Home
Disease Vault Drug Index & Alternatives Drug Interactions Pregnancy Drug Safety Drug Recalls & Alerts OTC Navigator
Medical Bot Med Lab Clinical Cases Nutrition & Macro Calculator
Questo Insights Global Opportunities Paper Helper Citation Generator Scholarships & Travel Visa & Mobility Navigator English Assessment Test
Med Jobs Med Lounge Pharma Swap Pharmacies Directory Medical Case Donations Opportunity Crowdfunding
Courses Academy Quiz Arena Memory Palace STEM Academy Toolkits
About Us Contact Us Privacy Policy Terms of Service
Back to Memory Palace
Nephrology Acute Kidney Injury Intermediate
2,023 views

Prerenal vs Intrinsic AKI Differentiation

BUN:Cr
&
FENa
CHEAT
SHEET
Diagnostic markers differentiating prerenal hypoperfusion from intrinsic acute tubular necrosis (ATN).

In prerenal azotemia, structurally intact renal tubules avidly reabsorb sodium and water to restore intravascular volume. In intrinsic ATN, tubular epithelial cell necrosis prevents reabsorption, leading to salt wasting.

Systematic Breakdown

BUN:Cr > 20:1

Prerenal Azotemia

Urea is reabsorbed passively along with sodium and water; creatinine is not, elevating the ratio above 20:1.

BUN:Cr < 15:1

Intrinsic Renal (ATN)

Damaged tubular epithelium cannot reabsorb urea; both urea and creatinine excretion are impaired in tandem.

FENa < 1%

Prerenal (Avid Na+ Reabsorption)

Fractional excretion of sodium < 1% indicates intact tubular function conserving sodium.

FENa > 2%

Intrinsic / ATN (Tubular Salt Wasting)

Damaged tubules cannot reabsorb sodium, so more filtered sodium appears in the urine.

Urine Osm > 500

Prerenal Concentrated Urine

ADH actively concentrates urine; intact tubules generate high medullary osmotic gradient.

Urine Osm < 350

Intrinsic Isosthenuric Urine

Loss of medullary concentration gradient results in urine osmolality mirroring plasma (~300 mOsm/kg).

High-Yield Clinical Pearls & Exam Tips
  • If patient is on diuretics, FENa is unreliable (diuretics force sodium into urine); use Fractional Excretion of Urea (FEUrea < 35% indicates prerenal).
  • Urine microscopy: Hyaline casts = Prerenal azotemia; Muddy brown granular casts / tubular epithelial cells = Acute Tubular Necrosis (ATN).
Related Pharmaceuticals
Furosemide IV Crystalloids ACE Inhibitors NSAIDs
Related Clinical Conditions
Prerenal Azotemia Acute Tubular Necrosis Hepatorenal Syndrome
Authoritative Sources: KDIGO Clinical Practice Guideline for Acute Kidney Injury · Brenner and Rector's The Kidney 11e

More Nephrology Mnemonics

Nephrology Intermediate
HARD-UP

Normal Anion Gap Metabolic Acidosis

H A R D - U P

Causes of hyperchloremic (normal anion gap) metabolic acidosis (AG 8-12 mEq/L) characterized by reciprocal chloride retention.

Harrison's Principles of Internal Medicine 21e 182
Nephrology Intermediate
INFLAMMATION vs PODOCYTE PODIUM

Nephritic vs Nephrotic Syndrome Cheat Sheet

INFLAMMATION vs PODOCYTE PODIUM

Core clinical and urinalysis distinctions between glomerular inflammation (Nephritic) and glomerular charge/barrier breakdown (Nephrotic).

Brenner and Rector's The Kidney 11e 2,047
Nephrology Intermediate
C BIG K DROP

Hyperkalemia Management Protocol: C BIG K DROP

C BIG K DROP

Step-by-step emergency medical stabilization and total body elimination of severe hyperkalemia (K+ > 6.5 mEq/L or with ECG changes).

AHA Guidelines on Electrolyte Emergencies 2,225

System Notice

Confirm Action