The method used to measure medication adherence impacts the results of that measurement, and therefore researchers should use various methods depending on the overall goals of the study, according to researchers from the Accredo Health Group and Medco Health Solutions.
They compared 4 different ways of measuring medication possession ratio (MPR) in 8 specialty pharmacy therapeutic categories using 18 months of claims data. A patient’s first use of a drug was recorded (index date), and then all subsequent claims for 360 days were recorded. The 8 drug categories were multiple sclerosis, rheumatoid arthritis, hepatitis C, pulmonary hypertension, growth hormone, osteoporosis, oral oncology, and finally, the drug omalizumab (Xolair).
The 4 methods included (1) continuously eligible patients are followed for 360 days from the indexed prescription; (2) all patients with 2 or more prescriptions are included, but the last prescription is not included;
(3) the third approach is the same as the second one, but the last prescription is included; and (4) all patients with 1 prescription filled are included, which includes the last prescription.
In every case, the first method provides the lowest estimate of adherence rate, and each of the following methods provides lower estimates than the next one above it. The difference in adherence rate between method 1 and method 4 for each of the drug categories was: 13% (multiple sclerosis); 16.6% (rheumatoid arthritis); 44.4% (hepatitis C); 17.7% (pulmonary hypertension); 21.6% (growth hormone); 26.9% (osteoporosis); 16.4% (oral oncology); and finally, 21.6% (omalizumab).