Glomerular filtration rate in relation to extracellular fluid volume: Similarity between99mTc-DTPA and inulin

The rate constant ( ₂) of the terminal exponential of the technetium-99m diethylene triamine pentaacetic acid (DTPA) plasma clearance curve is close to the ratio of glomerular filtration rate (GFR) to extracellular fluid volume (ECV) and is therefore a convenient, already normalised, measure of filtration function. Since ₂ depends on the distribution volume of the tracer, our aim was to compare ₂ from inulin and^99mTc-DTPA and also to compare the equilibration kinetics of the two filtration markers. Fifty millititres of^99mTc-DTPA (250 MBq) and inulin (10%), mixed in the same syringe, were given by intravenous injection in 15 patients undergoing routine^99mTc-DTPA renography for a variety of clinical indications. Frequent antecubital venous blood samples were taken up to about 4 h after injection to construct plasma clearance curves from which GFR, ECV and GFR/ECV (i.e. the reciprocal of mean transit time through the distribution volume) were calculated.^99mTc-DTPA/inulin concentration ratio curves were also constructed after normalisation to the ratio in the syringe. GFR given by the two markers correlated closely (DTPA=0.98·inulin—0.4ml/min;r=0.98).^99mTc-DTPA had the same distribution volume as inulin, had a similar transit time through it and gave the same value of ₂ (r=0.98). GFR/ECV from^99mTc-DTPA accordingly correlated closely with GFR/ECV from inulin (DTPA=0.75·inulin+0.99 ml/min;r=0.95). Even though the distribution volumes and the times to equilibration (i.e. to reach the terminal exponential) were similar, the distribution volume of^99mTc-DTPA at about 10 min after injection was, after subtraction of the plasma volume, about twice that of inulin.We confirm the validity of^99mTc-DTPA for measuring GFR. ₂ is a convenient measure of GFR, can be based on the terminal exponential of inulin of^99mTc-DTPA curves and can be converted to GFR/ECV with an appropriate scaling factor. The kinetics or the two clearance curves with respect to anatomical correlates of the exponentials and the rates of diffusion throughout the respective distribution volumes requires further study.