Secondary database analysis: models for measuring medication adherence and persistence

Secondary database analysis is a common used process to evaluate patients’ drug taking behaviour. There are more standardized models for measuring adherence and persistence.

Medication adherence:

Medication Possession Ratio (MPR):

• MPR:    number of days of medication supplied within the refill interval / number of days in refill interval
• At least two fill dates are needed to calculate this ratio.
• Numerator: total X days of supply
• Denominator:
• last refill as the end point 

                        total X days of supply              

last X date – fisrt X date + last X days of supply

• fixed interval (e.g. seasonal diseases) 

total X days of supply

            fixed interval (X days)

• Dual or triple therapy (e.g. hypertension): the numerator must be / with 2 or 3
• MPR is a dichotomous various (adherent / non-adherent).
• Cut-off value is disease specific.
• Conventional: patients with adherence over 80% (MPR) can be called adherent.
• Overuse: MPR is over 1.0 (100%), these patients are also non-adherent !

Proportion of days covered (PDC):

• PDC: the number of days with all drug(s) on-hand / the number of days in the observation interval.
• PDC values range from 0 to 1.

MPR vs. PDC : Is the patient adherent?

Drug

Months 1-12

Supply (days)

A

x

x

x

x

x

x

x

x

x

9x30=270

B

x

x

x

x

x

x

x

x

8x30=240

C

x

x

x

x

x

x

x

7x30=210

↑

↑

↑

3x30=90

MPR: ((270+240+210)/3) / 360 = 0.66

PDC: 90 / 360 = 0.25

Medication peristence:

• The duration of time from the initiation (or at chronic disease from an optional date) to the discontinuation of drug therapy. (Permissible gap: is reported as the maximum allowable period of the refill interval without discontinuation of the therapy.)
• Percentage of individuals remaining on therapy (persistent) until a specified time interval.

Patients persisted x days vs. y patients were persistent for z days

Patient

Months 1-12 (1 month = 30 days)

Days persistent (gap=30 days) *

Persist 180 days (gap=30 days) **

A

x

x

x

x

x

x

x

120

no

B

x

x

x

x

x

x

x

x

180

yes

C

x

x

x

x

x

x

60

no

*: patients persisted on avarge 120 days ((120+180+60)/3) **: 33% (1/3) of the patients were persistent for 180 days

References

Martin BC, Wiley-Exley EK, Richards S, Domino ME, Carey TS, Sleath BL.Contrasting measures of adherence with simple drug use, medication switching, and therapeutic duplication. Ann Pharmacother. 2009 Jan;43(1):36-44.

Karve S, Cleves MA, Helm M, Hudson TJ, West DS, Martin BC: Prospective validation of eight different adherence measures for use with administrative claims data among patients with schizophrenia. Value Health. 2009 Sep;12(6):989-95.

Peterson AM, Nau DP, Cramer JA, Benner J, Gwadry-Sridhar F, Nichol M: A checklist for medication compliance and persistence studies using retrospective databases. Value Health. 2007 Jan-Feb;10(1):3-12.