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BUN-to-Creatinine Ratio Blood Test

Last updated May 20, 2019

Approved by: Krish Tangella MD, MBA, FCAP

Testing BUN levels therefore gives information about protein breakdown in the liver and urea excretion in the kidneys.

What are other Names for this Test? (Equivalent Terms)

  • BUN/Creatinine Ratio Blood Test
  • BUN:Creatinine Ratio Blood Test

What is the Bun-to-Creatinine Blood Test? (Background Information)

  • Creatinine is formed when a high-energy compound called creatine is expended by muscles to fuel their contraction. It is the end-product of creatine metabolism
  • After creatine is used up, the creatinine that is produced travels through blood and exits through the kidneys
  • The rate of creatinine formation depends on an individual’s muscle mass. Aside from this, creatinine formation rate is fairly constant during normal conditions. This makes it a useful indicator for muscle and kidney disorders. Creatinine levels usually decrease with age because of age-dependent decreases in muscle mass
  • Damage to muscles resulting from injury or degenerative diseases causes the release of creatinine in blood. This leads to a spike in the normally constant blood creatinine levels. The creatinine levels may be used to assess muscle and kidney health
  • Meanwhile, testing blood urea nitrogen (BUN) allows the clinician to determine urea nitrogen levels in blood. Urea nitrogen is formed when the liver breaks down protein into ammonia and then urea. Urea is transported to the kidneys via the blood and excreted out of the body through the urine
  • Testing BUN levels therefore gives information about protein breakdown in the liver and urea excretion in the kidneys. Comparing BUN and creatinine levels allows the clinician to determine if the cause of high blood nitrogen (azotemia) stems from malfunction of the kidneys or from some other disorder

What are the Clinical Indications for performing the Bun-to-Creatinine Blood Test?

Following are the clinical indications for performing the Bun-to-Creatinine Blood Test:

  • Differentiating prerenal and postrenal azotemia from renal azotemia
  • Fatigue
  • Abnormal urine production
  • Altered mental status
  • Pale appearance
  • Abnormal heartbeat

How is the Specimen Collected for the Bun-to-Creatinine Blood Test?

Following is the specimen collection process for BUN-to-Creatinine Blood Test:

Sample required: Blood

Process of obtaining blood sample in adults:

  • A band is wrapped around the arm, 3-4 inches above the collection site (superficial vein that lies within the elbow pit)
  • The site is cleaned with 70% alcohol in an outward spiral, away from the zone of needle insertion
  • The needle cap is removed and is held in line with the vein, pulling the skin tight
  • With a small and quick thrust, the vein is penetrated using the needle
  • The required amount of blood sample is collected by pulling the plunger of the syringe out slowly
  • The wrap band is removed, gauze is placed on the collection site, and the needle is removed
  • The blood is immediately transferred into the blood container, which has the appropriate preservative/clot activator/anti-coagulant
  • The syringe and the needle are disposed into the appropriate “sharp container” for safe and hygienic disposal

Preparation required: No special preparation is needed prior to the test.

What is the Significance of the Bun-to-Creatinine Blood Test Result?

The significance of the BUN-to-Creatinine Blood Test result is explained.

  • An increased value for the test, with normal creatinine levels, may indicate:
    • Prerenal azotemia
    • Gastrointestinal hemorrhage
    • Kidney disorder
  • An increased value for the test, with increased creatinine levels, may indicate:
    • Postrenal azotemia
    • Prerenal azotemia superimposed on liver disease
  • A decreased value for the test, with decreased  BUN levels, may indicate:
    • Acute tubular necrosis
    • Decreased urea synthesis
    • SIADH
    • Congenital deficiency of urea cycle enzymes
  • A decreased value for the test, with increased creatinine levels, may indicate:
    • Rhabdomyolysis
    • Kidney failure

The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results, based on the overall clinical scenario.

Additional and Relevant Useful Information:

  • Certain factors interfere with the results of the BUN-to-Creatinine Blood Test. These include diet, hydration status, and pregnancy

Certain medications that you may be currently taking may influence the outcome of the test. Hence, it is important to inform your healthcare provider of the complete list of medications (including any herbal supplements) you are currently taking. This will help the healthcare provider interpret your test results more accurately and avoid unnecessary chances of a misdiagnosis.

The following DoveMed website links are useful resources for additional information: 



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References and Information Sources used for the Article:

Reviewed and Approved by a member of the DoveMed Editorial Board
First uploaded: Jan. 17, 2016
Last updated: May 20, 2019