A comparison of Pearl indices and cumulative incidences for use in meta-analysis of contraception trials

P. O’Brien

Westside Contraceptive Services, London, UK

Objectives: One of the issues encountered when performing meta-analysis of contraceptive trials is the variable way in which the outcomes are reported, as cumulative rates (Kaplan Meier curves), single or multiple decrement life-tables rates, or Pearl indices (events per 100 women-years). The latter decrease with time, while the others increase. However, it is usually the ratio of the rates that we use in meta-analysis. In this analysis we compare the ratio of the Pearl rates to the ratio of the cumulative incidence and their confidence intervals.

Design and methods: We used data from a large WHO randomised trial of a frameless IUD and TCu380A, to compared the ratio of Pearl indices to the ratio of cumulative incidences. We used the method described by Kleinbaum to calculate the standard error of the ratio of cumulative incidences and the method of Hasselblad for the standard error of the ratio of Pearl indices, to calculate their confidence intervals.

Results: For accidental pregnancies the difference in the ratio of Pearl ratios to ratio of incidence ratios was greatest at year 6 at 13% (Pearl rate ratio 0.95, 95%CI 0.64 to 1.47; incidence rate ratios 0.85, 0.56 to 1.29). In 4 of the 6 years of follow-up, the difference in ratios was less than 5%. The difference was smaller for removals for bleeding and pain, never exceeding 3% (Pearl ratio 1.10, 0.90 to 1.33; incidence ratio 1.13, 0.94 to 1.36 at 6 years). For total use-related discontinuations, the difference never exceeded 4% (Pearl ratio 1.26, 1.10 to 1.45; incidence ratio 1.22, 1.08 to 1.38 at 6 years).

Conclusions: The difference in rate ratios, whether we use the Pearl indices or cumulative incidence rates, is usually small. The difference in rate ratios from single and multiple decrement life-tables is likely to be smaller. The ability to incorporate trials using different reporting methods in meta-analyses enhances our capacity to systematically review the literature. Further research is required to understand the determinants of the magnitude and direction of the differences.