Renalclearance

Renal clearance is the volume of plasma from which a substance is completely removed by the kidneys per unit time. It is typically expressed in milliliters per minute (mL/min) and can be calculated as the product of urinary concentration of the substance and urine flow rate divided by the plasma concentration (Cx = (Ux × V) / Px).

Renal clearance reflects the net effect of glomerular filtration, tubular reabsorption, and tubular secretion. Substances that are freely filtered at the glomerulus and neither reabsorbed nor secreted have clearance equal to the glomerular filtration rate (GFR).

In practice, certain substances are used to estimate GFR or renal plasma flow. Inulin clearance is the classic measure of GFR because inulin is freely filtered and neither secreted nor reabsorbed. Creatinine clearance approximates GFR but is slightly higher due to modest tubular secretion of creatinine. Other markers include cystatin C for estimating GFR and PAH clearance as an index of renal plasma flow.

Methods for determining clearance include timed urine collections (for example, 24-hour urine) or single-sample formulas that use serum concentrations. Exogenous markers require controlled dosing and collection, whereas endogenous markers can be measured with routine blood and urine tests.

Clinical use and interpretation: Renal clearance is used to assess kidney function, guide drug dosing, and evaluate renal diseases. Limitations include variability in creatinine secretion, dietary influences, muscle mass, and measurement errors; therefore, clearance estimates may differ from the true GFR and should be interpreted in the clinical context.