This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General obstetrics, Subject name: Gynaecology & Obstetrics
Minimum level of Beta Hcg detected by radioimmunoassay is : 
Select and explain. 
A. 1.5-3.5 IU/ml, B. 0.5-1 IU/ml, C. 0.02-0.05 IU/ml, D. 0.001 IU/ml
D: 0.001 IU/ml
Answer: D: 0.001 IU/ml 
Explanation: * Radioimmunoassay (RIA) -- It using I125 ido hCG antibodies. It is more sensitive and can detect b subunit of hCG upto 0.002 IU/mL 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 IU/mL 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 dutta's Textbook of Obstetrics 7th edition page no 67