# Acute Tubular Necrosis (ATN): Ischemic & Nephrotoxic Patterns

Key Point

**Updated March 2026 to reflect KDIGO 2026 draft 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 Point

**Key feature of ATN:** Tubular epithelial cells DIE and slough into urine. Diagnosis confirmed by muddy brown casts (hemoglobin- or myoglobin-laden cellular debris).

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## 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

**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.

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## Ischemic ATN: Hypoperfusion Patterns

### **Classic Scenarios**

1.  **Cardiogenic shock** — MI, acute decompensated HF, severe bradycardia
2.  **Septic shock** — vasodilation + hypovolemia
3.  **Major surgery** — especially bypass, vascular surgery
4.  **Massive hemorrhage** — inadequate fluid resuscitation
5.  **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)

Clinical Warning

**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.**

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## 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

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### **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)

Clinical Warning

**Bicarbonate, N-acetylcysteine, theophylline:** NOT proven effective in recent RCTs. Hydration + careful dosing sufficient.

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### **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 assessment:** CK above 5 times the upper limit of normal or above 1,000 IU/L supports rhabdomyolysis, but a CK band does not define kidney injury severity. Assess creatinine trend, urine output, potassium, acid–base status, perfusion, and the cause.

**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:** Treat the cause and assess for compartment syndrome. Give isotonic crystalloid when resuscitation is indicated, with repeated assessment of perfusion, urine output, and lungs. Do not force a urine-output target with continued fluid in persistent anuria or overload. Routine bicarbonate or mannitol is not recommended to prevent AKI; acidic urine alone is not an indication. Monitor CK, creatinine, potassium/ECG, calcium, phosphate, and acid–base status. KRT is for refractory hyperkalemia, severe acidemia, pulmonary edema, or uremic complications. [AAST consensus](https://tsaco.bmj.com/content/7/1/e000836).

Clinical Pearl

**Clinical pearl:** Early recognition and treatment of hypovolemia can reduce risk, but fluids do not guarantee prevention of AKI. CK above 1,000 with dark urine calls for prompt assessment; the fluid prescription depends on perfusion, kidney function, and congestion.

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### **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:** Use the oncology regimen’s saline hydration and electrolyte protocol, with baseline and follow-up creatinine, magnesium, and other electrolytes and repeated volume assessment. Mannitol is an option in selected cisplatin regimens under local protocol, not a requirement for all patients; routine forced diuresis is not established. Review NSAIDs, other nephrotoxins, and medicines that affect perfusion with the treatment team. [eviQ cisplatin prevention guidance](https://www.eviq.org.au/clinical-resources/side-effect-and-toxicity-management/prophylaxis-and-treatment/184-prevention-and-management-of-cisplatin-induced).

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## 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:** Identify and treat the hemolysis cause urgently, support perfusion with reassessment, and manage electrolyte/AKI complications. Routine urinary alkalinization should not be inferred from the pigment mechanism alone.

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## Clinical Decision Tree: Ischemic vs Nephrotoxic ATN

    ATN Suspected (muddy brown granular casts on microscopy support tubular injury in context; 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%

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## Management Principles (Both Types) — Updated per KDIGO 2026 draft

### **Acute Phase**

1.  **Remove trigger:** Stop drug, restore perfusion, treat shock
2.  **Fluid resuscitation:** Buffered crystalloids preferred over 0.9% NaCl (KDIGO 2026 draft Rec 3.1.2, **1B**) — except in traumatic brain injury where NS remains appropriate
3.  **Hemodynamic targets:**
    - **MAP ≥65 mmHg** (KDIGO 2026 draft Rec 3.2.1, **2C**) — individualize; higher targets not proven beneficial
    - **No low-dose dopamine** for renal protection (KDIGO 2026 draft 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.
4.  **Avoid secondary hits:** No NSAIDs, ACE-I, nephrotoxins, contrast

### **Volume Management & Diuretics (KDIGO 2026 draft)**

Key Point

**KDIGO 2026 draft 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 draft 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 draft 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 draft)**

- **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 **recovery-phase 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

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## Related Resources

- Comprehensive ATN Review
- AKI Staging & Differential
- AIN vs ATN Differential
- AKI Workup Protocol
- Prerenal AKI Assessment

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## Board-Style Questions

1.  **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: assess trauma/compartment syndrome, potassium/ECG, perfusion, and kidney function; give volume-guided crystalloid if indicated with repeated lung/urine-output assessment. Do not routinely add bicarbonate for urine alkalinization; escalate for clinical KRT indications.
2.  **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
3.  **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

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*Version 2.0 \| PA/Medical student level \| Updated 2026-03-29 per KDIGO 2026 draft AKI/AKD Guidelines*

**References:** [Kellum et al. Acute kidney injury (2021)](https://pubmed.ncbi.nlm.nih.gov/34267223/); [KDIGO 2012 AKI guideline](https://kdigo.org/wp-content/uploads/2016/10/KDIGO-2012-AKI-Guideline-English.pdf); [Koyner et al. Furosemide Stress Test and Biomarkers for the Prediction of AKI Severity (2015)](https://pubmed.ncbi.nlm.nih.gov/25655065/).


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