Iodine-125 (125I)
Definition
The radioisotope most commonly used in RIA. It emits gamma radiation, has a half-life of approximately 60 days, and can be covalently attached to proteins and haptens. Its gamma emission is detected by a gamma counter without liquid scintillation fluid. The 60-day half-life limits reagent shelf-life and is a major logistical disadvantage in kit-based laboratories.
- Emission
- Gamma radiation
- Half-life
- Approximately 60 days
- Use
- Covalent labelling of proteins and haptens in RIA
- Detection method
- Gamma counter, no scintillation fluid needed
Common questions
Why is the 60-day half-life a practical problem?+
Labelled reagents lose usable activity over a matter of weeks, forcing laboratories to reorder or relabel tracer stock frequently, which raises cost and limits kit shelf-life compared with non-radioactive immunoassay labels.
Why did iodine-125-based RIA fall out of favour despite working well?+
Radioactive-waste handling, licensing and disposal requirements made it more burdensome to run than enzyme- or fluorescence-based immunoassays, which have largely replaced RIA in routine laboratories.
Related terms
- Calibration Curve
- In RIA, a set of standards containing known concentrations of unlabelled analyte that are run alongside test samples. Bound radioactivity is measured...
- Chemiluminescent Immunoassay (CLIA)
- A non-radioactive immunoassay that uses enzyme-catalysed chemiluminescent reactions to generate the detection signal. CLIA instruments measure light output in relative light units....
- Competitive Binding Assay
- An immunoassay format in which labelled and unlabelled forms of the same analyte compete for a fixed, limiting number of antibody binding...
- Cross-Reactivity
- The capacity of an antibody raised against one analyte to bind structurally related compounds. In RIA, cross-reactivity is the main driver of...
- Tritium (3H)
- A radioactive hydrogen isotope used as an alternative label in RIA, particularly for small organic molecules such as steroids and drugs where...