Blood Urea Nitrogen (BUN)

What is Blood Urea Nitrogen (BUN) ?

BUN is a laboratory test that measures the amount of nitrogen in the blood that comes from urea. Urea is a waste product produced by the liver during protein metabolism.

Protein metabolism produces ammonia, which is eventually converted to the less toxic waste product, urea. Urea is released by the liver into the blood, and eventually reaches the kidneys for excretion through the urine.

Urea is freely filtered and reabsorbed by the kidneys.⁽¹⁾

Serum Creatinine

Serum creatinine is a laboratory test that measures the amount of creatinine in the blood. Creatinine is a waste product produced by the muscles during the metabolism of creatine phosphate, a substance involved in energy production in muscles.

Creatinine is produced at a relatively constant rate every day, depending on muscle mass, which is relatively unchanged from day to day. It is released in the blood, and eventually reaches the kidneys for excretion through the urine. Creatinine is freely filtered but not reabsorbed in the kidneys.⁽²⁾

BUN/ Creatinine Ratio

BUN/creatinine ratio is the ratio of BUN to serum creatinine.⁽³⁾

Why are BUN, Serum Creatinine, and BUN/Creatinine Ratio Assessed?

BUN and serum creatinine are commonly assessed together to evaluate kidney function, to diagnose kidney disease, and to monitor patients with acute or kidney disease.

BUN/creatinine ratio is useful in determining the more specific causes of abnormalities in the levels of BUN and serum creatinine. It is particularly useful in differentiating acute and chronic kidney disease.^{(4, 5)}

When are the tests ordered?

BUN and serum creatinine may be ordered as part of the basic metabolic panel (BMP) or comprehensive metabolic panel (CMP), which are sets of tests ordered in:

• Routine medical checkup
• Acutely ill patients that come to the emergency room and/or is admitted to the hospital
• Confirmation of suspected kidney disease
• Monitoring of kidney function before starting on particular drug treatments
• Monitoring of kidney function in patients with chronic diseases such as diabetes, congestive heart failure, and heart attack
• Monitoring the effectiveness of dialysis^{(4, 5)}

How should you prepare for the tests?

For BUN

Do not eat too much meat or other protein-rich food for 24 hours prior to the test. Drink enough water during the 24 hours prior to the test.⁽⁶⁾

For Serum Creatinine

Do not exercise strenuously for 2 days (48 hours) prior to the test. Do not eat more than 8 oz (227 g) of meat or other protein-rich food for 24 hours prior to the test. Drink enough water during the 24 hours prior to the test.⁽⁷⁾

Image 1: Protein-rich foods.
Picture Source: izzihealth.com

How are the tests done?

Blood samples are withdrawn from the vein, usually in the arm.^{(4, 5)}

Picture 2: Blood being withdrawn from a vein in the arm.
Photo Source: laboratory-manager.advanceweb.com

What Do the Test Results Mean?

The normal range for BUN, serum creatinine vary among laboratories.

BUN

The normal value of BUN can range from 7 – 20 mg/dL.⁽¹⁾

Increased BUN may be due to:

• Acute or chronic kidney disease, damage, or failure, such as those caused by diabetes and high blood pressure
• Decreased blood flow to the kidneys, such as in congestive heart failure, shock (severe low blood pressure), burns, severe heart attack, and dehydration
• Excessive protein breakdown
• Increased dietary intake of protein
• Gastrointestinal bleeding, due to the proteins present in the blood
• Fever ^{(4, 6)}

Decreased BUN is less common. It may be due to:

• Severe liver disease
• Malnutrition
• Protein deficiency
• Overhydration^{(4, 6)}

Serum Creatinine

The normal value for serum creatinine can range from 0.7-1.2 mg/dL (62–106 μmol/L).⁽²⁾

Increased serum creatinine may be due to:

• Kidney disease or damage, such as those caused by infection, diabetes, hypertension, glomerulonephritis (damage to the glomerulus), and acute tubular necrosis (damage to the kidney tubules)
• Decreased blood flow to the kidneys, such as in congestive heart failure, shock, and dehydration
• Muscle injury, such as in crush injuries, burns, and strenuous exercise
• Muscle disease, such as rhabdomyolysis (breakdown of skeletal muscle), mild muscular dystrophy (progressive loss of muscle mass), or polymyositis (chronic inflammation of the muscles) ^{(5, 7)}

Decreased serum creatinine may be due to:

• Decreased muscle mass, such as in elderly people or people with severe muscular dystrophy
• Pregnancy
• Conditions that cause obstruction of urine flow after passing through the kidney, such as urinary stones and benign prostatic hyperplasia (enlargement of the prostate)^{(5, 7)}

BUN/Creatinine Ratio

The ratio of BUN to creatinine is usually between 10:1 and 20:1. Chronic kidneys disease usually causes elevation of both BUN and creatinine, but their ratio is relatively normal.^{(4, 8)}

On the other hand, the cause of acute kidney injury can be determined by the BUN/creatinine ratio.⁽⁹⁾

Increased BUN/creatinine ratio may be due to prerenal causes, or conditions that decrease the blood flow to the kidneys, such as dehydration, shock, congestive heart failure, and narrowing of the arteries to the kidneys. It can also be due to gastrointestinal bleeding or high protein intake.^{(3, 4, 6, 7, 9)}

Normal BUN/creatinine ratio could mean either normal kidneys or postrenal acute kidney injury, which is caused by urinary tract obstruction, such as in benign prostatic hyperplasia, kidney stones, bladder stones, or cancer of the bladder, ureters, or prostate.^{(3, 9)}

Decreased BUN/creatinine ratio may be due to intrarenal causes, or conditions that directly damage the kidney itself. The damaged kidney cannot reabsorb urea, and hence, decreases the BUN/creatinine ratio.

Common conditions that damage the kidneys are glomerulonephritis, acute tubular nephritis, and acute interstitial nephritis.

Decreased BUN/creatinine ratio may also be caused by muscle injury/disease, malnutrition, or liver disease.^{(3, 4, 6, 7, 9)}

Signs and Symptoms of Kidney Disease

• Fatigue, difficulty in concentrating, poor appetite, or difficulty in sleeping
• High blood pressure
• Swelling or puffiness (edema), especially in the face, wrists, abdomen, thighs, or ankles
• Urine that is frothy, bloody, or coffee-colored
• A decrease in the amount of urine
• Change in the frequency of urination, especially at night
• Problems in urination, such as a burning sensation or abnormal discharge during urination
• Flank or back pain, in the area below the ribs where the kidneys are nearly located^{(4, 5)}

Photo 3: Foamy urine, a sign of kidney disease.
Image Source: www.amarinbabyandkids.com

Image 4: Edema of the ankles and feet.
Picture Source: www.silverdale.com.au

References

1. https://en.wikipedia.org/wiki/Blood_urea_nitrogen
2. https://en.wikipedia.org/wiki/Creatinine#Serum_creatinine
3. https://en.wikipedia.org/wiki/BUN-to-creatinine_ratio#Interpretation
4. https://labtestsonline.org/tests/blood-urea-nitrogen-bun
5. https://labtestsonline.org/tests/creatinine
6. https://www.uofmhealth.org/health-library/aa36271#aa36274
7. https://www.uofmhealth.org/health-library/hw4322#hw4325
8. https://www.healthlabs.com/bun-creatinine-ratio-testing
9. https://en.wikipedia.org/wiki/Acute_kidney_injury#Prerenal
10. https://www.medicinenet.com/creatinine_blood_test/article.htm#why_are_blood_creatinine_levels_checked