Aspirin for Evidence-Based Preeclampsia Prevention trial: influence of compliance on beneficial effect of aspirin in prevention of preterm preeclampsia

Background: The Aspirin for Evidence-Based Preeclampsia Prevention trial was a multicenter study in women with singleton pregnancies. Screening was carried out at 11-13 weeks' gestation with an algorithm that combines maternal factors and biomarkers (mean arterial pressure, uterine artery pulsatility index, and maternal serum pregnancy-associated plasma protein A and placental growth factor). Those with an estimated risk for preterm preeclampsia of >1 in 100 were invited to participate in a double-blind trial of aspirin (150 mg/d) vs placebo from 11-14 until 36 weeks' gestation. Preterm preeclampsia with delivery at <37 weeks' gestation, which was the primary outcome, occurred in 1.6% (13/798) participants in the aspirin group, as compared with 4.3% (35/822) in the placebo group (odds ratio in the aspirin group, 0.38; 95% confidence interval, 0.20 to 0.74).

Objective: We sought to examine the influence of compliance on the beneficial effect of aspirin in prevention of preterm preeclampsia in the Aspirin for Evidence-Based Preeclampsia Prevention trial.

Study design: This was a secondary analysis of data from the trial. The proportion of prescribed tablets taken was used as an overall measure of compliance. Logistic regression analysis was used to estimate the effect of aspirin on the incidence of preterm preeclampsia according to compliance of <90% and ≥90%, after adjustment for the estimated risk of preterm preeclampsia at screening and the participating center. The choice of cut-off of 90% was based on an exploratory analysis of the treatment effect. Logistic regression analysis was used to investigate predictors of compliance ≥90% among maternal characteristics and medical history.

Results: Preterm preeclampsia occurred in 5/555 (0.9%) participants in the aspirin group with compliance ≥90%, in 8/243 (3.3%) of participants in the aspirin group with compliance <90%, in 22/588 (3.7%) of participants in the placebo group with compliance ≥90%, and in 13/234 (5.6%) of participants in the placebo group with compliance <90%. The odds ratio in the aspirin group for preterm preeclampsia was 0.24 (95% confidence interval, 0.09-0.65) for compliance ≥90% and 0.59 (95% confidence interval, 0.23-1.53) for compliance <90%. Compliance was positively associated with family history of preeclampsia and negatively associated with smoking, maternal age <25 years, Afro-Caribbean and South Asian racial origin, and history of preeclampsia in a previous pregnancy.

Conclusion: The beneficial effect of aspirin in the prevention of preterm preeclampsia appears to depend on compliance.