Effect of food and activity on the reproducibility of isotopic GFR estimation

The reproducibility of the plasma clearance of 99Tcm DTPA was studied in 26 patients under standardized conditions with the subject fasting and at rest. The coefficient of variation of duplicate measurements in patients with glomerular filtration rates (GRF's) ranging from 11-103 ml min-1 was 8%. Mean GFR following a breakfast containing 670 kcal and 31 g protein was increased significantly from 40.7 +/- 28.1 ml min-1 to 43.6 +/- 30.8 ml min-1. When fasted but permitted free exercise there was no consistent trend in GFR but the coefficient of variation of duplicate estimates increased significantly to 12.1%. It is recommended that routine GFR measurement should be carried out fasting or following a light diet with restricted activity.