99mTc-Mercaptoacetyltriglycine camera-based Measurement of renal clearance: Should the result be normalized for body surface area?

Abstract

Testing the rate of creatinine clearance by measuring the level of creatinine in the blood and in a 24-h rine collection is a common method of evaluating renal function. The result is routinely normalized for body urface area (BSA). Alternatively, renal clearance can be measured by 99mTc-mercaptoacetyltriglycine (MAG3) enal imaging without the need for urine collection. Frequently, the 99mTc-MAG3 camera-based result is also normalized for BSA. Methods: I evaluated the need for BSA normalization of renal clearance measurements in 9mTc-MAG3 imaging studies from both a conceptual and a mathematic point of view. Both approaches involved nalyzing the effect of patient size, that is, BSA, on the factors blood volume, renal blood flow, and amount f test substance present in the blood in the creatinine clearance method compared with the 99mTc-MAG3 camera- ased method. Results: Both the conceptual and the mathematic analyses were consistent with a significant ifference between the creatinine and 99mTc-MAG3 approaches to measuring renal clearance. Larger patients have arger kidneys, greater renal blood flow, higher renal clearances, larger blood volumes, more muscle mass, nd higher BSAs than smaller patients. However, the concentration of creatinine in the blood of patients of ny size with normal renal function is similar because the amount of creatinine released into the blood varies with patient muscle mass, which varies with blood volume. Because normalization for BSA is needed for reatinine clearance, a single reference range can be used for all patients. In the case of measurement of enal clearance with 99mTc-MAG3 imaging (assuming a constant dose), the concentration of tracer in the blood ill vary inversely with patient size because blood volume varies with patient size. Thus, as patient size ncreases, the blood concentration of tracer will go down and compensate for the increase in renal blood flow and renal clearance, and conversely. Consequently, the 99mTc-MAG3 renal imaging study is self-correcting for SA and no additional correction is needed.Conclusion: A conceptual and mathematic analysis suggests that, lthough normalization for BSA is necessary in the measurement of renal clearance by the standard creatinine learance test, such normalization is inappropriate in the 99mTc-MAG3 camera-based imaging study because the 9mTc-MAG3 method is inherently self-normalizing for BSA. © 2013 by the Society of Nuclear Medicine and Molecular Imaging, Inc.