Radioimmunoassay And its Application

Radioimmunoassay (RIA)

Radioimmunoassay is an in vitro assay. It calculates the presence of an antigen with very high sensitivity. Mainly, any biological product or substance for which a specific antibody exists can be measured, even in minute concentrations. Radioimmunoassay has been the first immune-assay procedure developed to analyze picomolar and nanomolar concentrations of hormones in biological fluids.

Method

The target antigen is labeled by radioactive isotope and it bound to its specific antibodies. For example, sample A & blood-serum, is then added to initiate a competitive reaction of the labeled radioactive antigens from the preparation.  The unlabeled antigens serum-sample react with the specific antibodies. The competition for the antibodies will release a certain amount of labeled antigen. This proportional to the ratio of labeled antigen & unlabeled antigen. A curve is plot which allows the amount of antigen in the patient's serum to be derived.

It means the concentration of unlabeled antigen is increased, more of it binds to the antibody, displacing the labeled radioactive variant. The bound antigens are separated from the unbound antigens. And the radioactivity of the free antigens remaining in the supernatant is measured. A binding curve can be generated using a known standard, which allows the number of antigens in the patient's serum to be derived.

Fig; Radioimmunoassay and radioactivity vs concentration graph.

Radioactive isotope uses for RIA           

Test Isotopes	Major Range of Gamma Emission (keV)
Iodine 131	80.2-637
Gallium 67	93.3-388
Thallium 201	135-167
Technetium 99	140
Cobalt 57	114-136
Iodine 125	25-36

Application of RIA

1.      Detection of narcotic Drugs.

2.      Radioimmunoassay of Hydromorphone & Hydrocodone in Human plasma.

3.      Radioimmunoassay of Flunisolide in human plasma.

4.      Measurement of Ferritin.

5.      Detection of digoxin.

6.      Thyroid testing.