99mTc-albumin can replace 125I-albumin to determine plasma volume repeatedly

Objective. Plasma volume assessment may be of importance in several disorders. The purpose of the present study was to compare the reliability of plasma volume measurements by technetium-labeled human serum albumin (Tc-99m-HSA) with a simultaneously performed plasma volume determination with iodine-labeled human serum albumin (I-125-HSA). Materials and methods. In 15 healthy volunteers, simultaneous plasma volume measurements with Tc-99m-HSA and I-125-HSA were performed after 1/2 hour in the supine position. Blood samples were obtained 10, 15, 20, and 30 minutes after the injection for accurate retropolation from the plasma counts to time zero to correct for leakage of the isotopes from the circulation. Results. The mean difference (bias) between plasma volume measured with I-125-albumin and Tc-99m-albumin was 8 ml (0.1 ml/kg) with limits of agreement (bias +/- 1.96 SD) ranging from -181-196 ml (-2.3-2.5 ml/kg). The tracer disappearance rate was significantly higher with Tc-99m-albumin (-23.1-7.1%/h) than with I-125-albumin (-6.7 +/- 3.6%/h) (p < 0.001). Conclusion. This study demonstrates that Tc-99m-HSA can replace I-125-HSA for single measurements of plasma volume in healthy volunteers. It needs to be emphasized however, that repeated blood sampling for 1/2 hour after injection of the tracer is required to correct for the disappearance of Tc-99m and Tc-99m-HSA from the circulation.