Adherence to therapy is difficult to measure accurately. Methods used include self-reporting by individuals, doctors' estimates, and tablet counting through pharmacy records, tablet returns or the use of sophisticated tablet container-closures that record the date and time of opening and closing. Problems with these approaches include assumptions about a person's honesty, the physician's judgment, and the fact that opening and closing a container means that the medication was then taken.
In clinical trials, adherence can be monitored with laboratory measurements, such as drug concentrations in the blood. These measures can be misleading, because people may be more adherent before clinic appointments.