Protein binding of 99Tcm-DTPA compared with other GFR tracers

99Tc m -Diethylenetriaminepentaacetic acid ( 99 Tc m -DTPA) has been postulated to be more bound to plasma proteins than are other radiopharmaceuticals used for measurement of glomerular filtration rate (GFR). The results of protein binding experiments are, however, highly related to methodology. The aim of the present study was to compare the protein binding of the 99 Tc m -DTPA preparation we use in our daily routine with that of the other five 99 Tc m -DTPA preparations available and with other GFR tracers by using the same ultrafiltration method. Protein binding was studied in vitro after incubation in donor plasma and in vivo after single injection in patients. A correction for the uneven distribution of ultrafiltrable ions ('Donnan effect') was included. Quality control of six 99 Tc m -DTPA preparations performed with thin layer chromatography showed a high labelling efficiency (96.2-99.9%). The in vitro protein binding ranged from 9.25% to 11.12%. The in vitro protein binding of 99 Tc m -DTPA was not significantly different from that of 51 Cr-ethylenediaminetetraacetic acid ( 51 Cr-EDTA) and 125 I-iothalamate (10.99% vs 12.15% and 9.55%). There was no difference in the in vivo protein binding of 99 Tc m -DTPA 5 and 40 min after injection (13.28% VS 12.58%), and there was no difference in the protein binding of 99 Tc m -DTPA and 51 Cr-EDTA (12.93% vs 12.54%). In conclusion, in vitro and in vivo protein binding of 99 Tc m -DTPA was 10-13%. The protein binding was not different from the binding of 51 Cr-EDTA and 125 I-iothalamate.