Iodine ¹²⁵I human albumin Other names: Iodinated I-125 albumin

Following intravenous injection, radioiodinated serum albumin is uniformly distributed throughout the intravascular pool within 10 minutes; extravascular distribution takes place more slowly. Labeled albumin also can be detected in the lymph and in certain body tissues within 10 minutes after injection, but maximum distribution of radioactivity throughout the extravascular space does not occur until two to four days after administration. The time at which extravascular activity is maximal has been designated as the “equilibrium time”. When this point has been reached, the radioactivity remaining in the intravascular and extravascular spaces decreases slowly and exponentially in parallel fashion.

The administered radioactivity is eliminated almost entirely in the urine, only about 2 percent of the total dose ultimately appearing in the feces. The biologic half-life of labeled albumin is dependent upon a number of factors, and published studies have varied considerably in their reporting of this figure. It has ranged, in the literature, from below 10 days to over 20 days. One important factor affecting the biologic half-life is the initial rate of excretion, and this depends in part on the quality of the labeled albumin. With iodine ¹²⁵I human albumin the biologic half-life in normal individuals has been reported to be approximately 14 days.