Functions of kidney: Kidneys perform various functions.

•• Excretion of waste products.

•• Regulation of acid-base balance.

•• Regulation of salt and water balance.

•• Secretion of renin (regulates blood pressure) and erythropoietin (involved in erythropoiesis).

TESTS FOR RENAL FUNCTION

•• Urine analysis (both routine and microscopic).

•• Blood chemistry.

•• Renal clearance tests to measure glomerular filtration rate (GFR).

•• Concentration and dilution tests.

Indications for renal function tests are listed in Box 64.1.

Urinalysis is discussed in detail in our previous post. Diseases associated with increased risk of chronic kidney disease are listed in Box 64.2.

Blood Chemistry

Impaired renal function causes elevation of end-products of protein metabolism which is known as azotemia.

These include: 

•• Blood urea (normal range 20–40 mg/dL).

•• Blood urea nitrogen (BUN): Normal range 10–20 mg/dL. Causes of raised blood urea nitrogen (BUN) are listed in Box 64.3.

•• Creatinine (normal range 0.6–1.2 mg/dL). 

Causes of raised serum creatinine are listed in Box 64.4.

Renal Clearance Tests

These tests measure the glomerular function and renal blood flow.

•• Glomerular filtration rate (GFR) is the rate at which fluid passes into the nephron after filtration and it measures renal excretory function. The rate of glomerular filtration is measured by the capacity of the kidneys to clear waste products or foreign materials (inulin, mannitol, creatinine and urea) from the blood into the urine.

•• Clearance of a solute from plasma into the urine equals GFR, if the solute is freely filtered through the glomerulus and neither secreted into nor absorbed from the renal tubules.

•• From the determination of blood concentration of the test material and the quantity eliminated in the urine, “clearance” can be calculated and is expressed in terms of milliliters of blood cleared per unit time.

The various tests are:

Inulin Clearance Test

Inulin is ideal for the measurement of GFR because it is neither secreted nor reabsorbed by the tubule. Mannitol can also be used.

Creatinine Clearance

Creatinine is normally released into the plasma by muscle metabolism and is filtered through the glomerulus. Normally, the clearance of endogenous creatinine approximates the glomerular filtration rate. It is easy and routinely employed method for GFR estimation.

Urea Clearance Test

Urea clearance test is less sensitive test.


Concentration/Dilution Tests

These tests are used to evaluate the tubular function. The ability of the nephron to concentrate or dilute urine is dependent on the tubular cells in the renal medulla as well as anti diuretic hormone (ADH).

Urine Specific Gravity

Urine specific gravity (normal range 1.003–1.035) is a measure of capacity of the tubules to reabsorb water, thus concentrating the urine. In chronic tubular damage it remains fixed at 1.010. If the patient’s urine contains no sugar or protein and has a specific gravity of 1.025 or higher, a concentration test is unnecessary.

Concentration Test

In this test, an artificial deprivation of fluid is induced in a patient for more than 20 hours. With normal tubular function, water is selectively absorbed and urine will have high solute concentration with specific gravity of 1025 or more. With tubular damage, the specific gravity remains constant irrespective of water deprivation.

Dilution Test

In this test, excessive fluid is given to the patient which will result in a lower solute concentration in a normal individual. The resulting specific gravity will be 1.003 or less. In tubular damage, the specific gravity remains constant irrespective of excessive water intake.


RENAL BIOPSY

It is used to establish the nature and extent of renal disease. Biopsy is performed transcutaneously with ultrasound guidance. Biopsy material is examined under light microscopy (fixed in neutral buffered formaldehyde), electron microscopy (fixed in glutaraldehyde) and by immunofluorescence (tested for IgG, IgA, IgM,C3, fibrin, and κ and λ light chains by using appropriate antibodies). Indications, for renal biopsy are presented in Box 64.5 and contraindications and complications of renal biopsy are presented in Table 64.1.

Points to note:

•• Renal function can be assessed by urine analysis (both routine and microscopic), blood chemistry, renal clearance tests, and concentration and dilution tests.

•• Impaired renal function results in raised values of end-products of protein and can be assessed by estimation of blood urea, blood urea nitrogen and serum creatinine.

•• Renal clearance tests are used to assess the glomerular function whereas concentration and dilution tests are for tubular function.

•• Renal biopsy is useful for establishing the nature and extent of renal disease.