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Renal
Urea / blood urea nitrogen (BUN)
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Urea is the main nitrogenous end-product of protein catabolism, excreted by the kidney. The US reports urea nitrogen (BUN, mg/dL); most other countries report urea in mmol/L or g/L.
Reference interval
| Conventional (mg/dL (BUN)) | SI (mmol/L (urea)) | |
|---|---|---|
| Adults | 7–20 mg/dL (BUN) | 2.5–7.1 mmol/L (urea) |
Specimen: Serum or plasma · Conventional × 0.357 = SI
Causes of high values
Reduced glomerular filtration (pre-renal, renal or post-renal), dehydration, gastrointestinal bleeding, high protein intake, corticosteroids and catabolic states.
Causes of low values
Low protein intake, severe liver disease, overhydration, pregnancy and SIADH.
Pre-analytical and method notes
Convert BUN (mg/dL) to urea (mg/dL) by multiplying by 2.14; urea in mmol/L = BUN in mg/dL x 0.357. A BUN-to-creatinine ratio above about 20:1 (mg/dL) suggests pre-renal azotaemia or upper GI bleeding. Reference intervals vary by laboratory and age; urea is a less reliable marker of GFR than creatinine because of non-renal influences.
References
- Rifai N, Horvath AR, Wittwer CT. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Elsevier, 6th edition (2018) — Tier 3 (textbook)
- McPherson RA, Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. Elsevier — Tier 3 (textbook)
- KDIGO 2012 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Disease: Improving Global Outcomes (2013) — Tier 2 (guidelines / primary literature)
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