Variability in repeat serum etonogestrel concentrations among contraceptive implant users during the steady-release pharmacokinetic period.

OBJECTIVE

To assess the variability of repeated measurements of serum etonogestrel concentration among contraceptive implant users.

STUDY DESIGN

We measured 3 consecutive serum etonogestrel concentrations, drawn weekly, in women using etonogestrel implants for 12 to 36 months. We used a repeated measures test to evaluate differences.

RESULTS

Among 20 participants, repeat serum etonogestrel concentrations did not differ from initial measurements (Friedman's test, p = 0.95). Mean serum etonogestrel concentrations had similar 95% confidence intervals at each time point: (134.09, 201.46), (135.08, 237.46), and (132.66, 192.45).

CONCLUSION

We confirm that single-time measurements of serum etonogestrel concentration are acceptable pharmacokinetic outcomes for etonogestrel implant studies.

IMPLICATIONS

Pharmacokinetic studies of the etonogestrel contraceptive implant assume single-time measurements are stable steady-state estimates based on small studies using older analysis methods. Our repeated measures study using modern liquid-chromatography mass-spectrometry analysis methods provides updated support for single-time pharmacokinetic measurements among etonogestrel implant users.