terms medication adherence, compliance or persistence
can be used interchangeably, the ultimate desirable
outcome is the patient “taking the medication like the
prescriber has ordered.”
Medication adherence is a major challenge to the
effectiveness of health care services. An estimated 15
to 20 percent of Americans don’t fill their prescriptions
at all. Of those who fill their prescriptions, 15
percent do not take their medication. In addition, an
average of 25 percent of Americans stop taking their
medication before the supply dispensed runs out, or
they take less of the medication than prescribed. Nonadherence
results in additional healthcare costs and lost
productivity.
How do we measure adherence?
To directly measure adherence, one would have to
directly observe the administration or consumption
of the medication, which is very time-consuming
and logistically difficult. Medication dosage counts,
either manual or automatic, are relatively accurate but
not practical. Indirect measures, from databases and
medical records, are more common. Clinical data (lab
tests results and other clinical markers) can indicate
some of the physiologic changes resulting from taking
medications. However, administrative data from medical
and pharmacy claims as well as other billing records
are used to calculate the Medication Possession Ratio
(MPR) or the Proportion of Days Covered (PDC), the
most common proxy measures for adherence.
Medication possession ratio is calculated as the
number of doses supplied divided by the dosing
intervals in study (MPR = # days supply/#days).
This represents the percentage of days for which a
patient has medication on-hand to treat as prescribed.
Although this calculation is relatively straightforward,
it does not account for the various possible gaps in
refills when applied to the same measurement period.
An MPR threshold of 80% is the most commonly
targeted value. Another proxy measure for adherence
is the proportion of days covered, calculated as
PDC = time with drug available/time of observation.
Both the medication possession ratio and the
proportion of days covered measure prescription fill
data. These calculations tell you how much medication
the patient receives and not if the patient actually
consumed or used the medication as prescribed.
With any calculation of adherence measures, it is
important to understand what factors can affect the
data used, which in turn, can affect any conclusions
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made from the calculations. Administrative data can
be affected by the completeness of the data, data entry
errors, the ability to link records across databases, the
available data elements, and the many assumptions that
are made. Doing “drill downs” deeper into the data
using stratifications by the specific lines of business
(e.g., commercial,  Medicaid,
Medicare), by age groups, by
specific disease/conditions or
by medication classes helps
to gain more insight on
what factors are behind the
calculated numbers.
The abundance of
medications currently
available and widely
prescribed makes it
difficult to understand how
medication non-adherence
is such a widespread
problem. Some of the main
reasons why adults age 50
and older are not getting
prescriptions filled include
the following: cost of the
medication; side effects of the medication; belief that
the medication would not help much or that it is not
needed; dislike for taking prescription medications, or
already taking several medications; or the condition
improved on its own or with the use of an over-thecounter,
non-prescription medication. Adherence is a
multidimensional phenomenon that is determined by
the interplay of the following sets of factors, several
listed above: patient-related factors; therapy-related
factors; condition-related factors; provider/patient/
health care system factors; and social/economic factors.
It is along these dimensions that with the proper
motivation, education and support the barriers to
medication adherence can be overcome.
Finding a Solution
There are several solutions that have been used to try
to improve adherence. Some of the tools currently
available include: medication reminder devices; refill
reminder programs; auto-refill programs; reduction
in patient’s share of the cost of the medication;
incentives tied to participation in medication or
disease management programs; various education
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