PREDICTION OF URINARY-EXCRETION OF TECHNETIUM-99M-MAG3

The urinary excretion of Tc-99m-mercaptotriacetylglycine (MAG3), like that of I-131-orthoiodohippurate (OIH), can be used to identify acute renal transplant rejection and measure its severity. This parameter is often quantitated as the excretory index (observed excretion/predicte d excretion). A new method for predicting the urinary excretion of Tc- 99m-MAG3 is presented. Methods: The expected excretion was calculated from multisample plasma time-activity curves in 122 subjects, with cor rection for the first pass of the initial bolus. The resulting formula was tested prospectively against actual urine measurements in an addi tional 466 subjects. Results: Least-squares fitting led to the followi ng equation: Predicted excretion = 0.79(1-exp(-0.0066C(MAG3)), with re sidual s.d. 0.06, where C-MAG3 is MAG3 clearance in ml/min and the pre dicted excretion is expressed as a fraction of the administered dose. Tested prospectively in the additional 466 subjects, the s.d, was 0.09 , Conclusion: A new formula to predict the urinary excretion of Tc-99m -MAG3 has been developed and prospectively validated. Based on our dat a, the normal range for the excretory index using MAG3 is the same as that of I-131-OIH, 0,8-1.2.