A 29-year-old woman presents to the emergency department with right lower quadrant abdominal pain, vaginal spotting and a history of 7 weeks since her last menstrual period. She has a positive urine pregnancy test. Vital signs are stable. Transvaginal ultrasound shows no intrauterine pregnancy and a small right adnexal mass; serum β-hCG is 2 500 IU/L. What is the most appropriate next step?