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

August 3, 2023Chemical pathology Lab Tests

Blood urea nitrogen (BUN)/Creatinine ratio

Sample Blood urea nitrogen/creatinine ratio

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

Indications for 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.

Causes of Uremia

Calculation of BUN/creatinine ratio

Blood urea nitrogen/Creatinine ratio formula

Normal BUN/creatinine ratio

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

Interpretations of BUN/creatinine ratio:

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

BUN/Creatinine ratio is maintained when BUN and creatinine are raised:

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

BUN/Creatinine ratio is raised; this is suggestive of:

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

BUN/Creatinine ratio is decreased, suggestive of:

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

Table differentiating 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

Increased ratio (BUN/Creatinine) >10:1 while creatinine is normal:

Prerenal azotemia, where BUN rises without the increase in creatinine, is due to decreased GFR and is seen in:

Heart failure.
Dehydration.
Blood loss.
Salt depletion.
GI tract hemorrhage.
High protein intake.
Certain drugs like tetracycline and glucosteroids.

Increased BUN/Creatinine ratio = >10:1 with raised creatinine level is seen in:

Postrenal azotemia where BUN rises without any ratio to creatinine increase.

Obstructive uropathy.
Prerenal azotemia is superimposed on renal disease.

Decreased BUN/Creatinine ration <10:1 with decreased BUN value is seen in:

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

Decreased BUN/Creatinine ratio <10:1 with increased creatinine level is seen in:

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/Creatinine ratio causes

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.