<prompt> Minimum level of Beta Hcg detected by radioimmunoassay is :1.5-3.5 IUml0.5-1 IUml0.02-0.05 IUml0.001 IUml  <response>: Radioimmunoassay RIA -- It using I125 ido hCG antibodies. It is more sensitive and can detect b subunit of hCG upto 0.002 IUmL in the serum. It can detect pregnancy as early as 8-9 days after ovulation day of blastocyst implantation. Radio receptor assay gives highest sensitivity of 0.001 IUmL in the serum. RIAs are quantitative, so can be used for determining the doubling time of hCG ectopic pregnancy monitoring. RIAs require 3-4 hours to perform. Reference : D C duttas Textbook of Obstetrics 7th edition page no 67 