Self-irradiation and ageing of radioactive tracers in immunoassays

To determine the importance of the self-irradiation parameters during the ageing of radiotracers, the antigens 3H-testosterone, 125I-thyroxine and 125I-anti-FSH monoclonal antibody were tested after self-irradiation had been simulated by exposure in a 137Cs irradiator at an intensity of 0.3 Gy/min. For 3H-testosterone the calibration curve obtained with an irradiated tracer with gamma radiation equivalent to one year of storage was shifted in the middle by comparison with a reference tracer. In addition, there was a 20% increase in the non-specific binding, probably due to radiolysis. For 125I-thyroxine the comparative assay between calibration curves set up with an irradiated tracer with gamma radiation dose equivalent to 4 months of storage and a reference tracer showed no difference, but a drop of 2.5% in the ratio Bo/T was noticed. For antibody anti 125I-FSH there was no significant change in the calibration curve when the antibody had a gamma dose equivalent to 6 or 12 weeks o storage. But when a two month out-of-date antibody was used the slope curve (B/T) was greatly shifted. The non-specific binding was four times higher and the control serum was overestimated by 7%. A self-irradiation process seems to be greatly involved in the ageing of 3H-testosterone even when the tracer is preserved with a radioprotector. Even so, this phenomenon is only involved for gamma dose irradiation equivalent to one year of self-irradiation. Concerning 125I tracers (thyroxine and antibody anti-FSH), self-irradiation is not the main ageing factor, compared to other phenomena such as loss of specific radioactivity and loss of immunoreactivity for antibodies. (author). 5 refs, 2 figs, 3 tabs