“…Actually, in most studies, the methods to determine the cutoff value (varying from 0.9 to 2.0 ng/mL) defining PPR were arbitrarily chosen (6,8,12), which undoubtedly undermines the reliability of any conclusion. In some other studies, receiver operating characteristic curve analysis was used to determine the cutoff value (5,11). Nevertheless, that is not a proper method, because our data showed that the relationship between P levels and live birth rates is not linear, which is consistent with Bosch et al's report (14).…”