Creative BioMart provides Radioimmunoassay (RIA) that uses antibodies to detect and quantitate the amount of antigen (analyte) in a sample. This assay is typically very sensitive and specific. It is possible to detect as low as a few picograms of analyte in the experimental tube when using antibodies of high affinity (Kd = 10-8 - 10-11 M). The basic principle of radioimmunoassay is competitive binding, where a radioactive antigen ("tracer") competes with a non-radioactive antigen for a fixed number of antibody or receptor binding sites.