LAUSR

OBJECTIVE To determine whether serum etonogestrel concentrations in contraceptive implant users are associated with certain individual patient characteristics. STUDY DESIGN We enrolled reproductive-age women using etonogestrel contraceptive implants between 12-36 months duration and measured a single serum etonogestrel concentration. Participants also completed a questionnaire about demographics. RESULTS We enrolled 350 participants; median age was 22.5 years (range 18.0-39.1), median months of implant use was 26.0 (range 12.0-36.0), and median body mass index was 25.7 kg/m2 (range 18.5-52.0). Our study population was primarily white/Caucasian (46.6% [163/350]) and Hispanic/Latina ethnicity (51.4% [180/350]). The median serum etonogestrel concentration was 137.4 pg/ml and etonogestrel concentrations varied 12.4 fold in the population (range 55.8-695.1 pg/ml). Using forward stepwise linear regression, months of implant use (β=-1.74, p<.001) and body mass index (β=-3.10, p<.001) were both significantly associated with decreased serum etonogestrel concentration with Black/African American race as a positive effect modifier (β=18.24, p=.099); R-squared for the model=0.13. CONCLUSIONS Individuals demonstrated a wide variability in serum etonogestrel concentrations, which can potentially affect side-effect profiles and efficacy. Increasing body mass index and longer duration of implant use were associated with small decreases in serum etonogestrel concentrations, while self-reported Black/African American race was associated with a non-significant increase. Despite these findings, most of etonogestrel variability was unaccounted for, suggesting that other clinical, pharmacologic, and genetic factors contributing to variability in etonogestrel concentrations remain to be determined. IMPLICATIONS Although increases in body mass index are associated with lower etonogestrel levels in contraceptive implant users, the majority of women will maintain serum concentrations that consistently suppress ovulation. Furthermore, certain patient characteristics can only explain a small portion (13%) of the variability in serum etonogestrel levels among contraceptive implant users.