How to carryout kidney Function Tests: Purpose, Types, and Procedure

The kidney’s primary function is to excrete water soluble waste products from the body.It also  has different filtration, excretion and secretary functions.The disruption of any of these functions would either lead to a reduction in the excretion of waste products and, consequently, their accumulation in the body or the loss of some vital nutrient.Based on the level of these excretory products and nutrients in the urine and blood, we can make an accurate calculation to decipher the efficiency of the kidney in its different functions.

The kidney functional unit is called a nephron. It consists of two main parts, the tube system and the glomerulus.The glomerulus consists of a capsule of a bowman and a tuft of leaky blood vessels encapsulated in the capsule of the bowman.Glomerulus ‘ primary purpose is filtration.The leaky vessels filter almost all water, electrolytes, small proteins, nutrients like sugar, etc . into the glomerulus and excretory products like urea, etc.

Filtrations depend on the particles ‘ size and load.The average pore size is 8 nm, so only smaller particles pass through.The basement membrane also carries a negative charge, which prevents the passage of negatively charged particles.

The tubular system is responsible for the reabsorption of most water, electrolytes, nutrients and the excretion of the remaining nutrients through tubular secretion. The urine concentration of these tubules is responsible.

Kidney and Nephrone functional unit
Kidney and Nephrone functional unit

What are the Components of Kidney function test

The Kidney function test are test for kidney disease or kidney failure test and includes test that measures the glomerular function and Kidney functions.The following are a list of all test found in the kidney function test panel.

  • Urine examination
  • Serum Urea
  • Serum creatinine
  • Blood urea nitrogen (BUN)
  • Calcium
  • Phosphorus
  • Protein
  • Albumin
  • Creatinine clearance
  • Urea clearance
  • Inulin clearance
  • Dilution and Concentration test
  • Serum electrolyte levels

URINE EXAMINATION(urine analysis test)

Before we carry out a quantitative urinalysis, a qualitative examination is necessary because it provides excellent indications of the nature and location of the lesion in the renal system.

This examination consists of a physical examination in which the colour, smell, quantity, specific gravity, etc . of the urine is recorded. A microscopic examination of the urine or urine microscopy is performed to exclude any pus cells, Rbc casts and crystals.

Also read : How to collect urine sample from male,female,Infants and children

SERUM UREA

  • Urea is the end product of catabolism of protein.
  • The urea is produced from the amino group of the amino acids and through the Urea cycle is produced in the liver.
  • Urea is filtered at the glomerulus as well as secretion and reabsorption at the tubular level. The increase in serum urea levels is generally considered to be a marker of renal dysfunction, particularly glomerular dysfunction. The urea level only increases when the glomerular function has fallen below 50 percent.
  • The normal urea level in the serum is between 20 and 45 mg / dl.However, the level can also be influenced by diet and certain non- kidney disorders.A high protein diet can increase the blood urea level.Similarly, a low- protein diet can lower the urea level in the blood.Other causes of protein catabolism such as hyper metabolic disorders, hunger, etc also cause elevated urea levels in the blood.Similarly, the urea level can be reduced if the liver is injured.
  • Although blood urea is not an excellent marker for renal dysfunction, as it rises relatively late and its rise does not exclusively trigger renal dysfunction, for practical purposes serum urea is still one of the most orderly tests and forms an important part of the renal function test.
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Urea is measured either by UV kinetic method using á ketoglutarate as an NH3 + acceptor in the presence of enzyme glutamate dehydrogenase in diagnostic laboratories.

It is also colorimetrically measured by the end point method of Berthelot and read in visible range using a calorimeter.

BLOOD UREA NITROGEN (BUN)

  • Sometimes the urea level of the serum is expressed as blood urea nitrogen abbreviated BUN.
  • It can be calculated easily from the urea level of the serum.
  • Urea has a molecular weight of 60 and contains two nitrogen atoms with a combined atomic weight of 28. Thus the contribution of nitrogen to the total weight of urea in serum 28/60 is 0,47.
  • Therefore, urea levels in serum can be easily converted to BUN by multiplication with 0.47.(normal bun levels)
  • An increase in blood nitrogen levels is referred to as azotemia.

CALCIUM

  • This test measures the calcium content in your blood and not the calcium in your bones.
  • The body needs it to build and fix bones and teeth, to help nerves work, to contract muscles, to help clot the blood and to help the heart work.
  • The Calcium test screens for parathyroid or kidney problems, certain types of cancer and bone problems, pancreatic inflammation and kidney stones.

Normal value : 8.5 to 10.2 mg/dl

PHOSPHORUS

  • Phosphorus is a mineral which accounts for 1% of the total body weight of a person.
  • The body needs phosphorus to build and repair bones and teeth, to help nerves work and to tighten or contract muscles.
  • The kidneys help to control the phosphate content of the blood.
  • Extra phosphate is filtered through the kidneys and is released into the urine from the body.
  • It plays an important role in the use of carbohydrates and fats by the body and in the synthesis of protein for cell and tissue growth, maintenance and repair.
  • High levels of phosphorus in the blood occur only in people with serious kidney disease or serious calcium regulation dysfunction. Although rare, excessively high phosphorus levels in the blood can combine with calcium to form deposits in soft tissues such as muscle.

Normal Results: Standard range not available

PROTEIN

  • The protein in the urine is significantly elevated in all kidney diseases except obstruction and is therefore a very sensitive, general screening test for kidney diseases, although not specific.
  • The magnitude of proteinuria also provides useful information.
  • The highest level of proteinuria is found in nephrotic syndrome( > 3 to 4 g / day).
  • In a kidney disease with nephrotic syndrome, the excretion rate of the urinary protein is usually about 1- 2 g / day
  • In a tubulointerstitial disease, the urine protein is generally less than 1 g / day.
  • Only in nephrotic syndrome is the protein loss in the urine large enough to cause hypoproteinemia.

Serum protein can generally be maintained at concentrations above the lower normal limit by increasing hepatic protein synthesis, provided that the protein loss is less than 3 g / day.

SERUM CREATININE LEVEL(creatinine blood test)

  • Creatine is a small muscle tripeptide.It remains phosphorylated and releases energy for muscle activity.It is released during regular wear and tear from the muscles and converted to creatinine( its inner anhydride).It should be remembered that creatinine is not a toxic waste, unlike urea.It is simply used as a renal function marker.
  • Creatinine is freely filtered in the glomerulus and is also secreted into the tubules to a very small extent. Any problem with glomerular filtration has a significant effect on creatinine excretion, leading to a significant increase in serum creatinine.
  • The normal level of serum creatinine or creatinine normal range is 0.6 to 1.5 mg / dl.
  • Serum creatinine is a better indicator of renal function and, in particular, of glomerular function than urea.
  • The level of creatinine depends on the muscle mass and wear and tear of the muscle.
  • The creatinine level of individuals with a vastly different muscle mass may differ significantly.For example, a bodybuilder or athlete has higher levels of creatinine than a sedentary worker on the desk.
  • In the case of muscle trauma or excessive wear, creatinine levels will also increase, as appears to be the case with athletes and hard- working physicians.
Also read  Urine Dipstick principle and significance of parameters

Creatinine is most commonly measured colorimetrically in laboratories using Jaffe’s method

CREATININE CLEARANCE RATE

  • Creatinine is filtered at the glomerulus and its reabsorption at the tubular level is insignificant. For this reason, creatinine clearance can be used to measure glomerular filtration rate( GFR).
  • It is measured over a period of 24 hours. The urine is collected over a period of 24 hours and blood samples are taken. Creatinine concentration is measured in both urine and serum samples.

The following formula is used to measure the Creatinine clearance

Calculating creatinine clearance
Calculating creatinine clearance

The Normal creatinine clearance range is

Male100 – 120 ml/ min
Female95 – 105 ml/min

This is very similar to the glomerular filtration rate.

UREA CLEARANCE

  • Urea clearance is the hypothesis that the kidney clears urea in a minute. This is measured by measuring the urea concentration in the blood, the urine concentration and the quantity of urine excreted over an hour.
  • Urea clearance is less than glomerular filtration because some of the urea filtered in the glomerulus is reabsorbed in the tubules.

To measure urea clearance, the patient first voids urine and then drinks two glasses of water.After an hour, the urine is collected and a blood sample is collected simultaneously.The urine sample is then collected after another hour.The urea level is measured in the two urine samples and the blood sample.The volume of urine is calculated as urine output per minute.If the urine output exceeds 2 ml / minute, the urea clearance is measured as:

If urine output is below 2 ml / minute, the urea clearance( in ml / min) is measured as:

The maximum urea clearance of an average surface area of 1.73 square meters is 75 ml / min and the standard urea clearance is 54 ml / min. A clearance of the urea below 60% of the standard is regarded as impaired.

INULIN CLEARANCE

Inulin is a small, low- molecular fructose polysaccharide. The substance used should have the following qualities to measure glomerular filtrate:

  1. It shouldn’t be toxic
  2. Should not be metabolized in the body.
  3. Should be completely filtered at the level of the  glomerulus.
  4. Should not be secreted or reabsorbed into the tubules.

Inulin meets all these criteria and is therefore a suitable candidate for the measurement of GFR. The inulin clearance thus corresponds to the GFR. GFR is the amount of blood filtered through the glomerulus in one minute.

  • To measure the inulin clearance, inulin is first introduced into the blood by means of a slow continuous infusion, in order to achieve a constant concentration of inulin in the blood.First, 30 ml of 10% inulin is infused in 250 ml of normally infused saline solution at a rate of 20 ml / min to achieve the desired concentration.In order to maintain the desired concentration, 70 ml of 10% inulin in 500 ml of saline is infused at a rate of 4 ml / min.
  • The patient is asked to micturate 20 minutes after the second infusion and discard the urine and record the time. After exactly 60 minutes, take another sample of urine and blood will be taken. Measure the amount of urine and the conc. of inulin in both serum and urine.
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The inulin clearance is then measured by the formulae:

Normal inulin clearance for an average person with a body surface area of 1.73 sq m is 120 to 130 ml / minute. This approximates the GFR closely.

Under normal inulin clearance, the glomerular function is impaired.

CONCENTRATION TEST

If the body lacks water, the kidney can concentrate urine and preserve water.This is achieved by increasing the reabsorption of water at the tubular level from the glomerular filter.The measurement of the kidney’s ability to conserve water and concentrate urine is therefore a measure of tubular function.

The patient is not allowed to take any food or water for this test after the dinner.The first three urine samples collected in the morning are measured by their specific gravity.In a normal person, the specific gravity of one sample at least should be more than 1.025 or higher.If the specific gravity is below 1.025, it is a sign of tubular dysfunction.

DILUTION TEST

As with the concentration test, the dilution test is also a measurement of the tubular function. When our body is overloaded with fluid, the tubules reabsorb less water, resulting in the excretion of diluted urine.

For this test, the subject is quickly put on overnight and the subject drinks 1200 ml of water for a period of 30 minutes in the morning.The urine samples are then collected for 4 hours every hour.The specific gravity of the samples is measured and the specific gravity of one sample should be 1,003 or less.If none of the samples is 1.003 or less, this is a sign of tubular dysfunction.

ELECTROLYTES MEASUREMENT

The purpose of the kidney is not only to balance water and excretion, but also to maintain our body’s electrolyte balance.The kidneys actively reabsorb or excrete electrolytes to maintain the body’s electrolyte balance.Almost all electrolytes are filtered in the glomerulus due to their small size.After filtration, most electrolytes are absorbed back to the tubular level, but any problems at the tubular level lead to non- absorption and excessive loss of electrolytes in the urine.

Serum electrolytes that are measured for this purpose are:

Serum Sodium levels (Na+) 135 to 145 mmols/liter
Serum Potassium level (K+)3.5 to 5 mmols/liter
Serum Chloride level (Cl- )95 to 105 mmols/liter

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About the Author: Arthur Westmann

DEFFE ARTHUR (AMOEBAMANN) is the founder and author of MLTGEEKS and MLTEXPO.He’s from Cameroon and is currently a Final year State Medical Laboratory Technician (MLT MA). Beyond lab works, he’s a passionate internet user with a keen interest in web design and blogging. Furthermore He likes traveling, hanging around with friends and social networking to do in his spare time.

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