-specified level. Results: We identified 149 (92%) adherent- and 12 (8%) non-adherent patients. Descriptives and results are listed in Table 1. Non-adherent patients were significantly younger and had lower diastolic BP and daytime ambulatory heart rate. Mean total adherence scores obtained from the questionnaire did not differ between groups (5.9 ± 1.7 vs. 5.9 ± 1.0, p = 0.84). Among the non-adherent patients, 33% had daytime ambulatory BP < 135 mmHg, and 58% stated that they never forgot to take their prescribed antihypertensive drugs compared to 44% in the adherent group. Only 17% were correctly classified as non-adherent by investigators, while 8% of adherent patients were erroneously classified as non-adherent.Conclusions: Our results suggest that indirect methods of assessing antihypertensive drug adherence are inadequate when compared to direct serum drug concentration measurements.