A woman complains of right-sided lower abdominal pain and mild bleeding per vagina for one day after a period of amenorrhoea of 6 weeks. Her general condition is satisfactory. There is tenderness in the right iliac fossa. The urine hCG test is positive. Transvaginal ultrasound scan does not reveal an adnexal mass or an intrauterine pregnancy.What is the next step in the management?