<prompt> Hcg can be estimated earliest by :RadioimmunoassayELISA.Radioreceptor assayBloassay  <response>: Radioimmunoassay is more sensitive and can detect ss subunit of hCG up to 0.002 IUml in the serum. It can detect pregnancy as early as 8-9 days after ovulation. Reference:DC Duttas Textbook of obstetrics,8th edition,page no:76p 