Renal Function Test

Renal Function test (RFT) is a group of tests that may be performed together to evaluate kidney function. The tests measure levels of various substances, including several minerals, electrolytes, proteins, and glucose (sugar), in the blood to determine the current health of the kidneys.

If the kidneys are not functioning properly, waste products can accumulate in the blood, and fluid levels can increase to dangerous volumes, causing damage to the body or a potentially life-threatening situation. Numerous conditions and diseases can result in damage to the kidneys. The most common causes of and main risk factors for kidney disease are diabetes and hypertension.

Indication

Indications for the assessment of renal function are varied and range from acute emergency to chronic settings.

1. Primarily, renal function tests are performed to identify the renal disease to determine appropriate patient management and prevent further deterioration of renal function.
2. Further indications in patients in whom the renal disease has been identified are to stage level or type of renal disease and to monitor the progression of renal disease to ensure that optimal management occurs and to monitor response to interventions.

Components of RFT

Clinically, the most practical tests to assess renal function are to get an estimate of the glomerular filtration rate (GFR) and to check for proteinuria (albuminuria).

Glomerular Function- Clearance test

Within the kidneys are about a million tiny blood-filtering units called nephrons. In each nephron, blood is continually filtered through a cluster of looping blood vessels, called a glomerulus, which allows the passage of water and small molecules but retains blood cells, proteins such as albumin, and larger molecules.

The best test to assess glomerular function is the Glomerular filtration rate (GFR), which is the rate in milliliters per minute at which substances in plasma are filtered through the glomerulus; in other words, the clearance of a substance from the blood. The normal GFR for an adult male is 90 to 120 mL per minute.

Testing for GFR can be a complicated and lengthy procedure, which is why doctors use a formula to estimate GFR or eGFR. The standard way to estimate GFR is with a simple blood test that measures creatinine levels. Creatinine is a waste product from the digestion of dietary protein and the normal breakdown of muscle tissue. Aside from chronic kidney disease, creatinine levels can be affected by other factors, including diet, muscle mass, malnutrition, and other chronic illnesses.

According to the Kidney Disease Improving Global Outcomes (KDIGO), The stages of chronic kidney disease (CKD):

• Stage 1 GFR greater than 90 ml/min/1.73 m²  
• Stage 2 GFR-between 60 to 89 ml/min/1.73 m²
• Stage 3a  GFR 45 to 59 ml/min/1.73 m²
• Stage 3b GFR 30 to 44 ml/min/1.73 m²
• Stage 4 GFR of 15 to 29 ml/min/1.73 m²
• Stage 5-GFR less than 15 ml/min/1.73 m² (end-stage renal disease)

Serum Creatinine Test

Creatinine is a chemical waste product of creatinine Creatinine is an amino acid found in our muscles. This blood test is performed to measure the level of creatinine in our blood. Our kidneys normally eliminate creatinine from the body. By measuring the creatinine level in the blood, doctors can check the functioning of the kidney. If the level of creatinine in your blood is high, then it indicates that the kidney is not functioning properly. To check any problem with the functioning of your kidney, the serum creatinine  is performed along with other tests such as

• Basic metabolic panel test 
• Blood urea nitrogen test 
• Comprehensive metabolic panel test

You will be asked to undergo a serum creatinine test, to measure the level of creatinine, if you show signs and symptoms of kidney disease such as

• High blood pressure
• Swelling in the abdomen, wrists, face, or ankle
• Nausea and vomiting
• Changes in urine output and frequency
• Trouble falling asleep
• Fatigue and loss of appetite

Creatinine is normally measured in mg/dL or milligrams per deciliter of blood. The results of serum creatinine tests vary depending on gender and age. 

In men, the range of normal creatinine level in the blood is 0.9-1.3mg/dL, whereas, in women who are 18 to 60 years old, the range of normal creatinine level in the blood is 0.6-1.1mg/dL.

Creatinine levels higher than 1.4mg/dL in men and 1.2mg/dL in women indicate an anomaly in the normal pattern which might be a problem.

Below is the creatinine levels chart which indicates the normal creatinine for men, women, teenagers, and toddlers in blood and urine samples.

There are many reasons for an elevated level of serum creatinine in your blood. They are-

• Dehydration
• Urinary tract obstruction
• High-protein diet
• Kidney infection 
• Reduced blood flow to the kidney

Blood urea nitrogen (BUN)

The liver breaks down protein and creates a waste product known as Urea nitrogen in the liver. The blood urea nitrogen test is performed to measure the level of blood urea nitrogen in the blood. Urea nitrogen is normally filtered out by the kidneys and is removed from the body through urination. By measuring the urea nitrogen level in the blood, doctors can determine how well your kidneys are working. When the liver or kidney is damaged, the level of BUN in the blood will increase. Elevated levels of urea nitrogen indicate liver or kidney problems. 

For proper diagnosis, a BUN test is often done with a KFT test, such as a creatinine blood test. 

The medical conditions that can be diagnosed by a Blood Urea Nitrogen test include-

• Congestive heart failure
• Urinary tract obstruction(UTO)
• Bleeding in the digestive tract
• Liver damage
• Malnutrition

A blood urea nitrogen test is also used to check the effectiveness of dialysis treatment in patients. 

Urea nitrogen is normally measured in mg/dL or milligrams per deciliter of blood. The results of BUN vary depending on gender and age. 

The range of normal BUN levels in the blood is-

Men: 8-24mg/dL

Women: 6-21mg/dL

Children: 7-20mg/dL

A high level of blood urea nitrogen indicates-

• Kidney failure
• Kidney disease
• Congestive heart failure
• Heart disease
• Gastrointestinal bleeding
• Obstruction in the urinary tract

A low level of Blood urea nitrogen indicates-

• Protein deficiency
• Liver failure
• Malnutrition
• Overhydration

Albuminuria

Albuminuria refers to the abnormal presence of albumin in the urine

• Albuminuria is used as a marker for the detection of incipient nephropathy in diabetics.
• It is an independent marker for cardiovascular disease since it connotes increased endothelial permeability, and it is also a marker for chronic renal impairment.

Urine albumin may be measured in 24-hour urine collections or early morning/random specimens as an albumin/creatinine ratio. The presence of albuminuria on two occasions with the exclusion of a urinary tract infection indicates glomerular dysfunction. The presence of albuminuria for three or more months is indicative of chronic kidney disease.

Typical Tests

The individual tests included in a kidney function panel can vary by laboratory, but the tests typically performed include:

1. Electrolytes: Electrically charged chemicals that are vital to normal body processes, such as nerve and muscle function; among other things, they help regulate the amount of fluid in the body and maintain the acid-base balance. Electrolytes include:

• Sodium
• Potassium
• Chloride
• Bicarbonate (Total CO2)

2. Minerals

• Phosphorus – a mineral that is vital for energy production, muscle and nerve function, and bone growth; it also plays an important role as a buffer, helping to maintain the body’s acid-base balance.
• Calcium – one of the most important minerals in the body; it is essential for the proper functioning of muscles, nerves, and the heart and is required in blood clotting and the formation of bones.

3. Protein

• Albumin – a protein that makes up about 60% of protein in the blood and has many roles such as keeping fluid from leaking out of blood vessels and transporting hormones, vitamins, drugs, and ions like calcium throughout the body.

4. Waste products

• Urea – urea is a nitrogen-containing waste product that forms from the metabolism of protein; it is released by the liver into the blood and is carried to the kidneys, where it is filtered out of the blood and eliminated in the urine.
• Creatinine – another waste product that is produced by the body’s muscles; almost all creatinine is eliminated by the kidneys.

5. Energy Source

• Glucose – supplies energy for the body; a steady amount must be available for use, and a relatively constant level of glucose must be maintained in the blood.

Clinical Significance

• Creatinine: Serum creatinine is elevated when there is a significant reduction in the glomerular filtration rate or when urine elimination is obstructed.  About 50%  of kidney function must be lost before a rise in serum creatinine can be detected. Thus serum creatinine is a late marker of acute kidney injury.
• BUN: Serum urea/BUN level increases in acute and chronic renal disease.
• eGFR equations are used to determine the presence of renal disease, and stage of CKD, and to monitor response to treatment.

Symptoms Of Kidney Problems

• High blood pressure 
• Painful urination
• Frequent urge for urination
• Blood in urine
• Urinary hesitancy
• Swelling of hands and feet

Having a single symptom doesn’t indicate a serious problem, but when the symptoms occur simultaneously, then the kidneys might not be functioning properly.