Risk of bias within studies
QUADAS-2 criteria was used for assessing the risk of bias (figure 2). The reference standard refers to the final clinical outcomes of all patients. Reference standard always remains high risk in risk of bias, for it has to be included in differential verification bias. For example, the reference standard of EP could be ultrasound, non-visualized treatment or surgery, whereas that for an persistent pregnancy of unknown location(PPUL) could be non-declining hCG and a non-confirmed location of the pregnancy that were confirmed neither by TVS nor by laparoscopy between four consecutive measurements, or classified at D7 if the hCG was plateauing by D7 with no visible pregnancy on repeat TVS.