Study guide
8.1 KDIGO AKI Classification, Etiologies & Diagnostic Evaluation
ABIM Critical Care Medicine Study Guide Exam
Track your progress as you read
Finished sections get a check mark.
1 Chapter 1: Exam Orientation: Format, Blueprint & Study Strategy
1.1 ABIM Critical Care Medicine Exam: Format, Blueprint & Study Strategy
2 Chapter 2: Acute Respiratory Failure & ARDS
2.1 ARDS Definition, Pathophysiology & Phenotypes 2.2 Lung-Protective Mechanical Ventilation 2.3 Refractory ARDS: Prone Positioning, Neuromuscular Blockade & VV-ECMO
3 Chapter 3: Obstructive Airways, Pulmonary Vascular & Pleural Emergencies
3.1 Severe Asthma & COPD Exacerbations in the ICU 3.2 Massive & Submassive Pulmonary Embolism 3.3 Pulmonary Hypertension & Right Ventricular Failure in the ICU 3.4 Massive Hemoptysis, Complex Pleural Effusions & Thoracic Drainage
4 Chapter 4: Airway, Parenchymal & Nonthrombotic Pulmonary Emergencies
4.1 Airway Management, Upper-Airway Obstruction & Artificial-Airway Complications 4.2 Diffuse Alveolar Hemorrhage, Interstitial Lung Disease, Pulmonary Vasculitis & Noncardiogenic Edema 4.3 Severe Community-Acquired & Viral Pneumonia 4.4 Nonthrombotic Embolism (Air, Fat, Amniotic Fluid, Tumor, Septic) & Acute Chest Syndrome
5 Chapter 5: Mechanical Ventilation Modes, Dyssynchrony & Liberation
5.1 Mechanical Ventilator Modes & Advanced Breath Delivery 5.2 Patient-Ventilator Dyssynchrony & Waveform Analysis 5.3 Ventilator Liberation, Spontaneous Breathing Trials & Post-Extubation Support
6 Chapter 6: Shock Syndromes & Hemodynamic Principles
6.1 Shock Classification, Cellular Pathophysiology & Biomarkers 6.2 Hemodynamic Monitoring: Arterial Lines, CVP & Pulmonary Artery Catheter 6.3 Dynamic Assessment of Fluid Responsiveness & Resuscitation Targets
7 Chapter 7: Vasoactive Therapies & Mechanical Circulatory Support
7.1 Vasopressors & Inotropes: Pharmacology & Clinical Titration 7.2 Temporary Mechanical Circulatory Support: IABP, Impella & TandemHeart 7.3 Venoarterial ECMO (VA-ECMO) & Differential Hypoxemia 7.4 Post-Cardiac Arrest Care, Targeted Temperature Management & Neuroprognostication
8 Chapter 8: Acute Kidney Injury & Renal Replacement Therapy
8.1 KDIGO AKI Classification, Etiologies & Diagnostic Evaluation 8.2 Continuous Renal Replacement Therapy (CRRT) vs Intermittent Hemodialysis 8.3 Indications & Timing for RRT in Critical Illness: AKIKI & STARRT-AKI
9 Chapter 9: Electrolyte, Acid-Base & Endocrine Emergencies
9.1 Severe Dysnatremias: Hyponatremia, Hypernatremia & Osmotic Demyelination 9.2 Potassium, Calcium, Magnesium & Phosphate Disorders 9.3 Complex & Mixed Acid-Base Disorders: Boston vs Copenhagen vs Stewart 9.4 Endocrine Crises: DKA, HHS, Thyroid Storm, Myxedema Coma & Adrenal Crisis
10 Chapter 10: Glycemic Control, Hyperammonemia, Pituitary & Adrenal Disorders
10.1 ICU Glycemic Control & Severe Hypoglycemia 10.2 Non-Hepatic Hyperammonemia & Metabolic Encephalopathy 10.3 Pituitary Emergencies, Adrenal Excess & Critical Illness-Related Corticosteroid Insufficiency
11 Chapter 11: Acute Coronary Syndromes & Cardiac Arrhythmias
11.1 Acute Coronary Syndromes & Mechanical Complications of Myocardial Infarction 11.2 Critical Tachyarrhythmias: Atrial Fibrillation with RVR, VT & VF 11.3 Critical Bradyarrhythmias, Conduction Blocks & Temporary Cardiac Pacing
12 Chapter 12: Acute Heart Failure, Valvular & Pericardial Emergencies
12.1 Acute Decompensated Heart Failure & Cardiogenic Pulmonary Edema 12.2 Acute Valvular Regurgitation & Prosthetic Valve Emergencies 12.3 Pericardial Tamponade & Acute Aortic Syndromes
13 Chapter 13: Myocardial Disease, Hypertensive & Cardiac Device Emergencies
13.1 Fulminant Myocarditis, Stress & Peripartum Cardiomyopathy, and Hypertrophic Cardiomyopathy 13.2 Hypertensive Emergencies & Ruptured Abdominal Aortic Aneurysm 13.3 Implanted Cardiac Devices, LVAD Emergencies & Electrical Storm
14 Chapter 14: Sepsis & Systemic Infectious Diseases
14.1 Sepsis-3 Definitions & Surviving Sepsis Campaign 2026 Guidelines 14.2 Refractory Septic Shock & Adjunctive Therapies 14.3 Multidrug-Resistant Pathogens & Antimicrobial Stewardship in the ICU
15 Chapter 15: ICU-Acquired & Severe Focal Infections
15.1 Hospital-Acquired & Ventilator-Associated Pneumonia 15.2 Central Line-Associated Bloodstream Infections & Catheter-Associated UTIs 15.3 Intra-Abdominal Sepsis, Necrotizing Soft Tissue Infections & CNS Infections
16 Chapter 16: Focal, Special-Host & Systemic Infections
16.1 CNS Infections Beyond Bacterial Meningitis: Encephalitis, Brain Abscess & Epidural Abscess 16.2 Infections in the Immunocompromised Host, Tropical & Bioterror Infections 16.3 Infective Endocarditis, Device Infections & Other Focal Cardiovascular Infections 16.4 Antimicrobial Pharmacokinetics, Allergy, Toxicity & Infection Prevention 16.5 Gastrointestinal & Hepatobiliary Infections: Fulminant C. difficile, Typhlitis, CMV Colitis & Abscesses
17 Chapter 17: Neurocritical Care & Coma
17.1 Coma, Altered Mental Status & Acute Delirium in the ICU 17.2 Status Epilepticus & Non-Convulsive Status Epilepticus 17.3 Acute Ischemic Stroke & Intracranial Hemorrhage
18 Chapter 18: Cerebrovascular Syndromes, Encephalopathy & Withdrawal States
18.1 Cerebral Venous Sinus Thrombosis, PRES & Reversible Cerebral Vasoconstriction Syndrome 18.2 Alcohol & Sedative Withdrawal, Wernicke Encephalopathy & Drug-Induced Encephalopathy
19 Chapter 19: Intracranial Hypertension & Neuromuscular Weakness
19.1 Intracranial Pressure Monitoring, Cerebral Perfusion & Tiered ICP Management 19.2 Acute Neuromuscular Respiratory Failure, ICU-Acquired Weakness & Brain Death Determination
20 Chapter 20: Traumatic Brain Injury, Spinal Cord Injury & Toxin-Mediated Paralysis
20.1 Traumatic Brain Injury Management & Paroxysmal Sympathetic Hyperactivity 20.2 Acute Spinal Cord Injury, Neurogenic Shock & Autonomic Dysreflexia 20.3 Toxin-Mediated Paralysis & Neuromuscular Crises: Botulism, Tetanus, Tick Paralysis & Myasthenic Crisis
21 Chapter 21: Gastrointestinal, Hepatic & Abdominal Emergencies
21.1 Acute Upper & Lower Gastrointestinal Bleeding in the ICU 21.2 Acute Liver Failure & Severe Acute Pancreatitis 21.3 Abdominal Compartment Syndrome & Acute Mesenteric Ischemia
22 Chapter 22: Luminal GI Emergencies & Complications of Cirrhosis
22.1 Esophageal, Gastric & Bowel Emergencies: Perforation, Obstruction, Ogilvie Syndrome & Colonic Ischemia 22.2 Complications of Cirrhosis: Alcohol-Associated Hepatitis, Hepatorenal Syndrome & Pulmonary Vascular Syndromes
23 Chapter 23: Trauma, Burns & Perioperative Critical Care
23.1 Polytrauma Resuscitation, Damage Control & Massive Transfusion 23.2 Severe Burn Injury, Inhalation Injury & Environmental Emergencies 23.3 Perioperative Critical Care for High-Risk Surgical Patients
24 Chapter 24: Thoracic Trauma, Environmental Injury & Obstetric Critical Care
24.1 Thoracic Trauma: Flail Chest, Pulmonary Contusion, Great Vessel & Tracheobronchial Injury 24.2 Environmental Injury: Drowning, Diving, Altitude, Heat, Cold, Electrical & Radiation 24.3 Obstetric & Genitourinary Emergencies and the Physiology of Pregnancy
25 Chapter 25: Transplantation, Donor Management & Post-Cardiac Surgery Care
25.1 Solid Organ Transplant Recipients & Deceased Donor Management 25.2 Post-Cardiac Surgery Critical Care: Bleeding, Tamponade, Low Output & Vasoplegia
26 Chapter 26: Hematologic Emergencies, Toxicology, Critical Ultrasound & ICU Liberation
26.1 Hematologic Emergencies in the ICU: HIT, DIC, TMA & Anticoagulant Reversal 26.2 Critical Care Toxicology, Toxidromes & Specific Antidotes 26.3 Critical Care Point-of-Care Ultrasound (POCUS) 26.4 ICU Liberation, Analgosedation, Goals of Care & Critical Care Ethics
27 Chapter 27: Transfusion Medicine, Hematologic & Oncologic Disorders
27.1 Transfusion Medicine: Products, Thresholds, Reactions & Apheresis 27.2 Red and White Cell Disorders, Hemoglobinopathies, Factor Deficiencies & Hypercoagulable States 27.3 Oncologic Emergencies, Hematopoietic Cell Transplant Complications & Cellular Therapy Toxicity
28 Chapter 28: Clinical Pharmacology, Adverse Drug Reactions & Specific Poisonings
28.1 Pharmacokinetic Principles, Dose Adjustment & Drug Interactions in Critical Illness 28.2 Adverse Drug Reactions: Anaphylaxis, Severe Cutaneous Reactions & Hyperthermic Syndromes 28.3 Specific Poisonings: Sodium-Channel Blockers, Lithium, Valproate, Theophylline, Iron, Sulfonylureas & Methemoglobinemia
29 Chapter 29: Ethics, Research Methods, Administration & Abdominal Ultrasound
29.1 Advance Care Planning, Nonbeneficial Treatment, Informed Consent & Confidentiality 29.2 Critical Care Research Methods, Statistics, IRB Oversight & Quality Improvement 29.3 Abdominal, Vascular & Procedural Point-of-Care Ultrasound
8.1 KDIGO AKI Classification, Etiologies & Diagnostic Evaluation
Key Takeaways Explain This Topic AI
The Kidney Disease: Improving Global Outcomes (KDIGO) consensus staging categorizes acute kidney injury into Stages 1, 2, and 3 based on absolute or relative rises in serum creatinine or sustained reductions in hourly urine output.
Differentiating pre-renal azotemia from acute tubular necrosis (ATN) relies on urinary indices: fractional excretion of sodium ( F E N a < 1 % FENa < 1\% FEN a < 1% ) indicates pre-renal azotemia, but is falsely elevated by loop diuretics and falsely low in contrast nephropathy, acute glomerulonephritis, and rhabdomyolysis.
Fractional excretion of urea ( F E U r e a < 35 % FEUrea < 35\% FE U re a < 35% ) bypasses loop diuretic confounding because urea is reabsorbed in the proximal nephron, serving as a reliable index of pre-renal physiology in patients receiving active diuretic therapy.
Urine sediment microscopy is an indispensable bedside diagnostic tool: muddy brown granular casts confirm ATN, red blood cell casts and dysmorphic acanthocytes establish glomerulonephritis, and white blood cell casts with pyuria indicate acute interstitial nephritis.
The Furosemide Stress Test (FST) evaluates functional tubular reserve in euvolemic Stage 1 or 2 AKI; a 2-hour post-challenge urine output < 200 mL < 200\text{ mL} < 200 mL strongly predicts progression to Stage 3 AKI and the eventual need for renal replacement therapy.
Last updated: October 2026
Diagnostic Framework and KDIGO Staging
Acute kidney injury (AKI) affects over 50% of critically ill intensive care unit (ICU) patients and represents an independent predictor of short- and long-term mortality, cardiovascular events, prolonged mechanical ventilation, and transition to end-stage kidney disease (ESKD). Historical diagnostic frameworks, including the RIFLE (Risk, Injury, Failure, Loss of kidney function, End-stage kidney disease) criteria (2004) and the Acute Kidney Injury Network (AKIN) criteria (2007), standardized the definition of acute renal dysfunction. In 2012, the Kidney Disease: Improving Global Outcomes (KDIGO) consensus guideline synthesized these systems into a unified, globally adopted definition and staging framework.
KDIGO Diagnostic Definition
Under KDIGO criteria, AKI is definitively established by the presence of any one of the following:
An absolute increase in serum creatinine (SCr) by ≥ 0.3 mg/dL \ge 0.3\text{ mg/dL} ≥ 0.3 mg/dL ( ≥ 26.5 μ mol/L \ge 26.5\ \mu\text{mol/L} ≥ 26.5 μ mol/L ) within a 48-hour window; or
A documented or presumed increase in SCr to ≥ 1.5 × \ge 1.5\times ≥ 1.5 × baseline within the preceding 7 days; or
An hourly urine output (UOP) < 0.5 mL/kg/h < 0.5\text{ mL/kg/h} < 0.5 mL/kg/h for at least 6 consecutive hours.
KDIGO Severity Staging
Once AKI is diagnosed, severity is stratified across three progressive stages based on the maximum degree of serum creatinine elevation or the duration and severity of oliguria. When creatinine and urine output parameters yield discordant stages, the higher (more severe) stage dictates classification:
KDIGO Stage Serum Creatinine (SCr) Criteria Urine Output (UOP) Criteria
Stage 1 Increase ≥ 0.3 mg/dL \ge 0.3\text{ mg/dL} ≥ 0.3 mg/dL within 48 hr OR SCr 1.5 to 1.9 × 1.5\text{ to }1.9\times 1.5 to 1.9 × baseline within 7 days < 0.5 mL/kg/h < 0.5\text{ mL/kg/h} < 0.5 mL/kg/h for 6 to 12 hours 6\text{ to }12\text{ hours} 6 to 12 hours
Stage 2 SCr 2.0 to 2.9 × 2.0\text{ to }2.9\times 2.0 to 2.9 × baseline < 0.5 mL/kg/h < 0.5\text{ mL/kg/h} < 0.5 mL/kg/h for ≥ 12 hours \ge 12\text{ hours} ≥ 12 hours
Stage 3 SCr ≥ 3.0 × \ge 3.0\times ≥ 3.0 × baseline OR SCr ≥ 4.0 mg/dL \ge 4.0\text{ mg/dL} ≥ 4.0 mg/dL ( ≥ 353.6 μ mol/L \ge 353.6\ \mu\text{mol/L} ≥ 353.6 μ mol/L ) OR Initiation of renal replacement therapy (RRT) < 0.3 mL/kg/h < 0.3\text{ mL/kg/h} < 0.3 mL/kg/h for ≥ 24 hours \ge 24\text{ hours} ≥ 24 hours OR Anuria for ≥ 12 hours \ge 12\text{ hours} ≥ 12 hours
Note
In pediatric patients ( < 18 years < 18\text{ years} < 18 years ), KDIGO Stage 3 also includes an acute reduction in estimated glomerular filtration rate (eGFR) to < 35 mL/min / 1.73 m 2 < 35\text{ mL/min}/1.73\text{ m}^2 < 35 mL/min /1.73 m 2 .
Clinical Nuances of Biomarker Kinetics in the ICU
Serum creatinine is an imperfect surrogate of acute changes in glomerular filtration rate. Creatinine is a steady-state kinetic marker: following an abrupt, complete cessation of bilateral renal filtration, serum creatinine rises at an absolute rate of approximately 1.0 to 1.5 mg/dL per day 1.0\text{ to }1.5\text{ mg/dL per day} 1.0 to 1.5 mg/dL per day . Consequently, a patient with fulminant septic shock and bilateral renal cortical necrosis may maintain a normal serum creatinine for the initial 24 to 48 hours. Conversely, hourly urine output changes instantaneously in response to hemodynamic collapse, sympathetic activation, and neurohormonal tone, making oliguria the earlier and more sensitive marker of injury in monitored ICU patients.
Furthermore, aggressive crystalloid fluid resuscitation induces significant fluid overload and hemodilution , which artificially depresses measured serum creatinine concentrations. Clinicians should calculate adjusted serum creatinine using cumulative fluid balance to avoid under-recognizing AKI:
Adjusted SCr = Measured SCr × ( 1 + Cumulative Net Fluid Balance (L) Total Body Water (L) ) \text{Adjusted SCr} = \text{Measured SCr} \times \left(1 + \frac{\text{Cumulative Net Fluid Balance (L)}}{\text{Total Body Water (L)}}\right) Adjusted SCr = Measured SCr × ( 1 + Total Body Water (L) Cumulative Net Fluid Balance (L) )
Where total body water is estimated as 0.6 × weight (kg) 0.6\times\text{weight (kg)} 0.6 × weight (kg) in non-elderly males and 0.5 × weight (kg) 0.5\times\text{weight (kg)} 0.5 × weight (kg) in non-elderly females.
Etiological Taxonomy: Pre-Renal, Intrinsic, and Post-Renal
Determining the root cause of acute renal failure dictates therapeutic management. Etiologies are traditionally partitioned into three broad pathophysiological compartments:
Loading diagram...
1. Pre-Renal Azotemia (Functional Hypoperfusion)
Pre-renal azotemia arises when renal parenchymal integrity is entirely preserved, but diminished renal blood flow compromises the glomerular filtration pressure gradient. Causes fall into three functional subsets:
True Intravascular Hypovolemia : Massive blood loss, gastrointestinal losses (severe vomiting, secretory diarrhea), insensible cutaneous losses (severe burns, toxic epidermal necrolysis), or profound unreplaced urinary losses (diabetic ketoacidosis, hyperosmolar hyperglycemic state).
Decreased Effective Circulating Arterial Volume (ECAV) : Systemic arterial underfilling despite total body fluid overload, seen characteristically in acute decompensated heart failure with low cardiac index (cardiorenal syndrome Type 1), end-stage hepatic cirrhosis with splanchnic arterial vasodilation (hepatorenal syndrome Type 1 / HRS-AKI), and distributive septic vasodilation.
Disrupted Intrarenal Autoregulation : Normal autoregulatory mechanisms maintain glomerular capillary hydraulic pressure ( P gc P_{\text{gc}} P gc ) across mean arterial pressures ranging from 65 to 110 mmHg via afferent arteriolar vasodilation (mediated by local prostaglandins and nitric oxide) and efferent arteriolar vasoconstriction (mediated by angiotensin II):
Nonsteroidal Anti-inflammatory Drugs (NSAIDs) : Inhibit cyclooxygenase-1 and -2, abolishing vasodilatory prostaglandin ( P G E 2 , P G I 2 PGE_2, PGI_2 PG E 2 , PG I 2 ) synthesis at the afferent arteriole. The afferent arteriole constricts, precipitating an acute fall in P gc P_{\text{gc}} P gc .
Angiotensin-Converting Enzyme Inhibitors (ACEis) and Angiotensin Receptor Blockers (ARBs) : Inhibit angiotensin II-mediated efferent arteriolar vasoconstriction. The efferent arteriole dilates, depressurizing the glomerular capillary tuft and dropping GFR.
Calcineurin Inhibitors (Tacrolimus, Cyclosporine) : Induce severe, dose-dependent afferent arteriolar vasoconstriction.
2. Intrinsic Acute Kidney Injury (Parenchymal Damage)
Intrinsic AKI involves structural and cellular injury localized to the renal tubules, interstitium, glomeruli, or renal microvasculature:
Acute Tubular Necrosis (ATN) : Accounts for > 80 % > 80\% > 80% of intrinsic AKI cases in the ICU.
Ischemic ATN : Follows prolonged, uncorrected pre-renal hypoperfusion, severe septic shock, sustained intraoperative hypotension, or cardiopulmonary bypass. The medullary thick ascending limb of the loop of Henle and the S3 segment of the proximal tubule suffer marked hypoxic necrosis due to baseline high metabolic activity and tenuous medullary oxygen delivery.
Nephrotoxic ATN : Direct cellular injury from exogenous toxins (aminoglycosides, IV iodinated radiocontrast agents, vancomycin, amphotericin B, cisplatin, polymyxins) or endogenous pigment nephropathy (myoglobin in rhabdomyolysis, hemoglobin in massive intravascular hemolysis, or Bence-Jones light-chain cast nephropathy in multiple myeloma).
Acute Interstitial Nephritis (AIN) : An immune-mediated hypersensitivity reaction confined to the renal tubulointerstitium. Classically triggered by medications ( > 75 % > 75\% > 75% of cases), notably beta-lactam antibiotics, cephalosporins, fluoroquinolones, proton pump inhibitors (PPIs), NSAIDs, and sulfonamides. Less commonly driven by autoimmune conditions (sarcoidosis, systemic lupus erythematosus [SLE], Sjögren's syndrome) or systemic infections (Legionella, CMV, EBV). The traditional clinical triad of fever, maculopapular rash, and peripheral eosinophilia is present in < 10 to 15 % < 10\text{ to }15\% < 10 to 15% of biopsy-confirmed cases; microscopic pyuria and WBC casts are far more reliable.
Acute Glomerulonephritis (GN) and Systemic Vasculitis : Rapidly progressive glomerulonephritis (RPGN) characterized by immune crescent formation. Subdivided into: Pauci-immune ANCA-associated vasculitis (granulomatosis with polyangiitis, microscopic polyangiitis); Anti-glomerular basement membrane (anti-GBM / Goodpasture) disease; and Immune complex GN (lupus nephritis, post-streptococcal GN, cryoglobulinemia, IgA nephropathy).
Renal Microvascular Disease : Atheroembolic renal disease (cholesterol crystal embolization following endovascular or angiographic manipulation, presenting with livedo reticularis, blue toe syndrome, transient eosinophilia, and hypocomplementemia), thrombotic microangiopathy (TMA / TTP / atypical HUS), and acute renal cortical necrosis.
3. Post-Renal (Obstructive) Nephropathy
Mechanical impedance to urine outflow, which increases intra-tubular hydrostatic pressure, counteracts glomerular filtration, and incites medullary vasoconstriction. Obstruction may occur at the urethral/prostatic level (benign prostatic hyperplasia, obstructive Foley catheter, neurogenic bladder) or bilaterally at the ureteral level (retroperitoneal fibrosis, pelvic or retroperitoneal malignancies, bilateral nephrolithiasis, or unilateral obstruction in a solitary functioning kidney).
Diagnostic Evaluation: Urine Microscopy and Biomarkers
A disciplined diagnostic sequence distinguishes pre-renal physiology from intrinsic tubular injury, guiding fluid therapy versus dialytic preparation.
Urinalysis and Sediment Microscopy
Bedside urine sediment examination performed by the intensivist represents a non-invasive optical biopsy of the kidney:
Loading diagram...
Muddy Brown Granular Casts : Sloughed, degenerate tubular epithelial cells fused with Tamm-Horsfall mucoprotein. Pathognomonic for ischemic or nephrotoxic ATN.
Renal Tubular Epithelial (RTE) Cells and RTE Casts : Indicate direct, active necrotizing tubular cell death.
Red Blood Cell (RBC) Casts and Acanthocytes (Dysmorphic RBCs) : Pathognomonic for glomerular rupture and active proliferative glomerulonephritis or small-vessel vasculitis.
White Blood Cell (WBC) Casts and Pyuria : In the absence of urinary tract infection, diagnostic of interstitial inflammation (acute interstitial nephritis).
Broad, Waxy Casts : Reflect broad-caliber tubular dilation and stasis, pathognomonic for chronic kidney disease (CKD) rather than isolated hyperacute injury.
Urine Dipstick Discrepancy in Rhabdomyolysis : The orthotolidine reagent on the urine test strip cross-reacts with the heme moiety of both myoglobin and hemoglobin. When the dipstick shows 3 + to 4 + 3+\text{ to }4+ 3 + to 4 + blood but microscopic sediment examination reveals zero to rare intact red blood cells, pigment nephropathy (rhabdomyolysis or intravascular hemolysis) is confirmed.
- Fractional Excretion Indices: F E N a FENa FEN a vs. F E U r e a FEUrea FE U re a
- 1. Fractional Excretion of Sodium ( F E N a FENa FEN a )
The F E N a FENa FEN a quantifies the percentage of sodium filtered by the glomeruli that escapes tubular reabsorption and appears in the urine:
F E N a = U Na × P Cr P Na × U Cr × 100 % FENa = \frac{U_{\text{Na}} \times P_{\text{Cr}}}{P_{\text{Na}} \times U_{\text{Cr}}} \times 100\% FEN a = P Na × U Cr U Na × P Cr × 100%
In pre-renal azotemia , intact renal tubules maximally reabsorb sodium and water under intense aldosterone and angiotensin II stimulation: F E N a < 1 % FENa < 1\% FEN a < 1% (often < 0.1 % < 0.1\% < 0.1% ), and urine sodium concentration ( U Na U_{\text{Na}} U Na ) is typically < 20 mEq/L < 20\text{ mEq/L} < 20 mEq/L .
In established ATN , damaged tubular epithelial cells lose their active Na + / K + \text{Na}^+/\text{K}^+ Na + / K + -ATPase basolateral polarity, causing obligatory natriuresis: F E N a > 2 % FENa > 2\% FEN a > 2% and U Na > 40 mEq/L U_{\text{Na}} > 40\text{ mEq/L} U Na > 40 mEq/L .
Warning
Critical Confounders of F E N a FENa FEN a :
Diuretic Therapy : Loop diuretics (e.g., furosemide, bumetanide) directly block the Na + − K + − 2 Cl − \text{Na}^+-\text{K}^+-2\text{Cl}^- Na + − K + − 2 Cl − co-transporter in the thick ascending limb of Henle, forcing urinary sodium excretion. A patient with severe pre-renal volume contraction who received furosemide will frequently demonstrate an elevated F E N a > 2 % FENa > 2\% FEN a > 2% , falsely mimicking ATN.
Low F E N a FENa FEN a ( < 1 % < 1\% < 1% ) in Intrinsic Disease : Certain intrinsic renal etiologies characteristically maintain a F E N a < 1 % FENa < 1\% FEN a < 1% due to preserved tubular transport coupled with profound pre-glomerular or intrarenal vasoconstriction. These include:
- Contrast-induced nephropathy (intense afferent arteriolar vasospasm);
- Early rhabdomyolysis / myoglobinuric renal failure;
- Acute proliferative glomerulonephritis (tubules intact, filtration slit surface area plummeted);
- Acute abdominal compartment syndrome (elevated renal venous pressure);
Hepatorenal syndrome (splanchnic steal and intense intrarenal cortical vasoconstriction).
2. Fractional Excretion of Urea ( F E U r e a FEUrea FE U re a )
Urea reabsorption occurs passively in the proximal convoluted tubule and inner medullary collecting duct along with water and sodium, independent of the thick ascending limb. Loop and thiazide diuretics do not directly inhibit proximal tubular urea reabsorption. Therefore, the fractional excretion of urea ( F E U r e a FEUrea FE U re a ) is the index of choice when a patient has received diuretics within the previous 24 to 48 hours:
F E U r e a = U Urea × P Cr P Urea × U Cr × 100 % FEUrea = \frac{U_{\text{Urea}} \times P_{\text{Cr}}}{P_{\text{Urea}} \times U_{\text{Cr}}} \times 100\% FE U re a = P Urea × U Cr U Urea × P Cr × 100%
F E U r e a < 35 % FEUrea < 35\% FE U re a < 35% : Establishes pre-renal azotemia , even in the presence of active loop diuresis.
F E U r e a > 50 % FEUrea > 50\% FE U re a > 50% : Diagnostic of acute tubular necrosis .
Comprehensive Comparison of Diagnostic Urinary Indices
Diagnostic Parameter Pre-Renal Azotemia Acute Tubular Necrosis (ATN) Acute Interstitial Nephritis (AIN)
BUN / Serum Creatinine Ratio > 20 : 1 > 20:1 > 20 : 1 (avid urea reabsorption) 10 : 1 to 15 : 1 10:1\text{ to }15:1 10 : 1 to 15 : 1 (loss of reabsorption) 10 : 1 to 15 : 1 10:1\text{ to }15:1 10 : 1 to 15 : 1
Urine Specific Gravity > 1.020 > 1.020 > 1.020 ∼ 1.010 \sim 1.010 ∼ 1.010 (isosthenuric) Variable ( ∼ 1.010 \sim 1.010 ∼ 1.010 )
Urine Osmolality ( U Osm U_{\text{Osm}} U Osm ) > 500 mOsm/kg > 500\text{ mOsm/kg} > 500 mOsm/kg < 350 mOsm/kg < 350\text{ mOsm/kg} < 350 mOsm/kg (diluting/concentrating lost) < 400 mOsm/kg < 400\text{ mOsm/kg} < 400 mOsm/kg
Urine Sodium ( U Na U_{\text{Na}} U Na ) < 20 mEq/L < 20\text{ mEq/L} < 20 mEq/L > 40 mEq/L > 40\text{ mEq/L} > 40 mEq/L Variable ( > 20 mEq/L > 20\text{ mEq/L} > 20 mEq/L )
Fractional Excretion of Sodium ( F E N a FENa FEN a ) < 1 % < 1\% < 1% (without diuretics) > 2 % > 2\% > 2% Variable ( > 1 % > 1\% > 1% )
Fractional Excretion of Urea ( F E U r e a FEUrea FE U re a ) < 35 % < 35\% < 35% (valid with diuretics) > 50 % > 50\% > 50% Variable ( > 50 % > 50\% > 50% )
Urine Sediment Microscopy Normal, hyaline casts Muddy brown granular casts, RTE cells WBC casts, pyuria, eosinophils
- Renal Ultrasonography and Dynamic Assessment
- Bedside Renal Ultrasonography
Ultrasound should be performed promptly in unexplained AKI to evaluate three critical anatomical domains:
Hydronephrosis (Obstruction) : Dilation of the renal pelvis and calyces establishes post-renal obstruction. However, false-negative ultrasounds (absence of pelvic dilation despite true obstruction) occur in:
Early acute obstruction ( < 24 to 48 hours < 24\text{ to }48\text{ hours} < 24 to 48 hours before calyces dilate);
Severe intravascular volume depletion / hypovolemia (insufficient urinary volume to distend the collecting system);
Retroperitoneal fibrosis or retroperitoneal tumor encasement physically encasing the ureters and preventing radial expansion.
Renal Cortical Echogenicity : Normal renal parenchyma is hypoechoic relative to normal liver or spleen parenchyma. Increased parenchymal echogenicity (equal to or greater than liver) indicates medical renal parenchymal disease (diffuse intrinsic injury or glomerulonephritis).
Renal Dimensions : Normal bipolar renal length is 10 to 12 cm 10\text{ to }12\text{ cm} 10 to 12 cm . Small, contracted kidneys ( < 9 cm < 9\text{ cm} < 9 cm ) with cortical thinning ( < 1 cm < 1\text{ cm} < 1 cm ) identify pre-existing chronic kidney disease . Conversely, enlarged, edematous kidneys may be seen in acute glomerulonephritis, severe AIN, amyloidosis, HIV nephropathy, and acute renal vein thrombosis.
The Furosemide Stress Test (FST)
The Furosemide Stress Test is a validated, dynamic bedside challenge of functional medullary tubular integrity in non-anuric, volume-resuscitated critically ill patients with KDIGO Stage 1 or 2 AKI.
Loading diagram...
Physiological Basis : Furosemide is highly protein-bound ( > 95 % > 95\% > 95% ) and enters the tubular lumen almost exclusively via active transport by organic acid transporters (OAT-1, OAT-3) in the proximal convoluted tubule. Once inside the lumen, it must travel down to the thick ascending limb of Henle and bind the Na + − K + − 2 Cl − \text{Na}^+-\text{K}^+-2\text{Cl}^- Na + − K + − 2 Cl − co-transporter to block sodium reabsorption and trigger natriuretic diuresis. A brisk diuretic response demonstrates preserved proximal secretion, patent tubular lumens, and functional thick ascending limb epithelium.
Protocol :
Confirm that the patient is strictly euvolemic or fluid replete (never perform an FST in a hypovolemic patient);
Administer an intravenous bolus of furosemide: 1.0 mg/kg 1.0\text{ mg/kg} 1.0 mg/kg for loop-diuretic naive patients, or 1.5 mg/kg 1.5\text{ mg/kg} 1.5 mg/kg for patients with prior chronic loop diuretic exposure;
Replace hourly urinary output milliliter-for-milliliter with balanced crystalloid (e.g., Plasma-Lyte or Lactated Ringer's) for the duration of the test if the patient is not hypervolemic, to avoid inducing hypovolemia.
Interpretation : A cumulative urine output < 200 mL < 200\text{ mL} < 200 mL over the first 2 hours post-administration constitutes a failed test. It predicts progression to KDIGO Stage 3 AKI and the need for renal replacement therapy with a sensitivity of about 87 % 87\% 87% and specificity of about 84 % 84\% 84% (Chawla et al., 2013), and it compared favorably with static novel biomarkers (such as urinary NGAL, KIM-1, or TIMP-2 × \times × IGFBP7).