SIMULTANEOUS MEASUREMENT OF EXTRACELLULAR FLUID DISTRIBUTION AND RENAL-FUNCTION WITH A SINGLE INJECTION OF TC-99M DTPA

Background. The initial rate of renal uptake of Tc-99m DTPA is equal t o the product of injected dose and the glomerular filtration rate (GFR ):plasma volume (PV) ratio, whilst later the plasma clearance rate con stant is equal to the GFR:extracellular fluid volume (ECF) ratio. Expr essing GFR in terms of body fluid volumes has several attractions, and techniques for measuring it in this form may be useful. Methods. GFR/ PV was determined from a Patlak plot in 82 studies in 47 patients unde rgoing routine gamma-camera Tc-99m DTPA renography and compared with G FR/ECV, determined from the terminal slope of the plasma clearance cur ve. Results. 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:G FR/ECV is ECV/PV, which (x) correlated significantly with a regional e stimate (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 ini tial rate constant of disappearance of Te-99m DTPA from the blood, det ermined from a ROI over the left ventricle, reflects tracer clearance from plasma to interstitial fluid plus GFR/PV. Subtraction of GFR/PV y ields a rate constant which reflects extrarenal Tc-99m DTPA clearance, Z, as a fraction of PV. A regional estimate of Tc-99m DTPA clearance from plasma to interstitial fluid, rZ, was derived from the Patlak plo t 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 Te-99m DTPA clearance. T hus 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 ex trarenal Tc-99m DTPA clearance or distribution of ECF. Conclusions. GF R can be measured in terms of PV and ECF volume from a single injectio n of Te-99m DTPA. ECV/PV, rECV/rPV, and variables representing the cle arance of Tc-99m DTPA across the microvasculature can be measured at t he same time.