Renal function tests

Dr.S.Sethupathy,M.D.,Ph.D.,Professor of Biochemistry,

RMMC, AU

Kidney functions1 Excretion of waste products Kidney excretes metabolic waste products such as urea, creatinine and uric acid.

2 Maintains homeostasisKidney maintains water and electrolyte balance with the help of antidiuretic hormone (Water Balance ) and renin – angiotensin – aldosterone mechanism (Electrolyte balance) and maintains acid base balance by reabsorbing sodium bicarbonate .

3.Hormonal function It produces erythropoietin which helps in promoting erythropoiesis in bone narrow . It plays a role in calcium metabolismRenal enzyme 1- alpha-hydroxylase converts 25-hydroxy cholecalciferol to 1,25 dihydroxy cholecalcified (calcitriol) which is highly potent in promoting intestinal calcium absorption.

It has two components 1. Glomerular apparatus - the Filtration unit

2. Tubular system for selective reabsorption

The renal function tests are used to assess glomerular and tubular functional efficiency for which blood and urine samples are used.

Nephron is the functional unit of kidney.

When it is unusually more than 2500 ml / 24 hours it is called polyuria. It can be due to

1. Increase in water excretion 2. Increase in solute excretion causing osmotic water

loss

Increase in water loss due to either diminished tubular dysfunction with decreased concentration ability or ADH deficiency. ADH deficiency results in diabetes insipidus. In this case urine specific gravity will be lower.

In case of diabetic mellitus due to glucosuria, there will be polyuria. In this case urine specific gravity will be higher.

Urine volume

When urine output is lesser then 400 ml / 24 hrs, it is called oliguria. If no urine is passed, then it is called anuria. Oliguria and anuria can be the result of following: Diminished perfusion of the kidney due to Diminished blood volume (eg. Dehydration, hemorrhage) or Diminished blood flow (eg. Cardiac failure) Renal diseases such as acute glomerulonephritis, tubular necrosis.Obstruction to the outflow e.g. prostate tumor, bladder stones, renal stones etc.

Normally it is pale yellow or amber colour

Hematuria or hemoglobinuria produce a dark brown colour. By microscopy, hematuria can be detected.

Pyuria (pale, turbid ) can be due to infection .

Yellow colour can be due to jaundice or B complex vitamins intake or concentrated urine due to reduced water intake

Color of urine

pH of urine usually acidic pH6 (4.5-8-pH )

specific gravity Normally varies from 1.016 to 1.025

Osmolality On average fluid intake, it ranges from

300 to 900 mosmol/kg

Odour Foul smell indicates bacterial infection

Detectable amount of protein in urine indicates glomerular leak and is the first sign of glomerular injury.

Normally the urinary excretion of albumin is lesser then 30 mg/24 hrs.

When the excretion is between 30-299 mg/24 hrs, it is detected by special test called test for microalbuminuria or paucialbuminuria

If it is more than 300 mg/24 hrs, it is called macroalbuminuria which can be detected by heat coagulation test or uristicks.

In case of severe damage to glomerulus, hematuira also occurs.

Tests of glomerular function

The renal clearance of a substance is defined as the volume of plasma from which the substance is completely cleared by the kidneys per unit time.

Creatinine clearance test is based on the rate of excretion of creatinine, a metabolic waste product produced at a steady level.

For this test, 24 hours urine is collected. It's volume, the urinary creatinine concentration and the plasma creatinine concentration were measured.

Creatinine Clearance Test

Creatinine clearance is calculated as follows: C = u x v --------- P C = creatinine clearance in ml/min U= concentration of creatinine in urine (mg/dl) P = plasma creatinine concentration (mg/dl) V = volume of urine passed per minute (Volume of urine collected in 24 hours / (24 x 60 ) =

urine volume per minute) Normal value Men : 75-125 ml/min ; Women : 65-115 ml/min It is decreased in renal dysfunction and indicates

decreased glomerular filtration rate (GFR)

Inulin clearance is exogenous compound and the blood level is maintained, It is neither secreted nor reabsorbed. It gives true GFR.

Clearance test is useful in the early stages of renal disease

In moderate impairment, blood urea, serum creatinine are elevated. That condition is known as azotemia or uremia

Creatinine clearance is little higher because some amount is secreted.

Urea clearance is lesser because some amount of urea is reabsorbed.

Urine concentration (or) fluid deprivation test After 15 hrs of withholding fluid intake, the first

urine sample collected should have osmolality more than 850 mosm/kg or specific gravity more than 1.025

If it is lesser then these values, it could be due to 1. Renal tubular defect (nephrogenic diabetes

insipidus) 2. ADH deficiency (diabetes insipidus) On ADH stimulation test, if it becomes normal then

it is due to ADH deficiency and not due to tubular defect.

Tubular function tests

In dilution test, after emptying the bladder, 1200 ml of water is given. Urinary specific gravity should fall to 1.005 or an osmolality lesser the 100 mosml/kg

Urine specimens are collected hourly for next four hours

In renal tubular disease, there will be a fixed specific gravity.

Urine dilution test

To assess the ability of kidney to reabsorb bicarbonate and excrete hydrogen ions.

Ammonium chloride (100 mg/kg) in gelatin capsule is given.

Urine is collected hourly for eight hours. pH of urine normally falls between 4.6 and 5.0.

But in renal tubular acidosis, it does not fall below 5.3.

Urine acidification test

Patient is given 600 ml water initially. Phenolsulphthalein test dye 6 mg in 1 ml

saline is given intravenously and urine samples are collected at 15, 30, 60, 120 minutes.

If the 15 minute urine contains 25% or more, the test is normal.

If it is lesser than 25%, it indicates impaired renal excretory function.

Phenolsulphthalein (PSP) test

Blood urea 15- 40 mg/dL Serum creatinine 0.6 -1.2 mg/dL Serum uric acid 4-7 mg/dL F: 3-6 mg/dl Sodium 135 -145 mmol/L Potassium 3.5 - 5.0 mmol/dL Chloride 96 - 106 mmol /dL Bicarbonate 23 - 27 mmol/dL Arterial blood pH 7.35 - 7.45 Arterial pCO2 35 - 45 mm of Hg Arterial pO2 80 - 100 mm of Hg

Normal values

AlbuminGlucosepHspecific gravityOsmolalityculture and sensitivity Amino acids

Urine investigations

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