In the calculation of adherence, 2 measures can be used, the "Medication Possession Rate" (MPR) and the "Proportion of Days Covered" (PDC).

MPR (Medication Possession Rate)

MPR is a ratio calculated as the ratio of the number of days a patient is stocked for their medication to the number of days a patient should be stocked for their medication.
The MPR indicator can be expressed by the number of days of treatment over the number of days in the period observed and is generally calculated by year.
The MPR is an adherence approach in the sense that it does not allow to know if the patient is really taking his treatment. Another possible bias is when the patient gets supplies earlier than the end of his initial supply, which can lead to MPRs greater than 100%. Generally, the MPR ratio is in these cases reduced to 100%. In the case of treatments requiring the simultaneous intake of several drugs, the MPR can be calculated by the average of the MPR of each drug per patient. The flaw is that drugs with a high MPR compensate for those with a low MPR.
The MPR is still today the most used measure for calculating compliance.

PDC (Proportion of Days Covered)

The PDC, created plus is also a ratio but whose numerator changes compared to the MPR. it is no longer the sum of the days of treatment available, but the sum of the days covered. PDC is the same as moving the number of days to the first day the patient runs out of medicine from their previous supply.
For example, a patient who takes supplies 5 days before running out of his medication, will have an overlapping period of days. This will artificially increase the MPR, while the PDC will adjust and not allow it to exceed 100%. The PDC is also better suited for treatments requiring the simultaneous intake of several drugs.
The PDC is increasingly recommended to replace the MPR in the calculation of adherence.

Illustration of a treatment with several drugs

Let us take the example of a study over one year of treatment requiring the simultaneous intake of 2 drugs, for each a box allows 30 days of treatment, including the therapeutic follow-up of a given patient would be the following:

The " standard "MPR has 480 days of processing over 365 days of the period, or 132% which is reduced to 100% .
The "adjusted" MPR , average days, has 240 days of processing over 365 days of the period, or 66% .
The PDC has 180 days of simultaneous processing of the 2 products over 365 days of the period, ie 49% .

To go further

ISPOR (International Society for Pharmacoeconomics and Outcomes Research), an international pharmacoeconomics and research organization, is the benchmark organization for these indicators. You can find their research publications on MPR and PDC on their website, available at https: //portal.ispor .org / .

You will find there several possible formulas to calculate the MPR , concrete comparisons with the PDC , as well as new measures such as the DPR (Daily Possession Ratio), another alternative to the MPR.

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