If radioactivity in circulation from a prior nuclear medicine test is unaccounted for, how will it affect plasma volume determination?

When radioactivity from a prior nuclear medicine test remains in circulation and is unaccounted for, it can interfere with the measurement of plasma volume. The presence of additional radioactive material in the bloodstream means that the calculated plasma volume may reflect not only the volume of the patient's actual plasma but also the volume of distribution of the residual radioactivity.

This effectively leads to a situation where the measured radioactivity will be higher than what is attributable solely to the current plasma elements being assessed. Consequently, the calculation of plasma volume can yield a falsely decreased result because the measurement assumes that all detected radioactivity is exclusively from the current test, leading to misinterpretation of a lower plasma volume.

Understanding this concept is essential when interpreting nuclear medicine results, as residual radioactivity can skew calculations and lead to incorrect clinical decisions.