Figure 1. Measuring Adherence to Individual Medication Classes Using Prescription-Based and Interval-Based Approaches.

The figure shows a medication-filling pattern for a hypothetical patient. Prescription-based approaches measure adherence between 2 prescriptions within a class. The numerator is the number of days of medication supplied from all prescriptions in the class, including the first and last prescriptions, and the denominator is the number of days between the first and last prescriptions plus the days' supply from the last prescription (ie, x + y days). Interval-based approaches measure adherence during a given interval. The denominator is the number of days of medication supplied from all prescriptions (ie, x + y days) during the interval, and the denominator is the total number of days in the interval (ie, t days).