Final answer:
An elevated serum beta-human chorionic gonadotropin (hCG) level is the most significant laboratory test in ruling out an ectopic pregnancy.
Step-by-step explanation:
An elevated serum beta-human chorionic gonadotropin (hCG) level is the most significant laboratory test in ruling out an ectopic pregnancy. In ectopic pregnancies, the hCG levels are lower than expected for a normal pregnancy. However, it is important to note that low hCG levels do not definitively rule out an ectopic pregnancy.