Medication adherence is often determined by a ratio of doses
taken on schedule compared with the total number of doses
prescribed over a specified period.11 In observational studies,
this ratio can be determined through pill counts conducted
by an investigator or logged by a medication event monitoring
system (MEMS). Pill counts provide an accurate measure of
adherence by comparing the prescribed quantity of drug with
the actual quantity of drug taken by the patient over a period
of time. MEMS has the additional benefit of providing an electronic
time stamp that measures whether the drug is taken at
the prescribed dosage interval.12 Another way to determine
adherence is by calculating the medication possession ratio
(MPR) or proportion of days covered (PDC) using pharmacy
claims data. MPR is the sum of the days supply for all fill periods
divided by the total number of days in that period. PDC is
calculated as the number of days with drug on hand divided
by the number of days in the specified time interval.13 MEMS
is considered perhaps the most precise measure of adherence