51 H2Oil2 study protocol 3 Potential impact and implications As health care costs are increasing around the world, research focusing on costeffectiveness of healthcare will help clinicians and policy makers to determine the appropriate position of the HSG with oil-based contrast within the fertility workup, taking into account both its diagnostic potential as well as its therapeutic effect. A costeffectiveness analysis of the long term outcomes of the H2Oil study showed an increase in the cumulative pregnancy rate when oil-based contrast was used, compared to when water-based contrast was used (80,0 versus 75,0%) (36). The higher price of the oilbased contrast was compensated by a decrease in the need for ART to achieve these pregnancies in the group receiving oil-based contrast, resulting in comparable overall costs. The study concluded that tubal flushing with oil-based contrast was therefore costeffective in comparison to water-based contrast in women with unexplained subfertility (36). In the proposed study, a cost-effectiveness analysis will be performed incorporating medical consumption, absence from (paid) work and loss of productivity due to health problems (24). The mechanism of action of oil-based contrast leading to a fertility enhancing effect is not fully elucidated. Different hypotheses place the point of action in the Fallopian tube (37, 38), the endometrium, (39) and the peritoneum (40, 41). A post-hoc analysis of the H2Oil-study showed that in the group of women with higher pain scores, the ongoing pregnancy rate was higher in women that had received oil-based contrast during HSG when compared to women that had received water-based contrast (42). These result support the first hypothesis that when using oil-based contrast medium, the pain was caused by an increase in intrauterine pressure prior to dislodgment of pregnancyhindering debris from the proximal part of otherwise anatomically normal Fallopian tubes (42). Previous research associated pre-procedural anxiety to a higher experienced pain level during medical procedures (19, 43). To further investigate the relationship between discomfort or pain during HSG and ongoing pregnancies in the current study, the APAIS questionnaire will be used to score pre-procedural anxiety as a confounder for experienced pain. Despite reassuring recent research on the prevalence of complications after an HSG, fear of complications is still a reason for some clinicians to withhold use of oil-based contrast (11, 18). A recent review, without publication date or language restrictions, showed that the incidence of intravasation of contrast in the venous or lymphatic system is higher during tubal flushing with oil-based contrast in comparison to water-based contrast (2.8% and 1.8% respectively, odds ratio 5.05, 95% CI 2.27 to 11.22) in the included RCTs (18). However, when including only studies that used fluoroscopy screening during HSG, no serious consequences of intravasation were identified. Pelvic infection after HSG is
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