Examination of renal function

Assessment of the glomerular filtration rate - renal clearance
If we want to determine GFR, which is one of the basic function of our kidneys, we have to use a substance that is excreted from the body only by glomerular filtration (inulin, creatinine) and is not affected by tubular processes. As an example we can mention the calculation of the clearance (plasma volume that is per unit time completely cleaned of marker substances) of endogenous creatinine, whose formula has the following form :

$$C_X = \frac{U_X \cdot V}{P_X}$$

CX - clearance of substance X

Ux - concentration of substance X in urine

V - urine volume

Px - concentration of substance X in plasma

The smallest value of renal clearance is 0 (plasma is not purified from the substance at all), the maximum value is equal to the plasma flow through the kidneys (all plasma flowing through the kidneys is purified from the substance).

The clearance of substances that are freely filtered in the glomeruli and not secreted or resorbed in the tubules is used to determine the glomerular filtration rate (eg inulin or creatinine clearance).

In clinical practice, we use more complex calculations, corrected for body surface area (and other physical parameters) – e.g. equation by Cockroft and Gault, equation MDRD etc.

Examination of renal tubule function

 * Examination of concentration ability of kidneys
 * Concentration test using thirst
 * very unpleasant; 12 hours of thirstiness, urine sample every 4 hours – urine density and osmolality; also a blood sample
 * Adiuretin test
 * more pleasant for patient; no drinks and food during night, ADH application in the morning through the nasal mucosa – urine density and osmolality


 * Examination of dilution ability of kidneys
 * test of reaction on increased water intake – decreased ADH production + increased diuresis in healthy people

Filling and emptying of the bladder
Cystometrogram (under construction)