BUN/Creatinine Ratio - Kresser Institute Membership Site

REFERENCE RANGES FOR BUN/CREATININE RATIO:

Laboratory reference range:¹

BUN:Cr	Urea:Cr	Location	Mechanism
>20:1	>100:1	Prerenal (before the kidney)	BUN reabsorption is increased. BUN is disproportionately elevated relative to creatinine in serum. Dehydration or hypoperfusion is suspected.
10-20:1	40-100:1	Normal or Postrenal (after the kidney)	Normal range. Can also be postrenal disease. BUN reabsorption is within normal limits.
<10:1	<40:1	Intrarenal (within kidney)	Renal damage causes reduced reabsorption of BUN, therefore lowering the BUN:Cr ratio.

Functional reference range: same as conventional range

DESCRIPTION:

Urea is the water-soluble byproduct of protein metabolism through the urea cycle (ornithine cycle).² Blood urea nitrogen (BUN) is a measure of nitrogen incorporated within urea molecules. Creatinine is a water-soluble waste product of protein catabolism, specifically muscle proteins. Neither BUN nor creatinine has a known physiological action in the body; however, these biomarkers are routinely measured in clinical settings.^3,4 The ratio of BUN to creatinine can provide useful clinical information about blood volume and renal perfusion.²

Muscles produce creatinine at a constant rate, proportional to overall muscle mass.^5,6 Virtually all creatinine in the blood is completely filtered by the kidneys and not reabsorbed.⁵ Creatinine varies inversely with glomerular filtration rate (GFR) and is a useful biomarker for estimating GFR. BUN production, on the other hand, is not constant.^7,8 Once urea is formed, it is filtered by the kidney.⁹ However, roughly half of filtered urea is passively reabsorbed in the proximal tubule of the kidney.² While BUN and creatinine vary inversely with GFR, BUN levels may increase without concomitant renal disease.¹⁰ For these reasons, creatinine is usually a better measure of GFR than BUN. The exception is advanced kidney disease, in which GFR is more accurately estimated by averaging clearance rates of both BUN and GFR.^11,12

The BUN/creatinine ratio is mainly used to detect prerenal injury, as occurs from reduced blood flow to the kidneys.¹³ As intravascular volume decreases, the proximal tubule of the kidney retains sodium and water to compensate.¹³ This is accompanied by an increase in urea reabsorption by the kidney and elevations in the blood.² Creatinine levels, on the other hand, stay relatively stable in this state. Consequently, a BUN/creatinine ratio of 20 or greater most often indicates prerenal disease. There may be other causes of an abnormally high BUN/creatinine ratio, however. Gastrointestinal bleeding may result in a disproportionately large increase in BUN relative to creatinine.^11,14 Corticosteroid treatment may also disproportionately elevate BUN levels.¹⁴ In fact, a large number of medications may increase BUN without appreciably affecting creatinine.^2,5 Conversely, muscle wasting may reduce creatinine production to the point that the BUN/creatinine ratio is abnormally high despite a normal BUN level.¹¹

Normal and abnormally low BUN/creatinine ratio results must be considered within the clinical context and are often not clinically useful.¹³ For instance, prerenal disease may exist in a patient with a normal BUN/creatinine ratio if urea production is abnormally low for some reason. Likewise, an abnormally low BUN/creatinine ratio is usually caused by decreased BUN in the context of normal creatinine.¹⁵ This may be due to liver failure, diminished protein intake, or severe polyuria/polydipsia.¹⁵ Massive increases in creatinine could also result in an abnormally low BUN/creatinine ratio, such as myositis, rhabdomyolysis, or other severe muscle trauma.¹⁵ Cephalosporins, vitamin C, and flucytosine can spuriously increase serum creatinine levels without affecting measured BUN levels, decreasing the BUN/creatinine ratio.¹⁵

Creatinine and BUN are routinely measured as part of the basic metabolic panel or complete metabolic panel.⁵

PATHOLOGICAL/CONVENTIONAL RANGE INDICATIONS:

High in:^13,15

Prerenal injury
- Hypovolemia
- Hypotension
- Congestive heart failure
- Nephrotic syndrome
Gastrointestinal hemorrhage
Muscle wasting
Drugs
- Diuretics (e.g., furosemide, hydrochlorothiazide)
- Antibiotics (e.g., tetracyclines)
- Anti-gout medications (e.g., allopurinol, probenecid)
- Chemotherapeutics (e.g., cisplatin)

Low in:¹⁵

Low protein intake
Liver failure
Severe polyuria/polydipsia
- Diabetes insipidus
- Diabetes mellitus (uncontrolled)
- Cushing’s disease
Muscle injury
- Myositis
- Rhabdomyolysis

FUNCTIONAL RANGE INDICATIONS:

High in:

Same as conventional indications

Low in:

Same as conventional indications

References: