Objective: The contraceptive efficacy of perfect dosing cycles and imperfect dosing cycles has not been described previously. Method compliance determines the proportion of perfect and imperfect dosing cycles, and together can form the basis for evaluating differences in efficacy based on differences in compliance.
Materials and methods: The transdermal contraceptive delivery system (Ortho Evra) has been studied in a North American randomized trial vs. an oral contraceptive (OC) and in total has been evaluated in 3319 women in contraceptive clinical trials. This article explores the impact of perfect vs. imperfect compliance with the contraceptive method on contraceptive efficacy. Previously published data for a transdermal system (Patch, n = 812) and OC (Triphasil, n = 605) users from the North American comparative study were reanalyzed to determine the effect of imperfect use on the contraceptive efficacy of the different methods.
Results: Contraceptive efficacy was significantly better (p = 0.007) in cycles with perfect dosing (Pearl Index = 0.83) compared to those with imperfect dosing (Pearl Index = 6.32) for both methods. This difference is homogeneous (p = 0.62) across the Patch and OC groups. Pooled data for all Patch users confirm that perfect dosing cycles are associated with significantly better efficacy than imperfect dosing cycles (p = 0.047). In addition, compliance did not vary by age in the pooled Patch data, which are in agreement with the previously published Patch data from the comparative study. In the comparative study, the percentage of cycles with perfect dosing was significantly higher with the Patch than with the OC (88.7% vs. 79.2%, p < 0.001), and was consistently high in all age groups (range, 89.6-91.8%). By contrast, among OC users, the percentage of cycles with perfect dosing increased with increasing age (p < 0.001) from 67.7% in users aged 18-20 years to more than 80% in those aged 30 years and older.
Conclusion: In conclusion, deviations from perfect use (whether corrected or not) of a transdermal contraceptive system and of an OC increase contraceptive failures by approximately 5-10-fold when compared to perfect use. The weekly change schedule of the transdermal contraceptive delivery system is associated with a significantly greater proportion of cycles in which there is perfect dosing compared to an OC.