SN1 SHORT NOTE 5M ★ High-yield

Define renal clearance. Describe inulin and creatinine clearance and their significance.

Physiology · 1st Professional — Pre-Clinical

Renal clearance of a substance = the volume of plasma completely cleared of that substance by the kidneys per minute.

C = (U × V) ÷ PU = urinary concentration, V = urine flow (mL/min), P = plasma concentration

Clearance	Interpretation	Example
= GFR (125 mL/min)	Freely filtered, neither reabsorbed nor secreted	Inulin, creatinine
< GFR	Filtered and reabsorbed	Urea (70), Na+, glucose (zero)
> GFR	Filtered and secreted	PAH (625), penicillin
Zero	Completely reabsorbed	Glucose (below threshold), amino acids

Inulin is a fructose polymer from the dahlia tuber
Properties — freely filtered, not reabsorbed, not secreted, not metabolised, non-toxic
Therefore inulin clearance = GFR = 125 mL/min
Drawback — requires continuous intravenous infusion and timed collections, so it is a research rather than a bedside method

Endogenous, from muscle; production is fairly constant
Freely filtered, but a small amount is secreted → therefore it slightly overestimates GFR (about 10–20%)
Requires a 24-hour urine collection
Cockcroft–Gault formula estimates it from serum creatinine, age and weight

Para-aminohippuric acid is filtered and actively secreted → almost completely cleared in one circulation
PAH clearance = effective renal plasma flow = 625 mL/min
Renal blood flow = RPF ÷ (1 − haematocrit) = 625 ÷ 0.55 = 1200 mL/min
Filtration fraction = GFR / RPF = 125/625 = 20%

Serum creatinine is insensitive — it stays normal until GFR has fallen by about 50% ("creatinine blind range")
Clearance measurement is used to stage chronic kidney disease and to adjust the dose of renally excreted drugs
Cystatin C is a newer marker, independent of muscle mass

All questions in Renal System