Hcg can be estimated earliest by :
Radioimmunoassay is more sensitive and can detect ss subunit of hCG up to 0.002 IU/ml in the serum. It can detect pregnancy as early as 8-9 days after ovulation. Reference:DC Dutta's Textbook of obstetrics,8th edition,page no:76<\p>
Radioimmunoassay
ELISA.
Radioreceptor assay
Bloassay
General obstetrics