Which of the following is the most sensitive or gold standard test for assessing HCG in maternal serum?

The radio-immune assay stands out to be the most sensitive test to measure the beta sub-unit of human chorionic gonadotropin in which a quantitative assessment of levels more than 5mIU/ml is read as positive. Thus even very minute quantities of the hormone can be detected as following the implantation of the blastocyst. The antibodies used in this assay is highly specific for the beta subunit of the hCG. Other than pregnancy, radioimmuno assay is also used for follow-up of cases like hydatiform mole and choriocarcinoma. Ref: Human Chorionic Gonadotropin (hCG) By Larry Cole, Laurence A. Cole, Stephen Butle, Page 13-15; Textbook of Obstetrics, D C Dutta 6th edition, Page 61; Mudaliar and Menon's Clinical Obstetrics 9th edition, Page 53-55