Step by Step
1
Creatinine — the best simple GFR estimate
A muscle waste product (0.6-1.2 mg/dL) that is freely filtered by the kidneys, making it the best simple marker of GFR. Creatinine doubles when GFR is cut in half. It rises relatively slowly, taking 24-48 hours to reflect acute kidney injury.
2
BUN and eGFR
BUN (blood urea nitrogen, 7-20 mg/dL) is affected by protein intake, hydration status, and GI bleeding — making it a less pure kidney marker than creatinine alone. eGFR (estimated GFR), calculated from creatinine plus age, sex, and race, should normally be above 60 mL/min; chronic kidney disease is defined as eGFR below 60 for more than 3 months.
3
The BUN:creatinine ratio — localizing the problem
A ratio above 20 suggests a prerenal cause (dehydration, GI bleeding, or heart failure — where the kidney itself is fine but underperfused). A ratio below 10 suggests intrinsic renal damage (like acute tubular necrosis or glomerulonephritis). A ratio between 10-20 is considered normal.
4
Urine sodium and FENa — further localization
Urine sodium below 20 mEq/L suggests a prerenal cause (the kidney is appropriately conserving sodium). Urine sodium above 40 mEq/L suggests intrinsic renal damage (the tubules can no longer reabsorb sodium properly). FENa (fractional excretion of sodium) below 1% suggests prerenal; above 2% suggests intrinsic renal.
Applied Walkthrough
1
A dehydrated patient shows elevated creatinine and BUN, with a BUN:creatinine ratio above 20 — this pattern, combined with a urine sodium below 20 mEq/L, points toward a prerenal cause: the kidneys themselves are structurally fine, but they're underperfused due to volume depletion.
2
A different patient with acute tubular necrosis (direct kidney tubule damage) shows a BUN:creatinine ratio below 10, along with a urine sodium above 40 mEq/L — this combination points toward intrinsic renal damage instead, since the damaged tubules can no longer effectively reabsorb sodium.
3
For both patients, eGFR would likely be reduced below 60 — but the additional BUN:creatinine ratio and urine sodium data are what actually distinguish a prerenal problem (potentially reversible with fluids) from intrinsic kidney damage (a more serious, structural issue).
Exam Application
Exams test whether you know normal creatinine, BUN, and eGFR ranges, and specifically whether you can use the BUN:creatinine ratio (along with urine sodium or FENa) to distinguish a prerenal cause of kidney dysfunction from an intrinsic renal cause.
⚠ Common Trap
The most common trap is using creatinine or BUN alone to diagnose the cause of kidney dysfunction — the BUN:creatinine RATIO (and supporting urine sodium/FENa data) is what actually localizes the problem as prerenal versus intrinsic renal, information neither value provides in isolation.
✓ Quick Self-Check
1. Why is creatinine considered the best simple marker of GFR?
Because it's a muscle waste product that is freely filtered by the kidneys, and it doubles when GFR is cut in half.
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2. What eGFR value defines chronic kidney disease, and over what time period?
Below 60 mL/min for more than 3 months.
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3. What does a BUN:creatinine ratio above 20 suggest?
A prerenal cause (dehydration, GI bleeding, or heart failure).
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4. What does a BUN:creatinine ratio below 10 suggest?
Intrinsic renal damage (like acute tubular necrosis or glomerulonephritis).
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5. How does urine sodium help distinguish prerenal from intrinsic renal causes?
Urine sodium below 20 mEq/L suggests prerenal (kidney appropriately conserving sodium); above 40 mEq/L suggests intrinsic renal (tubules can't reabsorb sodium).
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