GLOMERULAR-FILTRATION RATE IN RELATION TO EXTRACELLULAR FLUID VOLUME - SIMILARITY BETWEEN TC-99M-DTPA AND INULIN

The rate constant (alpha(2)) of the terminal exponential of the techne tium-99m diethylene triamine pentaacetic acid (DTPA) plasma clearance curve is close to the ratio of glomerular filtration rate (GFR) to ext racellular fluid volume (ECV) and is therefore a convenient, already n ormalised, measure of filtration function. Since alpha(2) depends on t he distribution volume of the tracer, our aim was to compare alpha(2) from inulin and Tc-99m-DTPA and also to compare the equilibration kine tics of the two filtration markers. Fifty millititres of Tc-99m-DTPA ( 250 MBq) and inulin (10%), mixed in the same syringe, were given by in travenous injection in 15 patients undergoing routine Tc-99m-DTPA reno graphy for a variety of clinical indications. Frequent antecubital ven ous blood samples were taken up to about 4 h after injection to constr uct plasma clearance curves from which GFR, ECV and GFR/ECV (i.e. the reciprocal of mean transit time through the distribution volume) were calculated. Tc-99m-DTPA/inulin concentration ratio curves were also co nstructed after normalisation to the ratio in the syringe. GFR given b y the two markers correlated closely (DTPA=0.98 . inulin-O.4 ml/min; r =0.98). Tc-99m-DTPA had the same distribution volume as inulin, had a similar transit time through it and gave the same value of a, (r=0.98) . GFR/ECV from Tc-99m-DTPA accordingly correlated closely with GFR/ECV from inulin (DTPA=0.75 . inulin+O.99 ml/min; r=0.95). Even though the distribution volumes and the times to equilibration (i.e. to reach th e terminal exponential) were similar, the distribution volume of Tc-99 m-DTPA at about 10 min after injection was, after subtraction of the p lasma volume, about twice that of inulin. We confirm the validity of T c-99m-DTPA for measuring GFR. a, is a convenient measure of GFR, can b e based on the terminal exponential of inulin of Tc-99m-DTPA curves an d can be converted to GFR/ECV with an appropriate scaling factor. The kinetics or the two clearance curves with respect to anatomical correl ates of the exponentials and the rates of diffusion throughout the res pective distribution volumes requires further study.