Post-use ring weight and residual drug content as potential objective measures of user adherence to a contraceptive progesterone vaginal ring.

Objectives: The primary aim was to investigate post-use ring weight as a potential measure of cumulative adherence to a progesterone-releasing vaginal ring.

Study Design: We weighed and quantified residual progesterone in 115 vaginal rings following 90-day use by participants in an acceptability trial conducted in Nigeria, Senegal and Kenya. The primary objective was to correlate residual progesterone content with post-use ring weight. Secondary objectives included correlating ring weight with putative duration of ring use, and, where participants used two rings consecutively in the study, correlating residual content between these paired rings.

Results: Mean ring weight and progesterone content of used rings was 8.62±0.24 g and 1245±245 mg respectively, versus 9.37±0.02 and 2058±21 mg for control rings. Most used rings (90.4%) had residual progesterone levels less than 85% of the nominal loading. Linear regression showed a strong positive linear trend between residual progesterone content and post-use ring weight for all rings (r=0.82). Duration of ring use was inversely associated (p=.00020) with ring weight.

Conclusions: Post-use ring weight is highly correlated with residual progesterone content, a benchmark objective cumulative measure of adherence, and thus potentially useful as a surrogate objective measure of cumulative adherence to a progesterone-releasing vaginal ring.

Implication Statement: For vaginal rings containing a high initial drug loading and releasing a relatively large fraction of the initial loading during clinical use, post-use ring weight may offer a simple and inexpensive alternative to residual content testing for accurate monitoring of user adherence.