Comparison of two self-rating instruments for medication adherence assessment in hypertension revealed insufficient psychometric properties.
OBJECTIVE
In cases of insufficiently controlled blood pressure, it is important for practitioners to distinguish between "nonadherence" and "nonresponse" to antihypertensive drug treatment. A reliable and valid adherence measurement based on the patient's self-report may be helpful in daily practice.


METHODS
In a primary care sample with 353 hypertensive patients, we applied two self-rating instruments to assess medication adherence (the "Hill-Bone Compliance to High Blood Pressure Therapy Scale" and Morisky's "Self-Reported Measure of Medication Adherence") and compared their psychometric properties.


RESULTS
Both scales showed low acceptability and insufficiency to moderate internal consistency (Cronbach's alpha=0.25 and 0.73, respectively). Their convergent validity as indexed by kappa=0.39 could be judged as "fair" at best. Testing the power to predict blood pressure >140/90mmHg, both scales showed an accuracy of 57% and 62%, respectively. The positive likelihood, that is, the increase in likelihood of high blood pressure in cases of nonadherence was 1.00 and 1.32, respectively.


CONCLUSIONS
The use of both scales cannot be recommended. They showed considerable floor effects, and their ability to identify medication adherence was inconsistent for nearly every third patient. The power of both scales to predict uncontrolled blood pressure was essentially a chance. The underlying conceptual framework of medication adherence therefore needs to be rethought.
Koschack
Janka
J
Department of General Practice, University of Göttingen, Göttingen, Germany. jkoscha@gwdg.de
Marx
Gabriella
G
Schnakenberg
Jörg
J
Kochen
Michael M
MM
Himmel
Wolfgang
W
D000368:Aged; D000959:Antihypertensive Agents / Q000008:administration & dosage*; D005260:Female; D006801:Humans; D006973:Hypertension / Q000188:drug therapy*; D008297:Male; D008875:Middle Aged; D011594:Psychometrics; D015203:Reproducibility of Results; D012646:Self Administration / Q000592:standards; D012650:Self Disclosure; D011795:Surveys and Questionnaires; D016903:Drug Monitoring
D000959:Antihypertensive Agents