Blood Urea Nitrogen (BUN)/Creatinine ratio, and Interpretations
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.
Calculation of BUN/creatinine ratio
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.
- The value is > 15:1 seen in dehydration and hypotension.
- Postrenal azotemia is due to obstruction.
- This value is <15:1.
BUN/Creatinine ratio is maintained when BUN and creatinine are raised:
- This is suggestive of renal diseases like:
- Intrarenal glomerulonephritis.
- Tubulointerstitial nephritis.
- This is referred to as renal azotemia.
BUN/Creatinine ratio is raised; this is suggestive of:
- Prerenal azotemia:
- There is poor renal perfusion like hypovolemia or hypotension.
- Postrenal azotemia:
- Seen in congestive heart failure.
- Urinary tract obstruction.
- Gastrointestinal bleeding.
BUN/Creatinine ratio is decreased, suggestive of:
- This is rare and seen in:
- Dietary protein deficiency
- 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.
- 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.
- Low-protein diet.
- Severe liver disease.
- Any etiology leading to decreased synthesis of urea.
- Repeated dialysis.
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.
Question 1: What is the limit of increased BUN/Creatinine ratio??
Question 2: What is the BUN/Creatinine ratio in acute renal failure.
It is <20:1