Effect of characteristics of women on attendance in blind and non-blind randomised trials: analysis of recruitment data from the EPHT Trial

OBJECTIVES: To analyse the effect of women's characteristics on their willingness to join a blind or a non-blind subtrial or to be excluded by physicians. DESIGN: Primary prevention trial of postmenopausal hormone therapy (HT). A 2×2, randomised design with a non-blind HT arm or control arm and a blind HT arm or placebo arm. SETTING: 3 clinical centres in Estonia. METHODS: Interest in joining the trial was asked in a questionnaire together with demographic and health status data. Interested and eligible women were invited to a health examination that also informed whether they belonged to a blind or to a non-blind subtrial; the arm was not revealed. Trial physicians made further exclusions when checking the women's eligibility. Thereafter, informed consent was asked as detailed in the flow chart. Comparisons were made between non-blind and blind subtrials. Analyses were carried out for each of the background variables. OUTCOME MEASURES: The proportion of willingness, eligibility and attendance. RESULTS: Women randomised to the non-blind subtrial were more willing to join (relative risk (RR) 1.17) and more likely to be found eligible by physicians (RR 1.10) than women in the blind subtrial, resulting in larger attendance (RR 1.29). Women with higher education were differentially more willing to join the non-blind trial (RR 1.29) than those with basic education (RR 1.08); the differential willingness of never-smokers (RR 1.20) was larger than that of current smokers (RR 1.07). The differential exclusion by physicians by education and smoking were small. Some subjective symptoms (eg, diarrhoea/constipation, stomach pain) had reverse differential effects on attendance in the non-blind subtrial in comparison to the blind subtrial. Menopausal symptoms did not affect the differential interest, eligibility or attendance. CONCLUSIONS: Blinding in RCT reduces attendance, due to decisions of the women and the trial physicians. Differential attendance by blinding may affect the generalisability of the results from trials. TRIAL REGISTRATION NUMBER: ISRCTN35338757.