title: 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 urine collection is a common method of evaluating renal function. The result is routinely normalized for body surface area (BSA). Alternatively, renal clearance can be measured by (99m)Tc-mercaptoacetyltriglycine (MAG3) renal imaging without the need for urine collection. Frequently, the (99m)Tc-MAG3 camera-based result is also normalized for BSA.
  
  
  METHODS
  I evaluated the need for BSA normalization of renal clearance measurements in (99m)Tc-MAG3 imaging studies from both a conceptual and a mathematic point of view. Both approaches involved analyzing the effect of patient size, that is, BSA, on the factors blood volume, renal blood flow, and amount of test substance present in the blood in the creatinine clearance method compared with the (99m)Tc-MAG3 camera-based method.
  
  
  RESULTS
  Both the conceptual and the mathematic analyses were consistent with a significant difference between the creatinine and (99m)Tc-MAG3 approaches to measuring renal clearance. Larger patients have larger kidneys, greater renal blood flow, higher renal clearances, larger blood volumes, more muscle mass, and higher BSAs than smaller patients. However, the concentration of creatinine in the blood of patients of any 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 creatinine clearance, a single reference range can be used for all patients. In the case of measurement of renal clearance with (99m)Tc-MAG3 imaging (assuming a constant dose), the concentration of tracer in the blood will vary inversely with patient size because blood volume varies with patient size. Thus, as patient size increases, the blood concentration of tracer will go down and compensate for the increase in renal blood flow and renal clearance, and conversely. Consequently, the (99m)Tc-MAG3 renal imaging study is self-correcting for BSA and no additional correction is needed.
  
  
  CONCLUSIONS
  A conceptual and mathematic analysis suggests that, although normalization for BSA is necessary in the measurement of renal clearance by the standard creatinine clearance test, such normalization is inappropriate in the (99m)Tc-MAG3 camera-based imaging study because the (99m)Tc-MAG3 method is inherently self-normalizing for BSA.
authors.affiliation: Division of Nuclear Medicine, Department of Radiology, University of Colorado School of Medicine, Aurora, Colorado.
authors.forename: William C
authors.initials: WC
authors.lastname: Klingensmith
mesh_terms: D001830:Body Surface Area; D003404:Creatinine / Q000097:blood / Q000652:urine; D006801:Humans; D007668:Kidney / Q000000981:diagnostic imaging / Q000502:physiology*; D007677:Kidney Function Tests / Q000592:standards*; D011877:Radionuclide Imaging; D012016:Reference Values; D017263:Technetium Tc 99m Mertiatide
chemical_list: D017263:Technetium Tc 99m Mertiatide; D003404:Creatinine
keywords: 99mTc-MAG3; clearance; creatinine clearance; image analysis; kidneys