Your Kidney and Hydration Marker
BUN measures urea in your blood—a waste product from protein breakdown. Your liver makes urea, and your kidneys filter it out. BUN rises when kidneys aren't filtering well OR when you're dehydrated (concentrated blood) OR when you're eating a lot of protein. The BUN-to-creatinine ratio helps distinguish these causes.
What is BUN (Blood Urea Nitrogen)?
BUN measures blood urea nitrogen. Urea is produced by the liver from ammonia (protein metabolism waste). Filtered by kidneys, with 40-50% passively reabsorbed. BUN/creatinine ratio: normal 10-20:1. >20:1 = prerenal (dehydration, GI bleed). <10:1 = liver disease, low protein intake.
↑ What High BUN (Blood Urea Nitrogen) Means
Either kidney dysfunction (rising with creatinine), dehydration (BUN rises more than creatinine → BUN/Cr ratio >20), GI bleeding (digested blood = protein load), or high protein intake.
Common symptoms:
If dehydration: thirst, dry mouth, dark urine, dizziness · If kidney failure (uremia): nausea, loss of appetite, fatigue, confusion, metallic taste · If GI bleed: dark tarry stools, fatigue, lightheadedness
↓ What Low BUN (Blood Urea Nitrogen) Means
Liver disease (liver can't make urea), malnutrition, overhydration. Not usually concerning unless very low.
Common symptoms:
Usually none
Why It Matters
When normal:
Part of basic metabolic panel
BUN/Cr ratio helps identify dehydration vs kidney disease
Elevated BUN from GI bleed (digested blood)
Monitors kidney function with creatinine
Risks if abnormal:
High: kidney disease, dehydration, GI bleed, high protein diet
Very high (>100): severe renal failure, may need dialysis
BUN alone is less specific than creatinine for kidney function
What Can Cause Abnormal Levels?
Dehydration
35% likelyBUN rises disproportionately to creatinine when dehydrated. BUN/Cr >20:1.
Kidney Disease
30% likelyBUN and creatinine rise together. BUN/Cr ratio stays 10-20:1.
GI Bleeding
Digested blood in the gut is absorbed as protein → liver converts to urea → BUN rises. Classic elevated BUN/Cr ratio.
High Protein Diet
More protein metabolism = more urea production.
Medications
Corticosteroids increase protein catabolism → higher BUN.
Heart Failure
Reduced renal perfusion → prerenal elevation.
What You Can Do
If elevated with BUN/Cr >20: hydrate (dehydration is likely)
Impact: IV or oral fluids correct prerenal azotemia quickly \u00B7 Timeline: Hours
If elevated with creatinine proportionally: evaluate kidney function (eGFR)
Impact: BUN alone is not a good kidney function marker \u00B7 Timeline: With creatinine
If lifestyle changes aren't enough:
If elevated BUN/Cr ratio without obvious dehydration: consider GI bleed
Impact: Check stool for blood (guaiac or FIT) \u00B7 Timeline: Promptly
Recommended retest: Part of every BMP/CMP; per kidney disease monitoring schedule
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