Renal Function Tests

M.PRASAD NAIDUMsc Medical Biochemistry,Ph.D Research scholar.

Kidney Functions:1. Urine formation2. Regulation of fluid and electrolyte balance3. Regulation of acid-base balance4. Excretion of waste products of nitrogen metabolism5. Hormonal function6. Protein conservation

Urine formation:

Normal glomerular filtration rate (GFR) is approximately 125ml/min.

About 190 liters of glomerular filtrate is formed daily(24Hours).Most of it is reabsorbed(99%) except 1 to 1.5 liters excreted as urine.

In PCT

60 to 80% of glomerular filtrate is reabsorbed.Na+ , Cl- , K+ , HCO3

- , glucose, amino acids are reabsorbed.

90% of H+ is secreted.Renal threshold for glucose – 180mg/dl.TMG - 350 mg/minute Renal threshold for bicarbonate – 30mequ/L.

Passive reabsorption of water, urea, chloride also occurs.

Tubular secretion Potassium, H+ ions , ammonia , uric acid, drugs.

Loop of Henle: Concentration of Urine

DCT Aldosterone increases sodium reabsorption and

excretion of potassium.

Ammonia is secreted in the DCT & buffers H+ ions & secreted as ammonium ions.

ADH acts on DCT & increases reabsorption of water.

Excretion of Waste Products

The End products of protein and nucleic acid metabolism like urea, creatinine , sulphate , uric acid, phosphate are excreted by the kidney.

Retention of useful compounds

Glucose, aminoacids, sodium etc are reabsorbed

Protein Conservation:

Normal urine contains < 200mg of protein / 24 hour urine.

Most of the filtered proteins are reabsorbed by PCT.

Note: Albumin excretion < 20mg/24hours urine.

Hormonal Functions:

Erythropoietin – A peptide hormone stimulates hemoglobin synthesis and formation of erythrocytes.

Calcitriol - (1,25 –DHCC)Active form of vitamin D

Renin A Proteolytic enzyme secreted by kidney Which converts angiotensinogen to angiotensin II.

Which in turn increases the secretion of aldosterone

Tests for Glomerular Function:Clearance Tests Clearance may be defined as volume of plasma in ml which contains the amount of substance which is excreted in urine in 1minute.

UV U= Substance in urine mg/100ml

P P= Substance in plasma mg/100ml

V= Volume of urine ml/mt

% normal Caluculation

eg : Urea UV X 100 X 1.73 P 75 BS

Inulin clearance 125 ml/mt (110 – 135)

Urea clearance 75 ml/mt (60 – 90)

Creatinine clearance 120 ml/mt (110 – 130)

Inulin clearance : 125ml/mt

Inulin is a plant polysaccharide contain fructose.

Inulin has to be given I.V to perform the test.

Inulin clearance test is called gold standard.Theoretically most ideal test because it is not secreted into the renal tubule and it is not reabsorbed by the renal tubules.

Inulin is excreted as such through urine.

But the test is cumbersome, needs I.V administration of inulin.

Imp: This test is not used routinely.

Urea clearance : 75ml/mt

Urea is the end product of protein metabolism.

after being filtered by the glomeruli, it is partially

reabsorbed by the renal tubules.

Its clearance is < GFR.

It is also influenced by dietary protein content.Blood urea levels tend to increase only when urea clearance value falls below 50% normalNot an Ideal test.

Creatinine Clearance Test: 120ml/mtMost commonly done test Creatinine is a waste product derived from creatine ( present in muscle). Creatinine formation and excretion mainly depends upon muscle mass.

Not influenced by dietary factors.The value of creatinine clearance is very close to GFR.

Procedure:

In the traditional test 24 hour urine is collected and creatinine concentration estimated both in urine and serum and clearance is calculated.

Modified Procedure:

Urine is collected for 1 hour after giving water.Volume of urine is noted. Serum creatinine and urine creatinine are estimated and clearance is calculated.

The creatinine clearance is lower in women(less muscle mass).

Normal serum creatinine levels 0.7 to 1.4mg/dl.Serum creatinine levels increase beyond the normal range only when more than 50% of Glomerular Function is lost.

RENAL TUBULAR FUNCTION TESTS

Normal urine specific gravity -1.015 to 1.025.

SPECIFIC GRAVITY is tested by URINOMETER.

Specific gravity increased in

diabetesmellitus,

excessive persipration.

Specific gravity decreased in

diabetes insipidus ,excessive water intake ,

chronic nephritis.

The simplest test for tubular function is the measurement of the specific gravity of urine.

Specific gravity depends on concentration of solutes .

Osmolality depends on number of osmotically active molecules.

Osmolality:

The concentration of a solution expressed as the total number of solute particle per kilogram.

Osmolarity:

the concentration of a solution expressed as the total number of solute particles per liter.

In moderate forms of kidney damage , blood level of urea and creatinine may be within normal limits.

The inability to excrete the waste products may be counterbalanced by large urine output.

Thus ,the earliest manifestation of renal disease may be DIFFICULTY IN CONCENTRATING THE URINE.

Concentration tests:

Principle : based on the ability of the kidneys to concentrate urine, and based on measuring specific gravity of urine.

Simple bedside procedure .

Easy to perform

Tests are conducted either under conditions of restricted fluid intake

or

By inhibiting diuresis by injection of ADH.

Procedure for concentration tests:

Patient is not allowed to take food or water after a meal at 6pm.

Next day at 7am bladder is emptied and specimen is discarded.

A second specimen is collected at 8 am and specific gravity is measured.

Adequate renal function: specific gravity – 1.022 and osmolality exceeds850 mosm/kg.

In renal tubular disease urine shows a fixed specific gravity around 1.010 (300mosm/kg) as the tubules loose their concentrating capacity.

ADH test ( concentration test):

Effectiveness of ADH decreases with IMPAIRED tubular function.

Procedure:

5 units of ADH injected.

Hourly samples of urine collected for next 24 hrs and tested for specific gravity.

At least one sample should have a specific gravity MORE THAN 1.020 and osmolality 800mosm/kg.

Measurement of osmolality:

Normally urine osmolality varies from 80 mosm/kg to 1200 mosm/kg .

Normal plasma osmolality is 285 – 300 mosm/kg.

Simultaneous measurements of plasma and urine osmolality and calculation of ratio of urine /plasma is more useful.

Normally the ratio varies from 3 – 4.5 .

Osmolality is measured with an osmometer.

Urine Dilution tests:

Principle: the ability of the kidneys to eliminate water is tested by measuring the urinary output after ingestion of a large volume of water.

Procedure:

Patient is not allowed to take water after midnight.

Bladder is emptied at 7 am.

Water load is given -1200ml over next 30 minutes.

Urine samples are collected hourly over next four hrs.

Volume , specific gravity and osmolality of each sample measured .

A noraml person will EXCRETE ALMOST ALL THE WATER LOAD WITHIN FOURS HOURS and specific gravity of at least ONE SAMPLE SHOULD FALL to 1.003 and osmolality to 50 mosm/kg .

The test is MORE SENSITIVE .

Urine volume,appearance ,odour , colour

Urine specific gravity

Urine pH

Urine osmolality

Blood urea

Serum creatinine

Clearance tests – creatinine clearancesss

Abnormal constituents : proteins, blood , reducing sugars , ketone bodies , bilesalts and

bile pigments.

Test for Tubular Function: PSP test (Phenol sulfo phthalein)

Give 600ml of water.Inject 6mg of dye I.V.Urine – 15 mt – 35% excreted In 2 hours 70% of dye eliminated through urine.This test indicates secretary function of tubules. Less than 50% of dye elimination in 2 hours.Indicated tubular defect.

Tubular function test:

PAH clearance 700ml/mt

Glomerular filtration rate(GFR)The rate in milliliters per minute at which plasma substances are filtered through the glomeruli into thel proximal tubule.

Renal threshold The plasma concentration of a substance above which it will be present in urine.

Specify gravityThe ratio of the weight in grams per milliliter of a body fluid compared with water.

Titratable acid:The combination of hydrogen ion with phosphate present in final urine.

Urine:The aqueous liquid and dissolved substances excreted by the kidney.

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