For renal plasma flow measurement

For renal plasma flow measurement,why inulin can’t be used as it is also extracted 100% by the kidneys and getting excreted but PAH is only 90% extracted?

Difference between inulin and PAH lies in secretion into the tubules by peritubular capillaries… PAH is secreted by peritubular capillaries…

Following filtration the amount of pah dat goes to peritubular capillaries is also eventually secreted back into tubules and so complete estimate of blood flow can be made