Cost analysis of therapeutic interchange of calcium channel blockers for the treatment of hypertension: unexpected results from a conversion program

Objective — To examine drug expenses and overall healthcare resource utilization associated with a calcium channel blocker therapeutic interchange and conversion program.

Design — A pre/post analysis involving a retrospective cohort study design and primarily using administrative databases was done. The study was conducted from an institutional perspective using hospital-specific drug acquisition costs in 1997 dollars.

Setting — Veterans Administration Hospital, Saginaw, Michigan.

Patients — One hundred and one patients receiving extended-release nifedipine (Procardia XL) were converted to either amlodipine (Norvasc) or felodipine (Plendil) through the institution's conversion program.

Main Outcome Measures — Change in total drug cost, secondary health resource utilization, study drug costs, cardiovascular drug costs, noncardiovascular drug costs, and change in severity of hypertension.

Results — Unexpectedly, the total cost of drug therapy was significantly higher during the nine-month postconversion period relative to the nine-month preconversion period (pless than0.001). Neither the total number of clinic visits nor the number of hospitalizations (cardiovascular or noncardiovascular) differed significantly between the preconversion and postconversion periods. No significant differences in the grade of hypertension was observed from the preconversion period to the postconversion period, although there were statistically significant drops in both systolic (mean reduction of 7.5 mm Hg; pless than0.001) and diastolic blood pressures (mean reduction of 5.6 mm Hg; pless than0.001).

Conclusions — The increased drug expenses and significant change in blood pressure appear to have been due to an increased number of prescriptions filled in the postconversion period relative to the preconversion period. Further study is warranted to determine whether this unexpected finding was due to a pharmacological factor such as improved tolerability of the medications or to an effect of the process of conversion itself.