Kidney Function Test: Components, Types, and Functional Unit

Kidneys are the two organs present inside our human body that are shaped in the form of a bean on both sides of our spine, at the back of our stomach. There are a number of functions that the kidneys serve in our body. To know whether or not our kidneys are in a good state and whether they are working properly, we need to go through some urine & blood tests. These tests help us in identifying if there is an issue with the health of our kidneys, if the waste and toxic products are being filtered by the kidneys very slowly or if the proteins are being leaked by the kidneys in the urine. In this article, we are going to learn about various kidney function tests which show the condition in which our kidneys are.

Keyterms: Kidney, Organ, Human body, Urine, Blood test, Blood, RBc, Plasma, WBC, Blood pressure, Stomach

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Kidney

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A pair of kidneys exist in our abdomen which helps in clearing out the excess water and waste from our bodies. Kidneys help in maintaining a balance of various chemicals such as calcium, sodium, and potassium in our bodies. They generate hormones that help in regulating the levels of blood pressure in our body and stimulate the bone marrow so it can produce red blood cells (RBCs) in our body. A single kidney weighs around 160 grams and has a size of approximately 5 inches. The job of a kidney is to filter the blood having wastes and toxins in it and excrete them through urine. The kidneys also maintain homeostasis i.e. maintaining the balance of fluids, electrolytes, and acid-base (pH balance) in our body.

Kidneys in the human body

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Nephron: Functional and Structural unit of a kidney

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The functional and structural unit of a kidney is called the nephron also known as Uriniferous tubules. A nephron is a tubular structure that consists of two main parts; the renal corpuscle (malpighian body/corpuscle) & renal tubule. There are about 10 lakh nephrons in a single human kidney. 

Nephron as a functional unit of the kidney

The most important and main function of nephrons is to regulate or balance the concentration of water and soluble substances such as sodium salts in the blood by filtering the blood regularly, reabsorbing what is essential and useful for the body such as nutrients, and excreting the waste materials through urine.

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Kidney Function Test

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Kidney Function Test (KFT), also known as Renal Function Test (RFT) are the tests that clarify the condition of kidneys and help us in determining their health. 

Components of Kidney Function Test

There are several components/parts of the kidney function test which show if the kidneys are functioning normally. These tests include:

Blood Tests

• Serum urea
• Serum creatinine 
• Glomerular Filtration Rate (GFR)
• Blood Urea Nitrogen (BUN)
• Calcium
• Dilution test

Imaging Tests 

• Ultrasonography
• Computerized Tomography (CT Scan)

Kidney Biopsy

Urine Tests

• Urinalysis
• Urine microalbumin
• Creatinine clearance test

Healthy vs Damaged Kidney Blood Test

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Blood Tests

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• Serum urea

Urea (NH₂)₂CO is the final product of protein catabolism. It is produced in the liver from the amino group of the amino acids by the cycle of biochemical reactions i.e., the urea cycle also known as the ornithine cycle. The filtration of urea is done at the glomerulus and is secreted and reabsorbed at the tubular stage. The level of urea rises only if there is a decrease in glomerulus function by 50%. The increase in urea level is known as glomerular dysfunction that causes renal dysfunction. The level of urea should be between 20 to 45 mg/dL.

• Serum creatinine 

In the creatinine blood test, creatinine level in the blood is measured. Creatinine is a chemical compound, a waste material that comes from the normal aging/damage on muscles of the body. However, it gets eliminated from our body by the kidneys through urine. The level of creatinine in the blood can vary from person to person as it is different in males and females, it gets affected by a person's age, gender, and the size of his/her body.

The normal level of serum creatinine in the blood for males is 0.7 to 1.2 milligrams per deciliter, on the other hand, the normal level of serum creatinine in the blood for females is 0.5 to 1.0 milligrams per deciliter. 

As per National Kidney Foundation (NKF), creatinine levels exceeding the above-mentioned digits can be an indication of kidney disease. As the disease progresses, the level of creatinine in the blood increases.

• Glomerular Filtration Rate (GFR) 

This blood test measures if the kidneys are filtering waste products normally. GFR is calculated by Cockcroft-Gault, Modification of Diet in Renal Disease (MDRD), or Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equations. For calculating GFR, all the above-mentioned factors need some or the other elements mentioned below:

1. Serum creatinine test (level of serum creatinine in the blood).
2. Age of the patient.
3. Gender.
4. Weight.
5. Height.
6. Body size.

GFR decreases with age which is normal but it should not go below 60 mL/min/1.73 m². The normal range for GFR is above 60. A GFR less than 60 is an indication of kidney problems. A GFR less than 15 is a sign of kidney failure which urgently needs treatment.

Kidney disease and GFR

• Blood Urea Nitrogen (BUN) 

"BUN" is a blood test that measures the quantity of waste products such as urea nitrogen in the blood and can be computed from serum urea level.

Urea nitrogen is a waste material that is created by our body after eating. Our liver creates it when breaking down proteins from our food and it is removed from the blood by our kidneys. 7 to 20 mg/dL is a normal level of BUN. But BUN level higher than this can be a sign of kidney disease known as Azotemia.

•  Calcium 

This test is used to measure the amount of calcium not in our bones but in our blood cells. Calcium is needed by our body for fixing bones & teeth, working nerves, and constructing muscles. But this does not mean that calcium at a higher level will be more beneficial for our body. An increase in the level of calcium can cause headaches, nausea, itchy skin, mood swings, sore eyes, and teeth. Also, this can be a sign of the slow functioning of the kidneys.

• Dilution test 

This test measures the functioning of tubules. The fluids reabsorb less amount of water and excrete diluted urine, in case of fluid overload of the body. For this test, the patient needs to drink at least 1200 ml of water for a time period of 30 mins. After which urine samples are collected every single hour for at least 4 hours. The specific gravity of at least one of the urine samples should be 1.003 or less, if the specific gravity comes to be more than 1.003, it can be a sign of tubular dysfunction.

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Imaging Tests (Radiology)

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• Ultrasonography 

Ultrasonography or Ultrasound is an imaging test that takes images of the kidneys. If there is any abnormality in them like their shape, size, position, stones, cysts, tumors, it can be seen in the image and can be treated accordingly.

Normal Kidney Ultrasound

• Computerized Tomography (CT Scan) 

This type of test captures images of kidneys through an X-ray machine. If there is any structural abnormality in them, it can be seen in the X-ray and can be treated accordingly.

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Kidney Biopsy

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Kidney biopsy is a process in which a fine needle is entered through the skin to remove a few small pieces of kidney tissues to inspect them under a microscope for detecting any damage/disease of the kidneys. Kidney Biopsy is helpful to observe and identify if the disease caught can be treated or not, or if a kidney transplant can be done or not.

Kidney Biopsy Test

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Urine Tests

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• Urinalysis

Urinalysis is a urine test that includes inspection of a urine sample both with a microscope and a urine test strip (dipstick). If there is any protein present in the urine (which can happen if anyone is having a kidney disease), it will be detected by the dipstick changing its color. In this manner, a dipstick helps to identify if there is a kidney problem prevailing.

• Urine microalbumin 

Albumin is a kind of protein that is produced by the liver for tissue growth but if this albumin gets leaked into the urine, it is a sign of kidney disease. If the result comes to be below 30 mg then it is normal but if it is higher than 30 mg then it can be because of some kidney disease. Also if someone is having sugar and bp problems, they should get this test done.

• Creatinine clearance test 

Creatinine is a waste material that is filtered out by the kidneys in urine but if creatinine is not being filtered out, it means that it is there in the blood which means that there is a kidney problem existing. Creatinine clearance test examines a 24-hour urine sample and observes how much creatinine is being filtered by the kidneys.

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Things to remember

• Nephrons are the microscopic filters of the kidneys which filter the soluble substances in the blood, reabsorb the useful products, and pass the waste materials in the form of urine.

• To know the well-being or functioning of the kidneys, one needs to go through not just one but certain tests which include 
  • Blood tests, 
  • Imaging tests, 
  • Kidney biopsy and 
  • Urine tests. 

These tests together will assess the condition of the kidneys.

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Previous Year Questions

• Columns of Bertin is the structure … [JIPMER 2016]
• What causes the liquid part of blood filter … [JIPMER 1996]
• Ureotelic animals excrete out … [JIPMER 1996]
• In Ornithine cycle, which of the following wastes … [JIPMER 2007]
• Uricotelism is found in … [Manipal 2007]
• Mammalian kidney is … [AMUEEE 2013]
• A fall in glomerular filtration rate activates … [KCET 2015]
• Compare the statements A and B … [KCET 2010]
• Effective filtration pressure in glomerulus … [KCET 2011]
• Facultative absorption of water from primary urine … [KCET 2016]