Aspirin non-response in pregnant women at increased risk of pre-eclampsia.

Objectives: Low dose aspirin (LDA) is recommended for women at increased risk of preeclampsia (PE), however it is not always effective. The study sought to determine the prevalence of non-response to LDA and to ascertain the effect of increasing aspirin dose in non-responders.

Study Design: Single centre, cohort study of 166 women at increased risk of PE was conducted in a large maternity unit in the UK between 2013 and 2016. All women were prescribed 75 mg of aspirin and invited to attend study visits at 18-24 weeks' and 32-36 weeks' gestation. Non-response was defined as a serum thromboxane B2 (TXB) ≤10 ng/mL. Aspirin dose was increased to 150 mg if a bedside VerifyNow test suggested non-response (test value ≥ 550 arachidonic acid reactive units [ARU]) at 18-24 weeks. Adherence was assessed by self-report.

Results: Based on serum TXB, response rates were 85.3 % at 18-24 weeks and 79.3 % at 32-36 weeks' gestation. Compared to serum TXB, the VerifyNow test demonstrated moderate test performance (AUC 0.79 95 % CI 0.71-0.88, p < 0.0001) to detect non-response. High prevalence of non-adherence (6/10) was evident in persistent non-response group. Dose change from 75 to 150 mg of aspirin in adherent participants improved response (VerifyNow: 598 [95 % CI 550-665] ARU at 18-24 weeks on 75 mg aspirin, 509 [95 % CI 350-667] at 32-36 weeks on 150 mg of aspirin, [p < 0.0001]).

Conclusions: Non-response to LDA is common in pregnancy but appears to be largely attributable to non-adherence. Dose change could be useful to improve response to LDA in this cohort.