Acute Tubular Necrosis (ATN): Ischemic & Nephrotoxic Patterns
Updated March 2026 to reflect KDIGO 2026 AKI/AKD guideline recommendations.
Definition & Epidemiology
ATN is acute injury to the proximal and distal tubular epithelium from either: 1. Ischemia (shock, sepsis, major surgery) → 45–50% of ATN 2. Nephrotoxins (drugs, pigments, contrast) → 35–40% of ATN 3. Combined (sepsis + aminoglycosides) → 15–20%
ATN is the #1 cause of intrinsic AKI in ICU (50% of ICU AKI cases). Unlike AIN, damage is necrotic (tubular epithelial death), not inflammatory.
Key feature of ATN: Tubular epithelial cells DIE and slough into urine. Diagnosis confirmed by muddy brown casts (hemoglobin- or myoglobin-laden cellular debris).
Pathophysiology: The Three Phases
Phase 1: Initiation (0–24 hours)
Ischemic ATN: - Reduced renal perfusion → hypoxia in proximal tubule (S3 segment = most vulnerable) - ATP depletion → loss of Na-K-ATPase function - Cellular swelling, brush border loss, cell death
Nephrotoxic ATN: - Direct tubular epithelial toxicity (aminoglycoside, myoglobin, hemoglobin) - ROS generation, mitochondrial dysfunction - Tubular obstruction (pigment casts in pigmenturia) - Na reabsorption blocked → FeNa elevation
Phase 2: Maintenance (1–3 weeks)
- Continued GFR decline despite removal of offending agent
- Persistent tubular dysfunction
- Tubular obstruction from casts
- Back-leak of glomerular filtrate across damaged epithelium
- Urine output variable: Oliguric (40%) vs non-oliguric (60%)
- Oliguric = worse prognosis (higher mortality, longer recovery)
Phase 3: Recovery (1–4 weeks)
- Tubular epithelial regeneration (cells redifferentiate)
- Restoration of tight junctions, brush border, transporter function
- GFR gradually improves
- Timeline: Non-oliguric recovers in 1–3 weeks; oliguric in 3–8 weeks
Clinical pearl: If patient enters oliguric ATN phase, mortality risk ↑ 2–3× despite same initial AKI severity. Early RRT, tight fluid balance, and avoidance of secondary insults critical.
Ischemic ATN: Hypoperfusion Patterns
Classic Scenarios
- Cardiogenic shock — MI, acute decompensated HF, severe bradycardia
- Septic shock — vasodilation + hypovolemia
- Major surgery — especially bypass, vascular surgery
- Massive hemorrhage — inadequate fluid resuscitation
- Aortic dissection/cross-clamp — renal artery occlusion
Tubular Vulnerability: The S3 Segment
Proximal tubule has 3 segments: - S1 & S2: Glucose, amino acid reabsorption; better oxygenation - S3 (thick ascending limb): High Na reabsorption, low oxygen supply → FIRST to die in hypoxia
Result: S3 injury → natriuresis (tubular reabsorption loss) → FeNa >2% early
Laboratory & Urinalysis Signature
- Urine microscopy is the gold standard for ATN diagnosis — prioritize over FeNa/FeUrea
- Muddy brown granular casts are DIAGNOSTIC of ATN — high specificity, underused in practice
- Granular casts (coarse or fine — tubular protein + cells)
- Urine osmolality: <400 mOsm/L (tubular concentration failure)
- No hematuria or proteinuria >3 g/day (unless rhabdo)
FeNa is overrated. FeNa only reliably answers ONE question: Is this oliguric patient prerenal or ATN? Even then, it is confounded by diuretics (use FeUrea instead — but FeUrea has its own limitations), CKD (baseline tubular Na wasting), and sepsis (FeNa can be <1% in septic ATN). FeNa >2% supports ATN, but urine microscopy with muddy brown granular casts is more specific and should be your first-line diagnostic tool.
Nephrotoxic ATN: Drug & Pigment Mechanisms
Aminoglycoside-Induced ATN
Mechanism: - Uptake by proximal tubule via megalin/cubilin (high specificity) - Accumulation in lysosomes → generation of free radicals - Dose-dependent (correlates with cumulative dose) - Onset: 2–5 days (sometimes up to 2 weeks)
Risk factors: - Age >70, female - CKD or DM baseline - Volume depletion, sepsis - Concurrent nephrotoxins (vancomycin, amphotericin, NSAIDs)
Prevention: - Extended-interval dosing (15 mg/kg Q24-48h) safer than conventional divided dosing - Monitor aminoglycoside levels (peak, trough) - Check baseline renal function, adjust dose - Avoid polypharmacy with other nephrotoxins
Contrast-Induced Nephropathy (CIN) / Contrast-Associated AKI (CA-AKI)
Mechanism: - Osmotic load → tubular obstruction, hypoxia, ROS - Direct tubular epithelial toxicity (rare with modern iso-osmolar agents)
Risk stratification (KDIGO): - High risk: eGFR <30, DM + CKD, elderly, dehydration, CHF - Moderate risk: eGFR 30–60 - Low risk: eGFR >60, no DM, young
Prevention (evidence-based): 1. Hydration: IV isotonic saline 1–1.5 L before/after (best prevention) 2. Hold metformin 48h post-contrast if Cr ↑ 3. Avoid NSAIDs × 48h post-contrast 4. Use iso-osmolar or low-osmolar agents if possible 5. Minimize contrast volume (not >5 mL/kg body weight)
Bicarbonate, N-acetylcysteine, theophylline: NOT proven effective in recent RCTs. Hydration + careful dosing sufficient.
Rhabdomyolysis & Myoglobinuria-Induced ATN
Mechanism: - Muscle breakdown → myoglobin release (molecular weight 17 kDa, filtered freely) - Myoglobin precipitation in acidic tubular fluid (forms casts) - Tubular obstruction + direct cytotoxicity - Dark urine (cola-colored); dipstick+ for blood BUT no RBCs on microscopy
Severity spectrum: - Mild: CK 1,000–5,000, no AKI - Moderate: CK 5,000–50,000, risk of AKI - Severe: CK >50,000, very high AKI risk (~50% develop AKI)
Common causes: - Trauma, crush injuries, prolonged immobilization - Statin-induced (especially with gemfibrozil or other CYP3A4 inhibitor) - Statins + NSAIDs + AKI precipitant - Exertional (extreme exercise, heat stroke, malignant hyperthermia) - Infections (influenza, HIV, EBV), seizures
Management: 1. Aggressive hydration: IV NS 1–1.5 L/h until myoglobin clears (dark urine → clear) - Goal: Urine output >200 mL/h (maintain tubular flow to prevent precipitation) 2. Urine alkalinization: NaHCO3 to keep urine pH >6.5 (myoglobin soluble above pH 6.5) - Sodium bicarb 50–100 mEq in D5W until pH target met 3. Monitor: CK, K, Ca, PO4, Cr, urine myoglobin 4. RRT if AKI severe: Hyperkalemia, severe acidosis, uremia
Clinical pearl: Rhabdo-induced AKI is preventable with aggressive early hydration. Don’t delay — start IV NS immediately if CK >1,000 and dark urine.
Cisplatin & Other Chemotherapy-Induced ATN
Mechanism: - Cisplatin: Nephrotoxicity in S3 proximal tubule; cumulative dose-dependent; up to 35% patients develop AKI - Amphotericin B: Lipid formulations safer; conventional more toxic - Ifosfamide, methotrexate, gemcitabine: Variable toxicity
Prevention: - Pre-chemotherapy hydration - Cisplatin: mannitol diuresis + saline hydration - Monitor baseline Cr, electrolytes, Mg - Avoid NSAIDs, ACE-I during chemo
Hemoglobinuria-Induced ATN (Intravascular Hemolysis)
Mechanism: Hemoglobin precipitates in acidic tubular fluid; same mechanism as myoglobin
Causes: - Massive transfusion reaction - Prosthetic valve hemolysis - Microangiopathic hemolytic anemia (MAHA) - Snake bite envenomation
Diagnostic clue: - Dipstick+ for blood, but RBC count <3 on microscopy - Serum LDH ↑, haptoglobin ↓, indirect bilirubin ↑
Management: Similar to rhabdo — hydration + urine alkalinization
Clinical Decision Tree: Ischemic vs Nephrotoxic ATN
ATN Suspected (muddy brown granular casts on microscopy — diagnostic; FeNa supportive but not required)
│
├─ ISCHEMIC ATN?
│ History: shock, sepsis, major surgery, cardiogenic event
│ Timing: AKI coincides with hypotensive episode
│ Exam: Hypotension (current or recent), cool extremities
│ Labs: Lactate ↑, base deficit, CK normal, urine myoglobin−
│ Prognosis: 60–80% recover; ICU patients have 20–60% mortality (multiorgan failure)
│
└─ NEPHROTOXIC ATN?
History: aminoglycoside, contrast, rhabdo, hemolysis, cisplatin
Timing: AKI 2–7 days post-drug
Exam: Normal BP if volume replete
Labs:
└─ If myoglobin/hemoglobin: Dark urine, dipstick+/RBC−, CK ↑↑, LDH ↑
Prognosis: Better if toxic agent removed early; rhabdo/hemolysis: mortality 10–30%Management Principles (Both Types) — Updated per KDIGO 2026
Acute Phase
- Remove trigger: Stop drug, restore perfusion, treat shock
- Fluid resuscitation: Buffered crystalloids preferred over 0.9% NaCl (KDIGO 2026 Rec 3.1.2, 1B) — except in traumatic brain injury where NS remains appropriate
- Hemodynamic targets:
- MAP ≥65 mmHg (KDIGO 2026 Rec 3.2.1, 2C) — individualize; higher targets not proven beneficial
- No low-dose dopamine for renal protection (KDIGO 2026 Rec 3.2.2, 1A) — unchanged from prior guidelines but frequently violated in practice. Low-dose dopamine does not prevent, treat, or improve outcomes in AKI.
- Avoid secondary hits: No NSAIDs, ACE-I, nephrotoxins, contrast
Volume Management & Diuretics (KDIGO 2026)
KDIGO 2026 formally endorses diuretics in AKI — a shift from prior guideline ambiguity.
- Diuretics for volume overload in AKI (Rec 3.3.1, 2B) — use to treat fluid overload, not to “prevent” AKI or convert oliguric to non-oliguric
- Try diuretics BEFORE extracorporeal fluid removal (Rec 3.3.2, 1C) — give the patient a chance to respond to diuretics before initiating ultrafiltration or RRT for volume alone
- Intermittent IV boluses as initial strategy (PP 3.3.1) — start with bolus dosing before continuous infusion
Furosemide Stress Test (FST) — now guideline-recommended (KDIGO 2026 Rec 2.3.1, 2B–2C): - 1.0 mg/kg IV furosemide (1.5 mg/kg if prior loop diuretic use) - Urine output <200 mL in 2 hours = high risk for severe AKI / need for RRT - Useful for prognostication and triage in AKI Stage 2–3
Diuretic Escalation Pathway (KDIGO 2026 Table 22):
| Step | Strategy | Details |
|---|---|---|
| 1. Assessment | Evaluate volume status | Clinical exam, urine output trend, lung ultrasound |
| 2. Escalation | Increase loop diuretic dose | Double furosemide dose if inadequate response; switch to bumetanide if gut edema |
| 3. Synergism | Add thiazide (sequential nephron blockade) | Metolazone 5–10 mg PO or chlorothiazide 500 mg IV 30 min before loop diuretic |
| 4. Intensify | Continuous infusion | Furosemide 10–40 mg/h after bolus loading dose |
| 5. Failure → RRT | Diuretic resistance | If escalation pathway fails, proceed to extracorporeal fluid removal |
Maintenance Phase
- Monitor K, Ca, PO4, Mg daily
- Renal dosing medications
- Nutrition: 30 kcal/kg/day, 1–1.2 g/kg/day protein (catabolic state)
RRT Timing (KDIGO 2026)
- Deferred (watchful waiting) RRT preferred over early/preemptive initiation (Rec 5.1.1, 1C) — unless life-threatening indications exist (refractory hyperkalemia, severe acidosis, uremic symptoms, refractory volume overload after diuretic escalation)
- Initiate RRT for emergent indications regardless of timing strategy
Recovery Phase
- Watch for postobstructive diuresis if oliguric phase (may occur during recovery)
- Continue supportive care even as Cr improves
- Follow-up: ~50% develop CKD long-term; monitor at 3–6 months
Board-Style Questions
- 35M post-MVA with crush injury to leg; CK 120,000; dark urine; Cr 0.8 → 2.6 in 8h
- Diagnosis: Rhabdomyolysis-induced ATN
- Immediate management: IV NS 1–1.5 L/h until urine clear, NaHCO3 for alkalinization
- 72F post-cardiac angiography; baseline Cr 1.2; now Cr 1.8; muddy brown casts; FeNa 3%
- Diagnosis: Contrast-induced ATN (CA-AKI)
- Prevention for future: Hydration, minimize contrast, hold metformin
- Pt on gentamicin × 10 days for sepsis; AKI onset day 5; FeNa 2.5%; granular casts
- Diagnosis: Aminoglycoside-induced ATN
- Action: Switch to alternative AB, aggressive hydration, consider RRT
Version 2.0 | PA/Medical student level | Updated 2026-03-29 per KDIGO 2026 AKI/AKD Guidelines
References: [1] Schrier RW, Wang W. Acute tubular necrosis. Kidney Int. 2004;66(12). [2] Bellomo R, Ronco C. Acute tubular necrosis: ischemic and nephrotoxic. Crit Care Clin. 2015;31(4). [3] KDIGO 2026 Clinical Practice Guideline for Acute Kidney Injury and Acute Kidney Diseases. Kidney Int Suppl. 2026. [4] Koyner JL, et al. Furosemide stress test and biomarkers for the prediction of AKI severity. JASN. 2015;26(8):2023-2031.