What is the BUN/Creatinine Ratio?

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.

Image credit: National Institute of Diabetes and Digestive and Kidney Diseases

BUN stands for blood urea nitrogen. Creatinine is a natural product of muscle breakdown that occurs at a low level in the body. Both BUN and creatinine are filtered by the kidney and excreted in urine. For this reason, BUN and creatinine are used together to measure kidney function.

If kidney function begins to decline, BUN and creatinine rise. A normal creatinine depends on muscle mass and age.  In general, a normal creatine is 0.5 to 1.2 milligrams per deciliter (mg/dL). A normal BUN is 7 to 20 mg/dL.

A small, temporary increase in either BUN or creatinine can occur during illness or dehydration; the numbers usually return to normal during recovery.

Find out what it means if your BUN/Creatinine ratio is too high or low @

Causes of increased BUN/Creatinine ratio:

If Urea increased and Creatinine normal (both plasma):

  • Dehydration
  • Heart failure (without renal involvement)
  • Gastrointestinal bleed
  • High-protein diet
  • Catabolic state due to:
    – trauma
    – severe infection
    – starvation
    – corticosteroid drugs

If Urea normal and Creatinine reduced (both plasma):

  • Decreased muscle mass

If Urea disproportionately higher than increased Creatinine (both plasma):

  • shock
  • hypovolemia due to blood loss, vomiting, etc.
  • hypoperfusion due to: – cardiorenal syndrome, heart failure
    – severe hypotension

Serum/plasma urea is not recommended for routine assessment of renal function because it is a less specific marker of glomerular filtration rate (GFR) than plasma creatinine, the blood test of choice for assessing and monitoring renal function. Urea measurement does, however, have some clinical value, especially when measured in tandem with plasma creatinine.

Measurement of urea alone has proven value in assessment of patients with acute pancreatitis and monitoring effectiveness of hemodialysis.

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