9.1 Acute Kidney Injury Evaluation & Diagnostic Staging

Q: A 72-year-old male with CKD stage 3 (baseline creatinine 1.5 mg/dL) receives high-dose IV furosemide for acute decompensated heart failure. His serum creatinine increases to 2.8 mg/dL over 48 hours, and his 8-hour urine output is 120 mL (weight 80 kg, 0.19 mL/kg/h). Urinalysis shows a specific gravity of 1.022, no casts, and trace protein. Laboratory values reveal Serum Na 136 mEq/L, BUN 62 mg/dL, Urine Na 48 mEq/L, Urine Cr 90 mg/dL, and Urine urea nitrogen 420 mg/dL. The FENa is approximately 1.1%. Which additional calculation is useful for the etiologic assessment, and how should it be interpreted?

Calculate FEUrea: approximately 21.1%, which supports a hypoperfusion pattern but must be integrated with volume/congestion assessment and urine sediment. FENa = (48×2.8)/(136×90)×100 = 1.10%. Loop diuretics can elevate urine sodium/FENa despite hypoperfusion. FEUrea = (420×2.8)/(62×90)×100 = 21.1%, using urine urea nitrogen and BUN in matching units. This is below 35%, supporting a hypoperfusion pattern, but FEUrea is not a definitive etiologic test or fully independent of diuretics. Assess congestion, perfusion, medication exposures, sediment and creatinine/urine-output trends before attributing worsening function to overdiuresis. No supplied finding establishes AIN or bilateral obstruction; urine eosinophils alone do not confirm AIN.

Q: A 59-year-old female is admitted to the ICU following emergent repair of a ruptured abdominal aortic aneurysm complicated by prolonged intraoperative hypotension (MAP 45–55 mmHg for 75 minutes). On postoperative day 2, her serum creatinine has risen from a baseline of 0.8 mg/dL to 2.6 mg/dL, and urine output over the past 24 hours has totaled 280 mL (patient weight 60 kg, rate 0.19 mL/kg/h). Urinalysis reveals numerous muddy brown granular casts and renal tubular epithelial cells. According to the KDIGO clinical practice guidelines, what is the correct AKI stage and primary pathophysiological mechanism?

KDIGO Stage 3 AKI due to ischemic acute tubular necrosis with tubular epithelial necrosis and cast obstruction. According to KDIGO staging criteria, Stage 3 AKI is established by any of the following: serum creatinine increase to >= 3.0 times baseline, an absolute serum creatinine >= 4.0 mg/dL, initiation of renal replacement therapy, urine output = 24 hours, or anuria for >= 12 hours. This patient's urine output rate of 0.19 mL/kg/h over a full 24-hour period meets the oliguric threshold for KDIGO Stage 3, and her creatinine has increased > 3-fold (from 0.8 to 2.6 mg/dL, a 3.25-fold elevation). The presence of coarse muddy brown granular casts and tubular epithelial cells on urine microscopy is pathognomonic for acute tubular necrosis (ATN) resulting from prolonged renal ischemia during cross-clamping and systemic hypotension.

Q: A 66-year-old female with a history of diabetic nephropathy (baseline eGFR 32 mL/min/1.73m², serum creatinine 2.1 mg/dL) is scheduled for urgent coronary angiography following an NSTEMI. The acute care nurse practitioner is formulating a prophylactic regimen to prevent contrast-associated acute kidney injury (CA-AKI). Based on clinical trial evidence, which strategy is recommended for CA-AKI prevention?

Targeted intravenous volume expansion with isotonic crystalloids (0.9% normal saline or balanced crystalloid) and avoidance of nephrotoxic medications. Tailored isotonic IV volume expansion is usual prevention for selected high-risk angiography patients, while accounting for heart-failure/congestion risk and minimizing unnecessary nephrotoxins/contrast. PRESERVE found no benefit of IV bicarbonate over 0.9% saline or oral NAC over placebo; it did not compare hydration with no hydration or prove balanced crystalloids superior. Forcing diuresis with loop diuretics or mannitol worsens medullary hypoperfusion and increases nephrotoxicity. Prophylactic hemofiltration or hemodialysis is invasive, carries serious risks, and is not recommended for contrast prophylaxis.

Key Takeaways

KDIGO consensus criteria stage AKI across three progressive tiers based on serum creatinine kinetics relative to baseline (or an absolute increase of $\ge 0.3\text{ mg/dL}$ within 48 hours) and the duration/severity of oliguria.
Prerenal hypoperfusion often has a supportive urinary pattern: a BUN:Cr ratio > 20:1, FENa < 1%, FEUrea < 35%, and a concentrated urine osmolality > 500 mOsm/kg.
Loop diuretics can elevate FENa despite hypoperfusion. FEUrea <35% supports a prerenal pattern but cannot establish the cause alone; interpret it with perfusion, congestion, sediment and the clinical course.
ATN reflects tubular injury, often with muddy brown granular casts and tubular cells. Traditional urinary patterns include FENa >2%, FEUrea >50% and poorly concentrated urine.
Contrast-associated AKI follows contrast exposure but is not necessarily caused by it. Prevention in selected high-risk patients includes tailored isotonic IV fluid, contrast minimization and avoiding unnecessary nephrotoxins. PRESERVE found no advantage of bicarbonate over saline or oral NAC over placebo.

Last updated: November 2026

9.1 Acute Kidney Injury Evaluation & Diagnostic Staging

Clinical Scenario: A 68-year-old male with heart failure (EF 25%) and CKD stage 3a (baseline SCr 1.4 mg/dL) is admitted to the ICU with decompensated heart failure and pneumonia. He receives IV furosemide 80 mg BID, piperacillin-tazobactam, and vancomycin. On hospital day 3, labs demonstrate BUN 68 mg/dL, SCr 3.2 mg/dL, and potassium 5.1 mEq/L. Over the preceding 12 hours, cumulative urine output is 140 mL (weight 70 kg; 0.17 mL/kg/h).

KDIGO Consensus Staging & Diagnostic Criteria

Acute Kidney Injury (AKI) is an abrupt decline in renal function occurring within 48 hours to 7 days. Staging follows Kidney Disease: Improving Global Outcomes (KDIGO) criteria, classified by the most severe metric met:

KDIGO Stage	Serum Creatinine (SCr) Criteria	Urine Output (UOP) Criteria
Stage 1	$\ge 0.3$ mg/dL increase in 48h OR $1.5–1.9\times$ baseline in 7 days	$< 0.5$ mL/kg/h for 6–12 hours
Stage 2	$2.0–2.9\times$ baseline	$< 0.5$ mL/kg/h for $\ge 12$ hours
Stage 3	$\ge 3.0\times$ baseline OR SCr $\ge 4.0$ mg/dL OR initiation of RRT	$< 0.3$ mL/kg/h for $\ge 24$ h OR anuria for $\ge 12$ h

Serum creatinine lags an abrupt GFR change but rises before a new steady state. Diagnose AKI from serial changes and urine output; do not wait for equilibration. Fluid accumulation can dilute creatinine and obscure injury. Hourly urine output monitoring through an indwelling catheter detects evolving AKI substantially earlier.

Etiologic Classification: Prerenal, Intrinsic & Postrenal AKI

1. Prerenal Azotemia (Renal Hypoperfusion)

Prerenal failure results from reduced renal perfusion without structural tubular injury:

Etiologies: Intravascular volume depletion (hemorrhage, GI losses), decreased effective circulating volume (decompensated heart failure, cirrhosis with ascites, sepsis-induced vasoplegia), or renal artery stenosis.
Hemodynamic Autoregulation: Glomerular filtration is maintained at mean arterial pressures (MAP) $\ge 65$ mmHg via afferent arteriolar vasodilation (prostaglandins) and efferent arteriolar vasoconstriction (angiotensin II). NSAIDs inhibit prostaglandins, constricting afferent arterioles; ACE inhibitors and ARBs block efferent tone, inducing prerenal decompensation in hypoperfused states.

2. Intrinsic Renal Failure (Structural Parenchymal Injury)

Acute Tubular Necrosis (ATN): Accounts for >85% of intrinsic ICU AKI. Triggered by severe prolonged ischemia (shock, cardiac arrest) or nephrotoxins: exogenous (aminoglycosides, vancomycin, amphotericin B, IV contrast) or endogenous (myoglobin in rhabdomyolysis, hemoglobin in hemolysis). Sloughing of tubular cells forms obstructive casts and impairs filtration.
Acute Interstitial Nephritis (AIN): Allergic hypersensitivity involving the interstitium. Over 75% are drug-induced (beta-lactams, PPIs, NSAIDs, fluoroquinolones, sulfonamides). The classic triad (fever, rash, eosinophilia) occurs in <15% of cases. Findings include sterile pyuria, WBC casts, and urine eosinophils. Discontinue the culprit agent; initiate corticosteroids (prednisone 1 mg/kg/day) if renal recovery stalls after 3–5 days.
Glomerulonephritis: Glomerular inflammation presenting with dysmorphic red blood cells, RBC casts, and proteinuria.

3. Postrenal Acute Kidney Injury (Obstructive Uropathy)

Obstruction of urinary flow occurs from benign prostatic hyperplasia (BPH), neurogenic bladder, pelvic tumors, retroperitoneal fibrosis, or bilateral ureteral calculi. Prompt bedside point-of-care ultrasound (POCUS) reveals pelvicalyceal dilation and hydronephrosis. Relieving obstruction via bladder catheterization or ureteral stenting often provokes severe post-obstructive diuresis (>200 mL/h), requiring close electrolyte monitoring and 0.45% saline replacement to match 50–75% of urine output.

Traditional Urinary Patterns & Urine Sediment

Diagnostic Parameter	Prerenal Azotemia	Acute Tubular Necrosis (ATN)
BUN:Creatinine Ratio	$> 20:1$	$10–15:1$
Urine Sodium ( $U_{Na}$ )	$< 20$ mEq/L	$> 40$ mEq/L
Fractional Excretion of Sodium (FENa)	$< 1\%$	$> 2\%$
Fractional Excretion of Urea (FEUrea)	$< 35\%$	$> 50\%$
Urine Osmolality	$> 500$ mOsm/kg	$< 350$ mOsm/kg (Isosthenuria)
Microscopic Urine Sediment	Hyaline casts; benign	"Muddy brown" coarse granular casts, tubular cells

Warning

Fractional Excretion: Calculation and Limits: Loop diuretics promote natriuresis and can raise FENa despite hypoperfusion: $\text{FENa (\%)} = \frac{U_{Na}\times P_{Cr}}{P_{Na}\times U_{Cr}}\times100$ FEUrea may be useful, but diuretics and critical illness can also affect it. Pair urine and plasma urea nitrogen, or use total urea in both; do not mix them. Use matching units for each paired analyte: $\text{FEUrea (\%)} = \frac{\text{Urine urea nitrogen}\times\text{Serum creatinine}}{\text{BUN}\times\text{Urine creatinine}}\times100$ FEUrea <35% supports a hypoperfusion pattern; >50% may support tubular injury. Neither confirms the cause alone. In heart failure, assess congestion as well as perfusion rather than diagnosing overdiuresis from FEUrea.

Contrast-Associated AKI (CA-AKI) Prevention

CA-AKI is AKI temporally associated with intravascular iodinated contrast, usually assessed within 48 hours; another illness may be responsible. Contrast-induced AKI is the subset actually caused by contrast, with proposed vascular/tubular injury mechanisms. Temporal association alone does not establish causation.

High-Risk Profiles: Baseline eGFR $< 30–45$ mL/min/1.73m², diabetic nephropathy, heart failure, hypovolemia, and high contrast volume.
Evidence-Based Strategy: PRESERVE compared IV bicarbonate with 0.9% saline and oral NAC with placebo. Neither intervention improved the primary death/dialysis/persistent-kidney-impairment outcome or CA-AKI. The trial did not compare hydration with no hydration or establish balanced-fluid superiority.
Preventive Guidelines:
1. Volume expansion with isotonic crystalloid (0.9% normal saline or balanced crystalloid at 1.0–1.5 mL/kg/h for 3–12h pre- and 6–12h post-procedure).
2. Minimize contrast volume using iso-osmolar or low-osmolar agents.
3. Withhold nephrotoxic agents (NSAIDs, aminoglycosides) 24–48 hours prior to elective contrast.

Test Your Knowledge

A 72-year-old male with CKD stage 3 (baseline creatinine 1.5 mg/dL) receives high-dose IV furosemide for acute decompensated heart failure. His serum creatinine increases to 2.8 mg/dL over 48 hours, and his 8-hour urine output is 120 mL (weight 80 kg, 0.19 mL/kg/h). Urinalysis shows a specific gravity of 1.022, no casts, and trace protein. Laboratory values reveal Serum Na 136 mEq/L, BUN 62 mg/dL, Urine Na 48 mEq/L, Urine Cr 90 mg/dL, and Urine urea nitrogen 420 mg/dL. The FENa is approximately 1.1%. Which additional calculation is useful for the etiologic assessment, and how should it be interpreted?

Renal biopsy to evaluate for acute interstitial nephritis induced by loop diuretics

Non-contrast CT of the abdomen and pelvis to evaluate for bilateral ureteral obstruction

Calculate FEUrea: approximately 21.1%, which supports a hypoperfusion pattern but must be integrated with volume/congestion assessment and urine sediment

Urine eosinophil assessment; an elevated percentage confirms allergic interstitial nephritis

Test Your Knowledge

A 59-year-old female is admitted to the ICU following emergent repair of a ruptured abdominal aortic aneurysm complicated by prolonged intraoperative hypotension (MAP 45–55 mmHg for 75 minutes). On postoperative day 2, her serum creatinine has risen from a baseline of 0.8 mg/dL to 2.6 mg/dL, and urine output over the past 24 hours has totaled 280 mL (patient weight 60 kg, rate 0.19 mL/kg/h). Urinalysis reveals numerous muddy brown granular casts and renal tubular epithelial cells. According to the KDIGO clinical practice guidelines, what is the correct AKI stage and primary pathophysiological mechanism?

KDIGO Stage 3 AKI due to ischemic acute tubular necrosis with tubular epithelial necrosis and cast obstruction

KDIGO Stage 2 AKI due to prerenal hypoperfusion with intact tubular autoregulation

KDIGO Stage 1 AKI secondary to acute interstitial nephritis from perioperative antibiotics

KDIGO Stage 2 AKI due to postrenal obstructive uropathy from retroperitoneal hematoma mass effect

Test Your Knowledge

A 66-year-old female with a history of diabetic nephropathy (baseline eGFR 32 mL/min/1.73m², serum creatinine 2.1 mg/dL) is scheduled for urgent coronary angiography following an NSTEMI. The acute care nurse practitioner is formulating a prophylactic regimen to prevent contrast-associated acute kidney injury (CA-AKI). Based on clinical trial evidence, which strategy is recommended for CA-AKI prevention?

Intravenous infusion of sodium bicarbonate combined with high-dose oral N-acetylcysteine (1200 mg BID) initiated 24 hours pre-procedure

Targeted intravenous volume expansion with isotonic crystalloids (0.9% normal saline or balanced crystalloid) and avoidance of nephrotoxic medications

Prophylactic continuous venovenous hemofiltration initiated immediately before and continued for 6 hours after contrast exposure

Administration of intravenous furosemide 80 mg with mannitol 20% to force active osmotic diuresis during the angiographic procedure

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9.2 Severe Electrolyte Emergencies & Continuous Renal Replacement Therapy

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9.1 Acute Kidney Injury Evaluation & Diagnostic Staging

Key Takeaways

9.1 Acute Kidney Injury Evaluation & Diagnostic Staging

KDIGO Consensus Staging & Diagnostic Criteria

Etiologic Classification: Prerenal, Intrinsic & Postrenal AKI

1. Prerenal Azotemia (Renal Hypoperfusion)

2. Intrinsic Renal Failure (Structural Parenchymal Injury)

3. Postrenal Acute Kidney Injury (Obstructive Uropathy)

Traditional Urinary Patterns & Urine Sediment

Contrast-Associated AKI (CA-AKI) Prevention