Para-aminohippurate (PAH) is a substance that is fully secreted by the kidney and therefore provides the best estimate of total renal plasma flow.
Renal plasma flow (RPF) is defined as the volume of plasma that flows by the nephrons. Since plasma is the portion of blood that is not occupied by the red blood cells, this value can be defined as RPF = renal blood flow (RBF) * (1 - hematocrit). The important issue to note is that only a fraction of the total renal plasma flow will be filtered in the glomeruli. A substance that evaluates renal plasma flow has to not only be filtered but also be actively secreted. PAH is often used because it is recognized by transporters and almost completely secreted by the nephron.
Snyder and Pendergraph review evidence for the evaluation of chronic kidney disease. They present common methods of estimating glomerular filtration rate (GFR) such as the Cockcroft-Gault equation; however, they claim that more formal evaluation may be needed in patients suspected of having chronic kidney disease. Some of the most common causes are diabetes, hypertension, and other systemic disorders. They also present some of the substances that are used for evaluation, such as inulin.
Yasumoto et al. studied the correlation of glomerular function indices with glycemic status in nondiabetic subjects. The main substances they used in this study are inulin for estimation of GFR and PAH for estimation of RPF. They identified that higher levels of hemoglobin A1c, which indicates a higher chronic level of glucose, was correlated with markers such as postglomerular resistance. They suggest that this may be a mechanism for glomerular hypertension even in non-diabetic subjects.
Answer 1 and 2: Creatinine and inulin are both substances that are used to estimate the GFR given that they are freely filtered. Inulin is more accurate than creatinine because about 10% of creatinine is further secreted after filtration.
Answer 3: Glucose is fully reabsorbed until all glucose transporters are saturated and therefore does not estimate renal plasma flow.
Answer 5: Urea is permeable through membranes and is reabsorbed in the loop of henle and therefore is not completely cleared by the kidney.
Snyder S, Pendergraph B. Detection and evaluation of chronic kidney disease. Am Fam Physician. 2005 Nov 1;72(9):1723-32. Review.
PMID:16300034 (Link to Abstract)
Yasumoto M, Tsuda A, Ishimura E, Uedono H, Ohno Y, Ichii M, Ochi A, Nakatani S, Mori K, Uchida J, Emoto M, Nakatani T, Inaba M. Significant association between glycemic status and increased estimated postglomerular resistance in nondiabetic subjects - study of inulin and para-aminohippuric acid clearance in humans. Physiol Rep. 2015 Mar;3(3).
PMID:25742958 (Link to Abstract)