ASSESSMENT OF THE PLASMA-VOLUME PRODUCT TO CALCULATE GLOMERULAR-FILTRATION RATE

To further validate the rate of renal uptake of the Tc-99m-DTPA plasma volume product (RUPV) method to estimate glomerular filtration rate ( GFR), 104 determinations were performed and compared to blood sample o f GFR assays. The interassay consistency was also studied in 42 patien ts. Methods: The studies were performed with 370-550 MBq (10-15 mCi) o f Tc-99m-DTPA and a gamma camera. The 3-min cumulative renal uptake wa s calculated from the renogram curves and expressed as the rate of ren al uptake in min(-1). The plasma volume, in milliliters, was estimated from the patients body weight. The GFR (ml/min) was calculated from [ RU] x [PV] and by using two blood samples. To study interassay consist ency, two determinations of GFR were performed on separate days. Resul ts: The regression equation relating the rate of renal uptake (RU) in the abscissa and the GFR obtained from plasma samples in the ordinate is: y = 3.13 + 10.5x (n = 104; r = 0.90). The regression equation of R UPV estimated GFR (x) compared to the GFR calculated from blood sample s (y) is: y = 6.9 + 0.91x (n = 104; r = 0.90). The interassay consiste ncy study showed no statistically significant difference between measu rements obtained on Days 1 and 2. The mean + s.e.m. GFR for each deter mination were 84.3 +/- 6.12 and 81.9 +/- 6.21. For the blood sample me thod, the mean s.e.m. for each day were 87.26 +/- 6.69 and 96.86 +/- 6 .58 (p < 0.05). The percent variation coefficient for the RUPV method was: CV% = 6.8 +/- 2.7 and 12.1 +/- 3.3 (p < 0.03) for the blood sampl e method. Conclusion: The observed accuracy of the determination is co mparable to that in our previous study of a separate patient populatio n at another hospital. This method would be suitable for interinstitut ional comparison and for longitudinal patient studies.