RENAL FUNCTIONS

TESTby

V. KUZHANDAI VELU

K I D N E Y:

Functions of the kidney

REGULATION

Homeostasis, water, acid/base

EXCRETION

urea, creatinine

ENDOCRINE

renin,erythropoietin, 1,25 dihydroxycholecalciferol

OBJECTIVES of RFTs:

To detect possible renal damage

Assessment of its severity.

To observe the progress of renal disease

To monitor the safe and effective use of drugs

which are excreted in the urine

CLASSIFICATION Of RFT’s: Urine Analysis

Physical examination Chemical examination Microscopic examination

Test for Glomerular Function Renal clearance test Blood analysis of urea

and creatinine Proteinuria and

heamaturia

Test for Renal Plasma Flow Para-aminohippurate

test Test for Tubular

Function Urine concentration test Urine dilution test Glycosuria Amino aciduria Acid load test Phenosulfonpthalein

test

Physical examination: Volume:

Normal output = 800 – 2,500 ml / day Polyuria > 2,500 ml / day Oliguria < 500 ml / day Anuria: complete cessation of urine

Appearance: - Normal urine is transparent pale yellow or

amber colour Blood Colour (haemoglobin, myoglobin,) Turbidity (infection, nephrotic syndrome)

pH

However depending on the acid-base status, urinary pH

may range from as low as 4.5 to as high as 8.0.

Specific Gravity

Normal – 1.016 to 1.022

Osmolality

Average is 300 – 900 mosm/kg.

Odour

Normal – aromatic

Foul smell – Bacterial infection

Chemical examination

Chemical examination includes detection

of the following

Glucose

Protein

Blood

Glucose

Normal urine contains small amount of glucose which can not

be detected by routing test

Excretion of detectable amounts of reducing sugar in urine is

called glycosuria. It may be benign or pathological.

Protein

It increased amount of protein in urine

Proteinuria – increased glomerular permeability or reduced

tubular reabsorption.

Most common type of proteinuria is due to albumin

Blood or Hematuria

Renal stone, cancer, tuberculosis trauma of kidney or acute

glomerulonephritis

Test for Glomerular Function

Renal clearance test

Blood analysis of urea and creatinine

Proteinuria and hematuria

Renal clearance test:

The renal clearance of a substance is defined as the

volume of plasma from which the substance is

completely cleared by the kidneys per minutes

Depends on the plasma concentration of the substance

and its excretory rate, which in turn, depends on the GFR

and renal plasma flow.

GFR can be measured by determining the excretion rate

of a substance which is filtered through the glomerulus

but subsequently, is neither reabsorbed nor secreted by

tubules.

C = (U x V)/P

C is the clearance of the

substance in ml/minutes

U is the concentration of

the substance in urine

(mg/L)

P is the concentration in

plasma (mg/L)

V is the volume of urine

passed per minute.

Lower GFR

Indicates:

Acute tubular necrosis

Glomerulonephrosis

Shock, Acute nephrotic

syndrome

Acute and chronic renal

failure

Selected substrate should be

Freely filtered by glomerulus

Should not be reabsorbed or secreted

Should not be metabolized by the kidney

Should not be toxic

Should not be affected by dietary intake

2 types of substance are used for GFR

Endogenous – creatinine and urea

Exogenous – Inulin

CREATININE CLEARANCE TEST Creatinine is freely filtered at the glomerulus and is not

reabsorbed by the tubule.

A small amount of creatinine is secreted by tubules.

Creatinine clearance is determined by collecting urine over

24-hr period and a sample of blood is during the urine

collection period.

Creatinine from plasma directly related to the GFR.

Clinical interpretation:

Normal:– 90 – 120 ml/mints

Decreased filtrated rate – acute and

chronic damage to the glomerulus,

reduced blood flow

UREA CLEARANCE TEST

Normal value – 75 ml/min

Its less sensitivity

Conc. Of urea affected by dietary protein,

fluid intake, infection, surgery, etc.

Approximately 40 % of the filtered urea is

normally reabsorbed by the tubules.

INULIN CLEARANCE TEST Fructose polymer inulin satisfies the criteria as an

ideal marker of glomerular filtration rate.

Normal value – 120 ml/min

Disadvantages:

Need intravenous administration

Difficulty of analysis

BLOOD ANALYSIS Blood analysis may

be more sensitive when the renal failure is advance

Impairment of renal function results in elevation of blood urea and creatinine.

Increase end products of these substances called Azotaemia.

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0 25 50 75 100 125Seru

m Cr

eatin

ine (µ

mol/L

)

Creatinine Clearance (ml/min)

Relationship between Serum Creatinine Concentration and Creatinine Clearance

Use of Formulae to Predict Clearance

• Formulae have been derived to predict Creatinine Clearance

(CC) from Plasma creatinine.

• Plasma creatinine derived from muscle mass which is related

to body mass, age, sex.

• Cockcroft & Gault Formula

CC = k[(140-Age) x weight(Kg))] / Creatinine (µmol/L)

k = 1.224 for males & 1.04 for females

• Modifications required for children & obese subjects

• Can be modified to use Surface area

PROTEINURIA The glomerular basement membrane does not

usually allow passage of albumin and large

proteins. A small amount of albumin, usually less

than 25 mg/24 hours, is found in urine.

When larger amounts, in excess of 250 mg/24

hours, are detected, significant damage to the

glomerular membrane has occurred.

Quantitative urine protein measurements should

always be made on complete 24-hour urine

collections.

Albumin excretion in the range 25-300 mg/24

hours is termed microalbuminuria

Normal < 200 mg/24h. Causes: -

overflow (raised plasma Low MW Proteins, Bence Jones, myoglobin)

glomerular leak decreased tubular reabsorption of protein (RBP,

Albumin) protein renal origin

Test for Tubular Function

Assessment of the concentration and dilution ability of

the kidney can provide the most sensitive means of

detection early impairment in renal function since the

ability to concentrate or dilute urine is dependent upon

Adequate GFR

Renal Plasma Flow

Tubular mass

Healthy tubular cells

Vasopressin hormone

URINE CONCENTRATION TEST The ability of the kidney to concentrate

urine is a test of tubular function that can be carried out readily with only minor inconvenience to the patient.

This test requires a water deprivation for 14 hrs and has replaced the previous 24 hrs water deprivation test.

The test should not be performed on a dehydrated patient.

URINE DILUTION TEST

This test is very simple, but because it is less sensitive than the

water deprivation test as test of renal damage, its use is not

often required.

METHOD

After an overnight fast the patient (who is not allowed to smoke)

empties his bladder completely and is given 1000 ml of water to

drink. Urine specimens are collected for the next 4 hours, the

patient emptying bladder completely on each occasion.

INTERPRETATION Unless there is renal functional impairment, the patient will

excrete at least 700 ml of urine in the 4 hours, and at least one specimen will have a specific gravity less than 1.004.

URINARY ACIDIFICATION TESTThis procedure tests the ability of the renal tubules to form an acidic

urine and to excrete ammonia. It is useful if there is doubt whether a

patient's acidosis (confirmed by plasma analyses) is due to a pre-renal

cause, or to kidney damage as in renal tubular acidosis.

METHOD

The patient fasts from midnight until the conclusion of the test, zero

time. The patient empties his bladder completely. The urine is collected.

The patient takes 0.1 g (1.9 m mol) of ammonium chloride/kg body

weight and drinks a liter of water. A standard dose of 5 g is sometimes

used. In children the dose should be proportional to the body surface

area. At 2 hours, 4 hours, and 6 hours; complete urine specimens are

collected.

INTERPRETATION

In a normal subject the urine will be acidified to pH 5.3 or

less, and will contain more than 1.5 m mol of ammonia per

hour, in at least one of the specimens.

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