Simultaneous measurement of extracellular fluid distribution and renal function with a single injection of <sup>99m</sup>Tc DTPA

Abstract

The initial rate of renal uptake of 99mTc DTPA is equal to the product of injected dose and the glomerular filtration rate (GFR):plasma volume (PV) ratio, whilst later the plasma clearance rate constant is equal to the GFR:extracellular fluid volume (ECF) ratio. Expressing GFR in terms of body fluid volumes has several attractions, and techniques for measuring it in this form may be useful. GFR/PV was determined from a Patlak plot in 82 studies in 47 patients undergoing routine gamma-camera 99mTc DTPA renography and compared with GFR/ECV, determined from the terminal slope of the plasma clearance curve. GFR/PV (y) correlated significantly with GFR/ECV (x): y = 3.2.x + 5.3 ml/min/l (r = 0.82, n = 82, P < 0.001). The ratio, GFR/PV:GFR/ECV is ECV/PV, which (x) correlated significantly with a regional estimate (y) of ECV/PV (rECV/rPV) derived from a Patlak plot applied to a subrenal ROI: y = 0.47x + 1.14 (r = 0.61, n = 52, P < 0.001). The initial rate constant of disappearance of 99mTc DTPA from the blood, determined from a ROI over the left ventricle, reflects tracer clearance from plasma to interstitial fluid plus GFR/PV: Subtraction of GFR/PV yields a rate constant which reflects extrarenal 99mTc DTPA clearance, Z, as a fraction of PV. A regional estimate of 99mTc DTPA clearance from plasma to interstitial fluid, rZ, was derived from the Patlak plot applied to the subrenal ROI and expressed in relation to rPV. Z/PV (y) correlated significantly with rZ/rPV (x): y = 2.2x + 0.02 min-1 (r = 0.56, n = 52, P < 0.001). The distribution of ECF between plasma and interstitial fluid was related to extrarenal 99mTc DTPA clearance. Thus rZ/rPV correlated significantly with ECV/PV (r = 0.55, n = 52, P < 0.001) and Z/PV correlated significantly with rECV/rPV (r = 0.6, n = 52, P < 0.001). There was no correlation between renal function and extrarenal 99mTc DTPA clearance or distribution of ECF. GFR can be measured in terms of PV and ECF volume from a single injection of 99mTc DTPA. ECV/PV, rECV/rPV, and variables representing the clearance of 99mTc DTPA across the microvasculature can be measured at the same time.