Blood Urea Nitrogen (BUN)/Creatinine ratio, and Interpretations

July 10, 2025Chemical pathology Lab Tests

Blood urea nitrogen (BUN)/Creatinine ratio

What sample is needed for the Blood urea nitrogen/creatinine ratio?

Fresh serum of the patient is needed.
Estimate serum BUN and Creatinine.

What are the indications for the blood urea nitrogen/creatinine ratio?

To find the cause of azotemia or renal failure.
It will differentiate prerenal and postrenal azotemia from renal azotemia.
BUN/Creatinine ratio is a rough guide for renal disease.

Uremia (Azotemia) causes.

How will you calculate the BUN/creatinine ratio?

Blood urea nitrogen/Creatinine ratio formula

What is the normal BUN/creatinine ratio?

BUN/Creatinine ratio = 10 :1
- On normal diet = 12 to 16
- Optimum adult level = 15.5

How will you interpret the BUN/creatinine ratio?

BUN/Creatinine ratio differentiates between acute and chronic renal disease.
Prerenal azotemia appears with poor renal perfusion, such as hypovolemia and hypotension.
1. A value greater than 15:1 is seen in dehydration and hypotension.
Postrenal azotemia is due to obstruction.
1. This value is <15:1.

Where will you find that the BUN/Creatinine ratio is maintained when BUN and creatinine levels are elevated?

This is suggestive of renal diseases like:
1. Intrarenal glomerulonephritis.
2. Tubulointerstitial nephritis.
3. This is referred to as renal azotemia.

What are the causes of a raised BUN/Creatinine ratio?

Prerenal azotemia:
1. There is poor renal perfusion, such as hypovolemia or hypotension.
Postrenal azotemia:
1. Seen in congestive heart failure.
2. Urinary tract obstruction.
3. Gastrointestinal bleeding.
4. Trauma.

What are the causes of decreased BUN/Creatinine ratio?

This is rare and seen in:
1. Dietary protein deficiency
2. Severe liver disease

Prerenal acute failure = BUN/Creatinine ratio > 20 : 1
Renal acute failure = BUN/Creatinine ratio < 20 : 1
Suggest renal azotemia or late postrenal azotemia when BUN / Creatinine ratio = < 10:1

How will you differentiate acute prerenal and renal failure?

Lab tests	Acute renal failure	Acute Prerenal Failure	Acute Postrenal Failure
Etiology	Renal (prerenal or postrenal)	Decreased renal perfusion	Obstruction of the renal collecting system
BUN/ Creatinine ratio	<20:1 (<10)	>20:1 (>20)	Raised
Urine specific gravity	<1.010	>1.020	Raised

What are the causes of an increased (BUN/Creatinine) ratio >10:1 while creatinine is normal?

Prerenal azotemia, where BUN rises without an increase in creatinine, is due to decreased GFR and is seen in:
1. Heart failure.
2. Dehydration.
3. Blood loss.
4. Salt depletion.
5. GI tract hemorrhage.
6. High protein intake.
7. Certain drugs like tetracycline and glucosteroids.

What are the causes of an increased BUN/Creatinine ratio =>10:1 with a raised creatinine level?

Postrenal azotemia, where BUN rises without any correlation to creatinine increase.
1. Obstructive uropathy.
2. Prerenal azotemia is superimposed on renal disease.

What are the causes of a decreased BUN/Creatinine ratio = <10:1 with decreased BUN value?

Acute tubular necrosis.
Starvation.
Low-protein diet.
Severe liver disease.
Any etiology leading to a decrease in urea synthesis.
Repeated dialysis.
Pregnancy.

What are the causes of a decreased BUN/Creatinine ratio = <10:1 with increased creatinine level?

Rhabdomyolysis (There is the release of muscle creatinine).
Drugs like phenacemide therapy (Will increase the conversion of creatine to creatinine).
Patients with a muscular body develop renal failure.

Blood Urea Nitrogen (BUN)/Creatinine ratio: BUN and creatinine ratio causes

Blood Urea Nitrogen (BUN)/Creatinine ratio: The BUN and creatinine ratio causes

Questions and answers:

Question 1: What is the limit of increased BUN/Creatinine ratio??

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Question 2: What is the BUN/Creatinine ratio in acute renal failure?