What does the Blood Urea Nitrogen (BUN) test measure?

Why is Blood Urea Nitrogen (BUN) measured?

In the CMP, BUN is primarily assessed as one component of the BUN:creatinine ratio and in conjunction with creatinine and eGFR for renal function characterisation. Elevated BUN with normal creatinine (BUN:creatinine ratio >20:1) suggests pre-renal azotaemia — dehydration, heart failure, gastrointestinal haemorrhage, or high dietary protein intake — rather than intrinsic renal disease. BUN below 7 mg/dL may indicate severe liver failure with impaired urea cycle function, or low protein intake. In the nutritional assessment context, low BUN (combined with low albumin) suggests protein-calorie malnutrition.

Reference ranges

Standard reference

7–20 mg/dL (2.5–7.1 mmol/L) # REVIEWER FLAG: Confirm Quest Diagnostics reference range for BUN — the upper limit varies slightly between platforms (some use 25 mg/dL). Confirm whether Quest applies age-stratified ranges — BUN rises modestly with age as renal mass declines.

Functional medicine optimal

10–16 mg/dL (functional medicine mid-range optimal reflecting adequate renal clearance and balanced protein metabolism without excess catabolism or volume depletion). # REVIEWER FLAG: Not standardised in mainstream nephrology — reviewer to confirm framing.

Units

mg/dL, mmol/L

Reference ranges vary by laboratory and assay platform. Interpret in context of the issuing laboratory's stated range.