Different Pearl Indices in studies of hormonal contraceptives in the United States: impact of study population.

OBJECTIVE

To examine the impact of subject characteristics on efficacy as measured by the Pearl Index (PI) in clinical trials and to make study populations similar by matching.

METHODS

Our analysis used US data from four large Phase III studies. We compared results from one fertility control patch study with pooled data from three studies with virtually identical design on oral hormonal contraceptives. First, we identified three characteristics that had the most impact on the PI. Second, we used these three variables and matched subjects from the patch study with those from the oral contraceptive (OC) studies. Finally, we calculated the PIs for matched and unmatched subjects from both the patch study and the OC studies.

RESULTS

A total of 3706 subjects were included in our analysis. The variables 'Hispanic ethnicity', 'previous pregnancy' and 'previous use of hormonal contraceptives' had the most impact on the PI. The PIs for the matched patch cohort and the matched OC cohort were 2.97 and 2.48, respectively. Those for the unmatched patch cohort and the unmatched OC cohort were 10.17 and 0.90, respectively.

CONCLUSION

Subject characteristics strongly influence the PI in clinical studies of hormonal contraceptives. In particular, Hispanic ethnicity, previous pregnancies and no previous use of hormonal contraceptives result in a higher PI.

IMPLICATIONS: PIs from different clinical trials cannot be meaningfully compared unless subject characteristics that have most impact on the PI are similar or are made to be similar statistically as we did here by matching.