BUN/Creatinine Ratio – Lab Results explained

What is the BUN/Creatinine Ratio?

The Blood urea nitrogen (BUN): Creatinine Ratio [BUN/Creatinine Ratio] is a renal (related to kidneys) function indicator, commonly used to diagnose acute or chronic renal (kidney) disease or damage.

Blood Urea Nitrogen (BUN) and creatinine are both waste products of normal metabolism in the human body. 

BUN represents the amount of nitrogen produced from the metabolism of proteins. 

Creatinine is a normal waste product of muscle. 

Once a person performs both a BUN and Creatinine test, doctors can use the combined results to find the BUN-to-creatinine ratio. The BUN and creatinine tests alone are excellent methods of testing liver and kidney health, but together, doctors get a much better understanding of your kidney health and can provide a more accurate diagnosis to kidney-related issues.

Most clinicians agree that creatinine is a more specific indicator of glomerular function than BUN. However, the BUN to creatinine ratio may be used as an indirect estimate of renal function. 

Note: If results for BUN and Creatinine are both within the normal reference range, the BUN/Creatinine ratio will not be reported (not applicable).

What is a BUN test?

A Blood Urea Nitrogen (BUN) test measures the amount of nitrogen in your blood, and is used to provide doctors with information on how well your kidneys and liver are functioning. The nitrogen comes from the waste product urea, which is converted from nitrogen and produced in the liver when protein is metabolized after eating. Healthy kidneys then filter the urea and remove other waste products like urea through urination. It is normal to have some urea present in the blood since your liver is always producing it, but too much may indicate an issue with your kidneys.

What is a Creatinine test?

Creatinine is another type of molecule that is generated from muscle metabolism. Like urea, creatinine is transported through the bloodstream and to the kidneys, where it is also filtered and excreted through the urine. The rate of creatinine formation depends on the individual’s muscle mass. Because muscle mass remains fairly consistent, creatinine levels remain essentially unchanged throughout the day. When results of a creatinine test reveal excess amounts, it could signify impaired kidney function or kidney disease.

Why are we measuring the BUN/Creatinine ratio?

The BUN/Creatinine ratio blood test is used to diagnose acute or chronic renal (kidney) disease or damage. It may also be used to determine gastrointestinal bleeding or trauma. BUN (blood urea nitrogen) and creatinine are both filtered in the kidneys and excreted in urine. The two together are used to measure overall kidney function.

The BUN/Creatinine ratio provides specific clinical information about the kidney that can be used for multiple purposes. The BUN/Creatinine ratio is obtained to assess normal kidney function, help identify possible kidney diseases, to monitor the progression of kidney disease, or to monitor the effectiveness of medications in treating kidney disease.

The BUN/Creatinine ratio is useful in the differential diagnosis of acute or chronic renal disease. Reduced renal perfusion, e.g., congestive heart failure, or recent onset of urinary tract obstruction will result in an increase in BUN/Creatinine ratio. Increased urea formation also results in an increase in the ratio, e.g., gastrointestinal bleeding, trauma, etc. When there is decreased formation of urea as seen in liver disease, there is a decrease in the BUN/Creatinine ratio. In most cases of chronic renal disease the ratio remains relatively normal.

Basic on your kidneys:

You have two kidneys, each about the size of an adult fist, located on either side of the spine just below the rib cage. Although they are small, your kidneys perform many complex and vital functions that keep the rest of the body in balance. 

For example, kidneys:

– Help remove waste and excess fluid

– Filter the blood, keeping some compounds while removing others

– Control the production of red blood cells

– Make vitamins that control growth

– Release hormones that help regulate blood pressure

– Help regulate blood pressure, red blood cells, and the amount of certain nutrients in the body, such as calcium and potassium.

BUN and creatinine are two metabolites steadily produced in the body. 

The Blood Urea Nitrogen (BUN) level is a reflection of the amount of protein breakdown in the blood and it will accumulate with kidney malfunction. Creatinine is a breakdown product of muscle metabolism. It will also accumulate in the blood with kidney malfunction. 

BUN is filtered in the nephrons and then reabsorbed in the blood, whereas creatinine is filtered and then secreted by the tubular cells into the tubular lumen.

Each of your kidneys is made up of about a million filtering units called nephrons. Those are the functional units of the kidney. They perform the filtering of the blood. Each nephron includes a filter, called the glomerulus, and a tubule. The nephrons work through a two-step process: the glomerulus filters your blood, and the tubule returns needed substances to your blood and removes wastes.

What is acute renal failure (ARF)?

Acute renal failure (ARF), also known as acute kidney injury (AKI), is a sudden episode of kidney failure or kidney damage that happens within a few hours or a few days. AKI causes a build-up of waste products in your blood and makes it hard for your kidneys to keep the right balance of fluid in your body. ARF can also affect other organs such as the brain, heart, and lungs. Acute kidney injury is common in patients who are in the hospital, in intensive care units, and especially in older adults.

What are the signs and symptoms of acute kidney injury?

Signs and symptoms of acute kidney injury differ depending on the cause and may include:

– Too little urine leaving the body

– Swelling in legs, ankles, and around the eyes

– Fatigue or tiredness

– Shortness of breath

– Confusion

– Nausea

– Seizures or coma in severe cases

– Chest pain or pressure

In some cases, AKI causes no symptoms and is only found through other tests done by your healthcare provider.

What are normal levels:

The ratio of BUN to creatinine is usually between 10:1 and 20:1.

A healthy individual typically has serum BUN levels higher than serum creatinine levels. A measurable increase in the serum creatinine levels in hospitalized patients is indicative of Acute renal failure (ARF).

Differentiating Acute renal failure (ARF) into 3 categories (prerenal, intrinsic renal, and postrenal failure) is of significant clinical utility.

→ BUN/Creatinine Ratio between 10 and 20 is considered a normal value or may be suggestive of a post-renal cause of ARF.

→ High BUN/Creatinine Ratio (>20) may be suggestive of a pre-renal cause of AFR. 

→ Low BCR (<10) may be suggestive of an intrinsic renal cause of AFR.

What is the BUN/Creatinine ratio used for?

Looking at the BUN/Creatinine ratio can further aid in distinguishing which disease state may be causing abnormal lab values. The BUN/Creatinine ratio usually remains normal in chronic kidney disease. A BUN:creatinine ratio that is >20 suggests dehydration or another state that causes decreased renal perfusion. 

A BUN:creatinine ratio >30 can suggest a GI bleed.

One may look at the BUN / Creatinine ratio to help determine the cause of renal failure. The ratio of BUN to creatinine is usually between 10:1 and 20:1. An increased ratio may be due to a condition that causes a decrease in the flow of blood to the kidneys, such as CHF (=Congestive Heart Failure) or dehydration. It may also be seen with increased protein, from BI bleed, or increased protein in the diet. The ratio may be decreased with liver disease (due to a decrease in the formation of urea) and malnutrition.

Reference Range(s):

Creatinine
Age	Male (mg/dL)	Female (mg/dL)
≤2 days	0. 79-1.58	0.79-1.58
3-27 days	0.35-1.23	0.35-1.23
1 month-9 years	0.20-0.73	0.20-0.73
10-12 years	0.30-0.78	0.30-0.78
13-15 years	0.40-1.05	0.40-1.00
16-17 years	0.60-1.20	0.50-1.00
18-19 years	0.60-1.26	0.50-1.00
20-49 years	0.60-1.35	0.50-1.10
50-59 years	0.70-1.33	0.50-1.05
60-69 years	0.70-1.25	0.50-0.99
70-79 years	0.70-1.18	0.60-0.93
≥80 years	0. 70-1.11	0.60-0.88

For patients >49 years of age, the upper reference limit for creatinine is approximately 13% higher for people identified as African-American.

Sources:

https://www.kidney.org/atoz/content/AcuteKidneyInjury

https://www.webmd.com/a-to-z-guides/blood-urea-nitrogen-test

What does it mean if your BUN/Creatinine Ratio result is too low?

A decreased ratio may be observed with liver disease and poor diet. Temporary levels that are high or low may not be a cause for concern and should be retested to confirm.

– Liver disease can cause a lower BUN level. 

– A low BUN level is common in the second or third trimester of pregnancy.

– May be related to malnutrition.

What does it mean if your BUN/Creatinine Ratio result is too high?

An increased ratio of BUN to creatinine may be due to conditions that cause a decrease in the flow of blood to the kidneys, such as congestive heart failure or dehydration. It may also be seen with high protein blood levels or from gastrointestinal bleeding.

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BUN/Creatinine Ratio: High & Low Levels + Normal Range

Written by Joe Cohen, BS | Last updated:

December 15, 2022

Medically reviewed by

Biljana Novkovic, PhD, Jonathan Ritter, PharmD, PhD (Pharmacology)
| Written by Joe Cohen, BS | Last updated:

December 15, 2022

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BUN (blood urea nitrogen) and creatinine are two blood tests that can reveal a lot about your metabolism, kidney, liver, and overall health. And while they can be used separately, the BUN/creatinine ratio can help pinpoint important issues. Learn more about the BUN/creatinine ratio and what it can reveal about your health.

What is the BUN/Creatinine Ratio?

BUN (blood urea nitrogen) and creatinine are two lab tests that are often ordered as part of a comprehensive metabolic panel. Your doctor may order this panel to get an idea of your overall health and metabolism.

BUN measures the amount of urea in your blood. Urea is a waste product made in the liver as the body processes protein. This protein is mostly derived from the diet, but it can also result from tissue protein turnover [1, 2, 3].

Urea is removed by the kidneys, but the rate of removal depends on the needs of the body. The kidneys can return different amounts of urea into the bloodstream depending on factors such as hydration and blood pressure. High urea is an important marker of kidney dysfunction as well [1, 2, 3].

Creatinine, on the other hand, is a waste product created from the normal wear and tear of muscles. It is produced from creatine, a protein that helps generate energy for muscle contractions. Creatinine production essentially reflects muscle mass, and because this mass changes little from day to day, creatinine production tends to be fairly constant [3, 4, 5].

Creatinine is removed from the body by the kidneys, which filter almost all of it from the blood into the urine, at a fairly constant rate. That is why blood levels are usually a good indicator of how well your kidneys are working [3, 4, 5].

So to recap, BUN levels fluctuate, while creatinine is removed at a constant rate and its blood levels are usually stable. That’s why the BUN/Creatinine ratio can be used to check for issues such as dehydration, kidney injury/disease, gut bleeding, and other problems [3].

BUN/Creatinine Ratio Normal Range

The normal range for BUN/Creatinine ratio is anywhere between 5 – 20 mg/dL.

BUN/Creatinine ratio increases with age, and with decreasing muscle mass [6].

Low BUN/Creatinine Ratio

A low BUN/Creatinine ratio indicates an underlying disease/disorder and will usually be accompanied by the symptoms of this underlying disorder. The causes shown here are commonly associated with low BUN/creatinine. Work with your doctor or other health care professional for an accurate diagnosis.

A BUN/Creatinine ratio lower than normal can be caused by:

• Low protein intake, seen in conditions of malnutrition and starvation. Less protein means lower BUN production [3].
• Advanced liver disease, when the liver can’t produce enough urea, resulting in lower BUN levels [3].
• Sickle cell anemia – in this condition kidneys reabsorb less urea and more of it is lost in the urine, resulting in lower BUN [3].
• Hypothyroidism, a condition where the thyroid glands do not produce enough thyroid hormone. This condition can increase creatinine levels [7, 8].
• Rhabdomyolysis, a condition in which damaged muscles breaks down rapidly, resulting in higher creatinine levels [3].
• Kidney damage and kidney failure. When kidneys become impaired for any reason, creatinine blood levels will rise [9].
• Drugs such as acetazolamide, a diuretic used to treat various conditions including glaucoma, epilepsy, altitude sickness, and heart failure [10].

High BUN/Creatinine Ratio

A high BUN/Creatinine ratio indicates an underlying disease/disorder and will usually be accompanied by the symptoms of this underlying disorder. The causes shown here are commonly associated with high BUN/creatinine. Work with your doctor or other health care professional for an accurate diagnosis.

A BUN/Creatinine ratio above the normal range can be caused by:

• Dehydration. Dehydration increases the blood levels of both BUN and creatinine but increases BUN more than creatinine [11].
• Gut bleeding. The blood in the gut gets digested and this increases the amount of protein and BUN levels [12, 13].
• Hyperthyroidism. This condition can increase BUN and lower creatinine levels [7].
• Congestive heart failure – heart failure increases the reabsorption of urea and increases blood BUN levels [14, 15].
• Kidney disease, it can increase BUN as well as creatinine levels [16].
• Drugs such as tetracycline (an antibiotic) or corticosteroids (used to treat inflammation) [10, 3, 17].

Factors that Increase BUN/Creatinine Ratio

It is important to address any health condition that may be causing the disbalance. Once the condition has been resolved, the BUN/creatinine ratio should return to a normal range. Your doctor will work to find an accurate diagnosis and an appropriate treatment plan, which may include some of the strategies below. Never use any of these in place of what your doctor prescribes.

Factors that Increase BUN

• Increasing dietary protein. Low BUN levels may mean that you are not consuming enough protein. If this is the case, try to increase your consumption of high-protein foods like lean meats and beans [1].
• Reducing alcohol consumption. Alcohol blocks the production of urea (BUN) [18].

Factors that Decrease Creatinine

• Avoiding creatine and creatine-based supplements
• Increasing dietary fiber. Vegetable and fruit fiber improves kidney health and can lower blood creatinine levels [19].
• Losing some weight if overweight. Weight loss can improve your kidney health and decrease creatinine levels [20].

How to Decrease Your BUN/Creatinine Ratio

Again, disbalance between BUN and creatinine is often due to a serious medical condition and it is important to address it. Once the condition gets resolved, the BUN/creatinine ratio will go back into the normal range. Your doctor will work to find an accurate diagnosis and an appropriate treatment plan, which may include some of the strategies below. Never use any of these in place of what your doctor prescribes.

Factors that Decrease Bun

• Drinking more water. Make sure you are properly hydrated.
• Losing weight if overweight. A high BMI can cause kidney dysfunction and increase BUN (urea) levels [21, 22, 23].

Factors that Increase Creatinine

• Increasing physical activity (unless it’s not recommended due to an existing medical condition) – exercise increases creatinine levels + it helps build muscle [24, 25].
• Avoid alcohol. It may decrease blood creatinine [26, 27].

Contents

• What is the BUN/Creatinine Ratio?

  • BUN/Creatinine Ratio Normal Range

  • Low BUN/Creatinine Ratio

  • High BUN/Creatinine Ratio

• Factors that Increase BUN/Creatinine Ratio

  • Factors that Increase BUN

  • Factors that Decrease Creatinine

• How to Decrease Your BUN/Creatinine Ratio

  • Factors that Decrease Bun

  • Factors that Increase Creatinine

Serum creatinine (with GFR)

Creatinine is a non-enzymatic breakdown product of creatine and creatine phosphate, formed in muscle. It is excreted from the body by the kidneys.

Synonyms Russian

1-methylglycocyamidine, a product of the conversion of creatine phosphate, creatinine in the blood.

Synonyms English

Research method

Kinetic method (Jaffe method).

Units

µmol/l (micromoles per litre).

What biomaterial can be used for research?

Venous blood.

How to properly prepare for an examination?

• Do not eat for 12 hours before the test.
• 30 minutes before the study, exclude physical and emotional overstrain.
• Do not smoke for 30 minutes before the test.

Study Overview

Creatinine is a waste product produced in muscles when a substance called creatine is broken down. Creatine is part of the cycle that provides the body with energy for muscle contraction. After 7 seconds of intense physical activity, creatine phosphate is converted to creatine, then turns into creatinine, which is filtered in the kidneys and excreted in the urine. Creatine and creatinine are stably produced by our body in a constant amount. Almost all creatinine is excreted by the kidneys, so its concentration in the blood is a good indicator of their function. The amount of creatinine produced depends on the total body weight and, in particular, muscle mass. Therefore, for example, creatinine levels in men will be significantly higher than in women and children.

A small part (15%) is secreted by the tubules, but it is mainly produced by filtration in the glomerulus. The level of creatinine in the blood does not go beyond the norm until the glomerular filtration rate drops to critical values, especially in patients with low muscle mass. Then the creatinine level rises.

Precisely because of the large number of factors (muscle mass, sex, age) that affect the concentration of creatinine in the blood, this study is not the best screening test for detecting kidney failure. At the same time, creatinine is a more sensitive indicator of kidney disease than urea.

What is research used for?

• To assess kidney function.
• To assess the function of the main organs and systems (in combination with other studies).
• To evaluate and treat renal impairment if creatinine or urea is abnormal and the patient has an underlying chronic disease such as diabetes that affects the kidneys.
• If blood and urine creatinine levels are known, creatinine clearance (Rehberg’s test) can be calculated – this test shows how efficiently the kidneys filter small molecules, such as creatinine, from the blood.
• To calculate glomerular filtration rate to confirm kidney damage.

When is the examination scheduled?

• At regular intervals for known kidney disease or a disease that can cause deterioration of kidney function (together with the test for urea and microalbuminuria).
• In the diagnosis of skeletal muscle diseases.
• Before and after hemodialysis.
• For symptoms of kidney dysfunction:
  • weakness, fatigue, decreased attention, poor appetite, sleep problems,
  • swelling of the face, wrists, ankles, ascites,
  • Foamy, red or coffee-colored urine,
  • decreased diuresis,
  • problems with the act of urination: burning, discontinuity, change in frequency (dominance of nocturnal diuresis),
  • pain in the lumbar region (especially on the sides of the spine), under the ribs,
  • high pressure.
• For any pathological conditions accompanied by dehydration.
• In preparation for a CT scan.
• Before prescribing drugs that can cause damage to the kidney tissue.

What do the results mean?

Reference values ​​(creatinine norm):

Creatinine

GFR (glomerular filtration rate): 60 or more.

Causes of increased creatinine levels

• Acute and chronic renal failure (amyloidosis, kidney damage in diabetes mellitus, etc.).
• Failure of the cardiovascular system (myocardial infarction, cardiogenic shock, myocardial dystrophy, etc.).
• Massive destruction of muscle tissue (crash syndrome) and release of creatinine from cells.
• Burns (massive necrosis of cells with the release of their contents into the intercellular substance).
• Acromegaly.
• Gigantism.
• Hyperthyroidism.
• Dehydration (blood clotting and relative hypercreatininemia).
• Excessive exercise.
• Excess consumption of meat products.
• Radiation sickness.
• Obstruction of the urinary tract.
• Taking nephrotoxic drugs (mercury compounds, sulfonamides, thiazides, antibiotics from the group of aminoglycosides, cephalosporins and tetracyclines, barbiturates, salicylates, androgens, cimetidine, trimethoprim-sulfamethoxazole).
• Damage to the vessels of the glomerular apparatus of the kidneys (glomerulonephritis), which may be the result of an infectious or autoimmune disease.
• Bacterial infection of the kidneys (pyelonephritis).
• Necrosis of tubular epithelium (acute tubular necrosis) caused, for example, by drugs or toxins.
• Diseases of the prostate, nephrolithiasis or other factors causing obstruction of the urinary system.
• Decreased blood flow to the kidneys due to shock, dehydration, acute heart failure, atherosclerosis, or complications of diabetes.

Causes of low creatinine levels

• Fasting.
• Hyperhydration (blood dilution – relative hypocreatininemia).
• Muscle atrophy.

What can influence the result?

• Confounding factors
  • Pregnancy (especially I and II trimesters).
• Performance enhancing factors
  • Excessive muscle mass, such as in some athletes (may increase creatinine levels despite normal kidney function).