Renal and extrarenal clearance of99mTc-MAG3: A comparison with125I-OIH and51Cr-EDTA in patients representing all levels of glomerular filtration rate

This investigation was undertaken in order to determine whether the clearance of technetium-99m mercaptoacetyltriglycine (^99mTc-MAG₃) is more closely correlated to the clearance of iodine-125 orthoiodohippurate (¹²⁵I-OIH) than to the clearance of chromium-51 ethylene diamine tetra-acetate (⁵¹Cr-EDTA) and whether there is a clinically significant extrarenal clearance of^99mTc-MAG₃ . Fifty-one patients with a glomerular filtration rate (GFR) of 4–132 ml/min were studied. After a simultaneous single injection of the three tracers, plasma clearance was measured from blood samples 0–5 h post injection (p.i.) (0–24h in patients with GFR<15 ml/min). Renal plasma clearance was measured 0–5 h p.i. The ratio between the renal plasma clearance of^99mTc-MAG₃ and¹²⁵I-OIH was 0.57. The ratio between the renal plasma clearance of^99mTc-MAG₃ and⁵¹Cr-EDTA was 2.57. The coefficient of variation (CV) of the^99mTc-MAG₃/¹²⁵I-OIH ratio was significantly smaller than the CV of the^99mTc-MAG₃/⁵¹Cr-EDTA ratio (13.4% versus 31.2%). The corresponding plasma clearance ratios were 0.59 (CV=14.8%) and 2.48 (CV=27.0%). Plasma clearance overestimated renal plasma clearance by 7.0 ml/min (5.4%) for^99mTc-MAG₃ and by 4.1 ml/min (8.4%) for⁵¹Cr-EDTA. The difference in plasma and renal plasma clearance of¹²⁵I-OIH of 5.5 ml/min did not reach statistical significance. Red blood cell binding of^99mTc-MAG₃,¹²⁵I-OIH and⁵¹Cr-EDTA was 2.0%, 14.6% and 0.2%, respectively. Protein binding of^99mTc-MAG₃,¹²⁵I-OIH and⁵¹Cr-EDTA was 86.3%, 61.1% and 5.9%, respectively. The volume of distribution of^99mTc-MAG₃,¹²⁵I-OIH and⁵¹Cr-EDTA was 16.3%, 27.0% and 19.4% of body weight. In conclusion, the clearances of^99mTc-MAG₃ and¹²⁵I-OIH are more closely correlated than is the clearance of^99mTc-MAG₃ with GFR. Extrarenal clearance of^99mTc-MAG₃ is relatively smaller than extrarenal clearance of⁵¹Cr-EDTA. Thus, plasma clearance of^99mTc-MAG₃ can be used as a measure of renal tubular function.