Medication adherence correlates with morbidity and mortality in patients with chronic heart failure (
CHF
), but is difficult to assess. We conducted a retrospective methodological cohort study in 3,808
CHF
patients, calculating adherence as proportion of days covered (
PDC
) utilizing claims data from 2010 to 2015. We aimed to compare different parameters’ influence on the
PDC
of elderly
CHF
patients exemplifying a complex chronic disease. Investigated parameters were the assumed prescribed daily dose (
PDD
), stockpiling, and periods of hospital stay. Thereby, we investigated a new approach using the
PDD
assigned to different percentiles. The different dose assumptions had the biggest influence on the
PDC
, with variations from 41.9% to 83.7%. Stockpiling and hospital stays increased the values slightly. These results queries that a reliable
PDC
can be calculated with an assumed
PDD
. Hence, results based on an assumed
PDD
have to be interpreted carefully and should be presented with sensitivity analyses to show the
PDC
's possible range.