BUN-to-creatinine ratio
| BUN-to-creatinine ratio | |
|---|---|
| Diagnostics | |
| LOINC | 44734-2, 3097-3 |
In medicine, the BUN-to-creatinine ratio is the ratio of two serum laboratory values, the blood urea nitrogen (BUN) (mg/dL) and serum creatinine (mg/dL) (Cr). Outside the United States, particularly in Canada and Europe, the truncated term urea is used (though it is still the same blood chemical) and the units are different (mmol/L). The units of creatinine are also different (umol/L), and this value is termed the urea-to-creatinine ratio. The ratio may be used to determine the cause of acute kidney injury.
The principle behind this ratio is the fact that both urea (BUN) and creatinine are freely filtered by the glomerulus, however urea reabsorbed by the tubules can be regulated (increased or decreased) whereas creatinine reabsorption remains the same (minimal reabsorption).
Contents |
[edit] Interpretation
| BMP/ELECTROLYTES: | |||
| Na+=140 | Cl−=100 | BUN=20 | / |
| Glu=150 | |||
| K+=4 | CO2=22 | PCr=1.0 | \ |
| ARTERIAL BLOOD GAS: | |||
| HCO3-=24 | paCO2=40 | paO2=95 | pH=7.40 |
| ALVEOLAR GAS: | |||
| pACO2=36 | pAO2=105 | A-a g=10 | |
| OTHER: | |||
| Ca=9.5 | Mg2+=2.0 | PO4=1 | |
| CK=55 | BE=−0.36 | AG=16 | |
| SERUM OSMOLARITY/RENAL: | |||
| PMO = 300 | PCO=295 | POG=5 | BUN:Cr=20 |
| URINALYSIS: | |||
| UNa+=80 | UCl−=100 | UAG=5 | FENa=0.95 |
| UK+=25 | USG=1.01 | UCr=60 | UO=800 |
| PROTEIN/GI/LIVER FUNCTION TESTS: | |||
| LDH=100 | TP=7.6 | AST=25 | TBIL=0.7 |
| ALP=71 | Alb=4.0 | ALT=40 | BC=0.5 |
| AST/ALT=0.6 | BU=0.2 | ||
| AF alb=3.0 | SAAG=1.0 | SOG=60 | |
| CSF: | |||
| CSF alb=30 | CSF glu=60 | CSF/S alb=7.5 | CSF/S glu=0.4 |
Normal serum values
| Test | SI units | US units |
|---|---|---|
| BUN (Urea) | 7-20 mg/dL | |
| Urea | 2.5-10.7 mmol/L | |
| Creatinine | 62-106 μmol/L | 0.7-1.2 mg/dL |
Serum Ratios
| 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. |
| 10-20:1 | 100-40:1 | 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. |
An elevated BUN:Cr due to a low or low-normal creatinine and a BUN within the reference range is unlikely to be of clinical significance.
[edit] Specific causes of elevation
[edit] Acute Kidney Injury (previously termed Acute Renal Failure)
The ratio is predictive of prerenal injury when BUN:Cr exceeds 20[1] or when urea:Cr exceeds 0.1 and urea is greater than 10.[2] In prerenal injury, urea increases disproportionately to creatinine due to enhanced proximal tubular reabsorption.
[edit] Gastrointestinal bleeding
The ratio is useful for the diagnosis of bleeding from the gastrointestinal (GI) tract in patients who do not present with overt vomiting of blood.[3] In children, a BUN:Cr ratio of 30 or greater has a sensitivity of 68.8% and a specificity of 98% for upper gastrointestinal bleeding.[4]
BUN level increases in upper GI bleeding because patients become prerenal, secondary to blood loss which decreases blood flow to the kidney.
A common misconception is that the ratio is elevated because of amino acid digestion. Since blood, which consists largely of the protein hemoglobin, is broken down by digestive enzymes of the upper GI tract into amino acids, which are then reabsorbed in the GI tract and broken down into urea. However, elevated BUN:Cr ratios are not observed when other high protein loads (e.g., steak) are consumed.[citation needed]
[edit] Advanced age
Because of decreased muscle mass, elderly patients may have an elevated BUN:Cr at baseline.[5]
[edit] References
- ^ Morgan DB, Carver ME, Payne RB (October 1977). "Plasma creatinine and urea: creatinine ratio in patients with raised plasma urea". Br Med J 2 (6092): 929–32. PMC 1631607. PMID 912370. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1631607.
- ^ "Acute renal failure: urea:creatinine ratio was not very helpful in diagnosing prerenal failure". Evidence-Based On-Call database. http://www.eboncall.org/CATs/1844.htm.
- ^ Witting MD, Magder L, Heins AE, Mattu A, Granja CA, Baumgarten M (May 2006). "ED predictors of upper gastrointestinal tract bleeding in patients without hematemesis". Am J Emerg Med 24 (3): 280–5. doi:10.1016/j.ajem.2005.11.005. PMID 16635697. http://linkinghub.elsevier.com/retrieve/pii/S0735-6757(05)00427-4.
- ^ Urashima M, Toyoda S, Nakano T, et al. (July 1992). "BUN/Cr ratio as an index of gastrointestinal bleeding mass in children". J. Pediatr. Gastroenterol. Nutr. 15 (1): 89–92. PMID 1403455.
- ^ Feinfeld DA, Bargouthi H, Niaz Q, Carvounis CP (2002). "Massive and disproportionate elevation of blood urea nitrogen in acute azotemia". Int Urol Nephrol 34 (1): 143–5. PMID 12549657. http://www.kluweronline.com/art.pdf?issn=0301-1623&volume=34&page=143.
[edit] External
- Agrawal M, Swartz R (April 2000). "Acute renal failure". Am Fam Physician 61 (7): 2077–88. PMID 10779250. http://www.aafp.org/afp/20000401/2077.html.
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