Learn › BUN to creatinine ratio
BUN to creatinine ratio: how to calculate it
To find the BUN to creatinine ratio, divide the BUN by the creatinine (both in mg/dL). A ratio of about 10:1 to 20:1 is typical; a ratio above 20:1 points to a problem before the kidney (prerenal), such as dehydration or a GI bleed, while a normal ratio with both values high suggests damage in the kidney itself.
How to calculate it
- Find the BUN (blood urea nitrogen) and the serum creatinine on the same lab draw.
- Divide BUN by creatinine.
- Write the result as a ratio. Example: BUN 40 mg/dL divided by creatinine 1.0 mg/dL = 40, so the ratio is 40:1.
Normal values
| Lab | Typical adult range | Note |
|---|---|---|
| BUN | About 6 to 20 mg/dL | Ranges vary by lab |
| Creatinine | About 0.6 to 1.2 mg/dL | Lower in smaller adults and older adults with less muscle |
| BUN to creatinine ratio | About 10:1 to 20:1 | Read it with both values, never alone |
What a high or low ratio means
| Pattern | Think | Examples |
|---|---|---|
| Ratio above 20:1 | Prerenal: less blood reaching the kidney, or more urea made | Dehydration, heart failure, upper GI bleed (digested blood raises BUN), high protein intake, corticosteroids |
| Ratio 10:1 to 20:1 with both values high | Intrarenal: damage to the kidney itself | Acute tubular necrosis, chronic kidney disease |
| Ratio below 10:1 | Less urea made | Liver disease, low protein intake, malnutrition |
What the nurse does
- Look at the whole client: blood pressure, heart rate, mucous membranes, intake and output and daily weight.
- A high ratio with signs of dehydration: expect fluids as ordered and recheck labs.
- A high ratio with black, tarry stools or coffee-ground emesis: think upper GI bleed and report it.
- Rising creatinine: check every medication for kidney dosing and watch urine output.
NCLEX tip: Creatinine is the better marker of kidney function. BUN rises with dehydration, bleeding and protein, so a high BUN with a normal creatinine is usually not kidney damage.
Practice this in NurseLingo
4,000+ NCLEX-style questions with rationales, 4,300+ flashcards on your lock screen and 380+ study podcasts, all in one app.
Get NurseLingo freeTest yourself: free lab values practice questions with answers and rationales.
Frequently asked questions
What is a normal BUN to creatinine ratio?
About 10:1 to 20:1 in adults. Labs vary, so compare with your lab's reference range.
What does a BUN to creatinine ratio over 20 mean?
It usually points to a prerenal cause such as dehydration, heart failure or an upper GI bleed, rather than damage to the kidney itself.
Why does a GI bleed raise BUN?
Blood digested in the gut is protein, and protein breaks down into urea, so BUN rises while creatinine stays about the same.
What does it mean when the BUN creatinine ratio is high?
A ratio above about 20:1 usually points to a problem before the kidney, such as dehydration, heart failure or an upper GI bleed, or to more urea being made (high protein intake, steroids). It is read together with the BUN, creatinine and the client's condition.
What level of BUN is concerning?
A typical adult BUN is about 6 to 20 mg/dL (labs vary). A rising BUN matters most alongside creatinine: a high BUN with a normal creatinine often means dehydration or bleeding, while both rising together suggests the kidneys are struggling. Report changes from the client's baseline.
How do you fix high BUN and creatinine?
By treating the cause. Dehydration is treated with fluids, an upper GI bleed with stopping the bleeding, and kidney injury by removing the cause, stopping kidney-harming drugs and adjusting doses. Nurses monitor intake and output, daily weight and repeat labs.
When to worry about creatinine ratio?
A BUN to creatinine ratio above about 20:1 suggests a prerenal problem such as dehydration, heart failure or a GI bleed, and a rising creatinine at any ratio suggests the kidneys are affected. Read it with the client's vital signs, urine output and history.
Keep studying
Sources
- BUN and Creatinine (Clinical Methods, 3rd edition), NCBI Bookshelf
- Prerenal Kidney Failure, StatPearls, NCBI Bookshelf
For study only, not medical advice. Ranges and protocols vary by facility; follow your program, your facility's policies and the provider's orders.