Acute Kidney Injury
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Acute kidney injury (AKI) is an abrupt decline in glomerular filtration rate (GFR) occurring over hours to days, characterized by rising serum creatinine and/or oliguria, with potential for partial or complete recovery of renal function. AKI represents one of the most common complications in hospitalized patients, affecting 5–7% of general ward admissions and up to 50% of intensive care unit (ICU) patients, with in-hospital mortality rates of 20–30% depending on severity and underlying etiology. The condition disproportionately affects elderly patients, those with pre-existing chronic kidney disease (CKD), and individuals with comorbidities such as diabetes mellitus and heart failure. Understanding the differential diagnosis and immediate pathophysiologic mechanisms of AKI is critical for clinicians because rapid identification and treatment of reversible causes can prevent progression to end-stage renal disease (ESRD) and reduce morbidity and mortality. AKI serves as a major USMLE board topic encompassing nephrology fundamentals, acute physiology, pharmacology, and critical care management.
AKI represents a spectrum of acute renal dysfunction resulting from multiple overlapping pathophysiologic mechanisms. The kidney's exquisite sensitivity to ischemic injury and nephrotoxic insults, combined with its high metabolic demand and critical role in fluid-electrolyte homeostasis, makes it particularly vulnerable to acute injury. Understanding the compartmentalization of AKI into prerenal, intrinsic renal, and postrenal categories provides a framework for mechanistic understanding and targeted intervention.
Prerenal AKI: Renal Hypoperfusion Mechanisms
Prerenal AKI results from reduced renal perfusion pressure without primary structural damage to the kidney. The fundamental mechanism involves inadequate delivery of blood to the glomerulus, which triggers activation of compensatory mechanisms that, when overwhelmed or inappropriately activated, contribute to further decline in GFR. In early prerenal disease, the afferent arteriole undergoes vasoconstriction mediated by increased sympathetic nervous system activity and activation of the renin-angiotensin-aldosterone system (RAAS). Angiotensin II preferentially constricts the efferent arteriole, thereby maintaining intraglomerular pressure and GFR despite reduced renal blood flow—this mechanism is fundamental to understanding why ACE inhibitors and angiotensin receptor blockers can precipitate AKI in volume-depleted states. When renal perfusion pressure falls below the lower limit of autoregulation (typically 60–80 mmHg mean arterial pressure), this protective mechanism fails. Additionally, prerenal hypoperfusion triggers increased renal tubular sodium reabsorption (mediated by aldosterone and sympathetic activation), resulting in oliguria with fractional excretion of sodium (FENa) <1% and urine osmolality >500 mOsm/kg—hallmarks of prerenal physiology. Lactic acidosis from tissue hypoperfusion and activation of hypoxia-inducible factors further contribute to tubular dysfunction.
Intrinsic Renal AKI: Acute Tubular Necrosis (ATN)
Acute tubular necrosis is the most common form of intrinsic AKI and reflects direct cellular injury to tubular epithelial cells. Ischemic ATN (accounting for ~50% of hospital-acquired AKI) occurs when severe or prolonged renal hypoperfusion exceeds the kidney's autoregulatory capacity, leading to mitochondrial dysfunction and ATP depletion. Proximal tubule cells, which are metabolically active and responsible for active sodium reabsorption, are particularly vulnerable. ATP depletion causes failure of the Na+/K+-ATPase pump, leading to intracellular sodium and calcium accumulation. Calcium influx activates proteases and phospholipases, resulting in cytoskeletal disruption, loss of epithelial cell polarity, and necrotic cell death. The damaged tubular epithelium sloughs into the lumen, creating casts and cellular debris ("muddy brown" granular casts on urinalysis) that further obstruct tubular flow. Loss of tight junctions allows back-leak of filtered ultrafiltrate across the damaged epithelium into the peritubular interstitium, effectively reducing net GFR. Additionally, tubular epithelial injury triggers release of inflammatory mediators (TNF-α, IL-6, IL-8) and generation of reactive oxygen species (ROS), propagating injury even after the initial ischemic insult resolves.
Nephrotoxic ATN results from direct chemical or immunologic injury to tubular cells. Contrast-induced AKI involves osmotic injury and ROS generation when high-osmolality contrast agents are filtered at the glomerulus and concentrated in the tubular lumen, drawing water out of tubular cells and causing cellular dehydration and death. The renal medulla, which relies on anaerobic metabolism and is already hypoxic, is particularly susceptible. Aminoglycosides undergo glomerular filtration and are reabsorbed via megalin-mediated endocytosis in proximal tubule cells; within lysosomes, they generate ROS and cause phospholipid accumulation, leading to cell death with a latency of 5–7 days after drug initiation. Rhabdomyolysis releases massive quantities of myoglobin, which is filtered at the glomerulus and precipitates in acidic tubular fluid, forming casts that obstruct flow; additionally, myoglobin generates ROS and directly damages tubular epithelial cells. Tumor lysis syndrome releases intracellular contents (potassium, phosphate, uric acid, nucleic acids); uric acid and light chains precipitate in renal tubules, causing obstruction and direct epithelial injury.
Intrinsic Renal AKI: Acute Glomerulonephritis and Vasculitis
Glomerular injury mechanisms include immune complex deposition (post-infectious GN, lupus nephritis), anti-GBM disease (linear IgG deposition on basement membrane), and ANCA-associated vasculitis (MPO-ANCA or PR3-ANCA binding to neutrophil granule antigens, triggering degranulation and glomerular destruction). Rapid proliferative lesions with crescent formation and fibrinoid necrosis rapidly reduce the glomerular filtration surface area. Infiltrating macrophages and T cells amplify injury through release of TNF-α, IL-1, and proteases.
Intrinsic Renal AKI: Acute Interstitial Nephritis (AIN)
Drug-induced AIN (accounting for ~10–15% of AKI) involves a T cell-mediated hypersensitivity reaction to the offending drug or its metabolite. Beta-lactam antibiotics, NSAIDs, and proton pump inhibitors are the most common culprits. The drug (often acting as a hapten) binds to tubular epithelial cells or basement membrane proteins, triggering recognition by T cells and activation of both CD8+ cytotoxic T lymphocytes and CD4+ helper T cells. Infiltration of T cells and macrophages into the interstitium causes tubular epithelial cell injury and inflammation. Eosinophils may also infiltrate, particularly with beta-lactams, leading to eosinophiluria. Unlike prerenal disease, the tubular epithelium is damaged and cannot concentrate urine appropriately, so FENa is typically >2%, and urine osmolality is low.
Postrenal AKI: Obstructive Mechanisms
Postrenal AKI results from obstruction to urine flow anywhere from the collecting ducts to the urethral meatus. Urine backs up into the collecting system, increasing intratubular pressure. This elevated pressure opposes filtration at the glomerulus (reducing effective glomerular filtration pressure), increases renal interstitial pressure, and activates inflammatory cascades within the kidney. Additionally, obstruction typically triggers activation of vasoconstrictor systems (angiotensin II, sympathetic nervous system) that further reduce GFR. Prolonged obstruction (>1–2 weeks) can lead to irreversible tubular atrophy, glomerular sclerosis, and permanent loss of renal function.
Systemic Mediators and Propagation Mechanisms
Regardless of initial insult, AKI involves systemic inflammatory activation with elevated IL-6, TNF-α, and complement activation. Sterile inflammation occurs even in non-infectious causes, perpetuating tubular damage. Oxidative stress from impaired electron transport and increased ROS production damages lipids, proteins, and DNA. Tubular epithelial cells undergo apoptosis and autophagy in response to cellular stress. Dysregulation of platelet aggregation and coagulation can lead to microthrombi, particularly in severe sepsis-associated AKI. Interestingly, surviving tubular epithelial cells initially undergo a dedifferentiation process to repopulate denuded areas, during which they are metabolically active and vulnerable to further injury.
AKI etiology varies by clinical setting. In community-dwelling patients, prerenal causes predominate (~55%), whereas in hospitalized patients, intrinsic renal disease (particularly ATN from sepsis or ischemia) is more common (~50%). Postrenal obstruction accounts for ~5–10% of AKI cases overall but should never be missed as it is often reversible.
Prerenal AKI
- Volume depletion: Hemorrhage (acute blood loss >15% intravascular volume), severe diarrhea or vomiting, burns, excessive diuretic use, or third-spacing into ascites, peritoneal cavity, or interstitium. Elderly patients and those with CKD have narrower margins of renal perfusion reserve.
- Decreased cardiac output: Acute decompensated heart failure with reduced ejection fraction, acute coronary syndrome with cardiogenic shock, severe arrhythmias, massive pulmonary embolism, constrictive pericarditis, or severe valvular disease. In cardiorenal syndrome, intrarenal vasoconstriction compounds the problem.
- Systemic vasodilation: Sepsis (the most common cause of AKI in ICU patients, accounting for ~50% of cases), anaphylaxis, or advanced cirrhosis with hepatorenal syndrome. In sepsis, endotoxin (lipopolysaccharide) and bacterial products trigger TLR4 signaling on endothelial cells and macrophages, causing widespread vasodilation and reduced systemic vascular resistance, compounded by increased vascular permeability and fluid sequestration.
- Renal artery stenosis or thrombosis: Critical bilateral stenosis (or unilateral stenosis in a solitary kidney) or acute thrombosis from atherosclerotic disease, dissection, or hypercoagulable state reduces renal perfusion pressure below the autoregulation threshold.
- Medications impairing RAAS compensation: ACE inhibitors and angiotensin II receptor blockers (ARBs) block the efferent arteriolar vasoconstriction that maintains GFR during hypoperfusion; risk is especially high in volume-depleted patients or those with renal artery stenosis. NSAIDs inhibit renal vasodilatory prostaglandins and reduce GFR, particularly dangerous in volume-depleted or septic patients. Direct renin inhibitors (aliskiren) carry similar risk.
Ischemic ATN
- Sepsis: The single most important cause of AKI in hospitalized patients; sepsis-associated AKI occurs in 19–23% of septic patients and carries mortality of 50–60%. Mechanisms include intrarenal vasoconstriction (despite systemic vasodilation), direct endotoxin-mediated tubular toxicity, inflammatory cascade activation, and microvascular thrombosis.
- Surgical or anesthesia-related hypotension: Perioperative hypotension, particularly in patients with pre-existing CKD or diabetes, can precipitate ATN. Major surgery itself triggers inflammatory cascades that contribute to AKI.
- Cardiogenic shock: Acute MI with mechanical complication, acute decompensated heart failure, or arrhythmias.
- Massive hemorrhage and transfusion: Beyond simple volume depletion, transfusion of stored blood (which leaches potassium) combined with ischemia increases rhabdomyolysis risk.
Nephrotoxic ATN
- Contrast-induced AKI (CI-AKI): Risk is 5–15% in patients with CKD; absolute risk increases with eGFR <30 mL/min/1.73m², diabetes, heart failure, and concomitant nephrotoxins. Iodinated radiocontrast agents (both high-osmolality and low-osmolality agents carry risk, though low-osmolality agents are preferred) cause osmotic injury and ROS generation.
- Aminoglycoside antibiotics: Gentamicin, tobramycin, amikacin cause dose-dependent, cumulative AKI in 5–15% of recipients; mechanisms include ROS generation and phospholipid accumulation. Renally cleared with narrow margin between therapeutic and toxic levels. Risk increased with volume depletion, concomitant nephrotoxins, and underlying CKD.
- Amphotericin B (conventional formulation): Causes dose-dependent AKI in up to 80% of recipients through direct tubular toxicity and intrarenal vasoconstriction. Lipid formulations are less nephrotoxic.
- Cisplatin and other chemotherapy agents: Cisplatin is filtered and accumulates in tubular cells; mechanisms include direct DNA damage, ROS generation, and apoptosis. AKI develops in 20–30% of recipients; risk increased with high cumulative dose and dehydration.
- Rhabdomyolysis: Crush injury, prolonged immobilization, statins combined with gemfibrozil or macrolides, severe exertion, hyperthermia, malignant hyperthermia, or certain viral infections (influenza, HIV, dengue). Myoglobin precipitates in tubules, particularly in acidic, hypovolemic conditions; threshold for clinically significant AKI is creatine kinase (CK) >5000 IU/L.
- Tumor lysis syndrome: Occurs within 12–72 hours of initiating chemotherapy for rapidly proliferating malignancies (particularly acute leukemias, lymphomas, and small cell lung cancer). Intracellular contents (potassium, phosphate, nucleic acids) are released; uric acid precipitates in tubules (urate nephropathy), and light chains in multiple myeloma precipitate in tubules ("myeloma cast nephropathy").
- NSAIDs: Beyond their prerenal mechanism, NSAIDs can cause papillary necrosis (particularly in chronic users with CKD) and direct tubular injury.
- Calcineurin inhibitors: Tacrolimus, cyclosporine cause dose-dependent AKI through vasoconstriction and direct tubular toxicity; trough levels should be monitored.
Acute Glomerulonephritis and Vasculitis
- Post-infectious GN: Most commonly post-streptococcal GN, occurring 1–3 weeks after group A Streptococcus pharyngitis or skin infection; immune complexes deposit in glomeruli.
- IgA nephropathy: Most common primary glomerulonephritis worldwide; episodes of gross hematuria can precipitate AKI.
- Lupus nephritis: SLE flare with Class III or IV (proliferative) lesions.
- ANCA-associated vasculitis: Granulomatosis with polyangiitis (GPA, formerly Wegener's) and microscopic polyangiitis (MPA) present with acute glomerulonephritis and pulmonary hemorrhage. Anti-PR3 and anti-MPO antibodies trigger neutrophil activation and glomerular destruction.
- Anti-GBM disease (Goodpasture syndrome): Linear IgG deposition along basement membranes; often presents with pulmonary-renal syndrome.
- Thrombotic microangiopathies (TMA): Hemolytic-uremic syndrome (HUS) from Shiga toxin-producing E. coli, thrombotic thrombocytopenic purpura (TTP), and atypical HUS (aHUS) from complement dysregulation.
Acute Interstitial Nephritis (AIN)
- Beta-lactam antibiotics: Penicillins, cephalosporins are the most common drug cause of AIN; allergic reaction typically develops 7–14 days after initiation (but can occur within 24 hours with prior exposure).
- NSAIDs and selective COX-2 inhibitors: Cause both prerenal AKI and AIN.
- Proton pump inhibitors: Omeprazole, pantoprazole increasingly recognized as AIN cause; incidence may be underestimated due to difficulty distinguishing from other mechanisms
The typical stem: an older hospitalized patient — often with CKD, diabetes, or heart failure — whose creatinine "bumped" after a hypotensive episode, a contrast study, a new antibiotic, or an NSAID/ACE inhibitor combination. Most AKI is detected on labs, not by symptoms, because the kidney has enormous functional reserve.
Symptoms by mechanism
- Oliguria (<400 mL/day) or anuria: reduced filtrate volume; anuria is a red flag for complete bilateral obstruction, bilateral renal artery occlusion, or cortical necrosis. Nonoliguric AKI is common with aminoglycosides and AIN and does not exclude AKI.
- Prerenal signs: thirst, orthostatic hypotension, tachycardia, dry mucous membranes, flat neck veins from true volume depletion — or, paradoxically, edema, ascites, and elevated JVP in heart failure and cirrhosis, where the effective arterial volume is low despite total-body fluid overload.
- Postrenal signs: an elderly man with BPH describing hesitancy, dribbling, and nocturia, with a palpable suprapubic bladder; flank pain and colic with stones; a pelvic malignancy or retroperitoneal fibrosis in bilateral ureteral obstruction.
- Intrinsic clues: cola-colored urine, hypertension, and periorbital edema in glomerulonephritis; muscle pain, weakness, and dark urine days after crush injury or prolonged immobilization in rhabdomyolysis; fever, rash, and eosinophilia in AIN (the full triad appears in a minority of cases, so its absence never excludes AIN); livedo reticularis, blue toes, and Hollenhorst plaques days to weeks after arterial catheterization in cholesterol embolization.
Uremic findings (late, severe AKI): nausea, anorexia, pruritus, asterixis and encephalopathy from retained organic solutes; a pericardial friction rub signaling uremic pericarditis; mucosal bleeding from uremia-induced platelet dysfunction; Kussmaul respirations with metabolic acidosis; and rales or hypoxemia from volume overload.
Step 1 — Define and stage the injury (KDIGO 2012 criteria): AKI is present with any of: a rise in serum creatinine of ≥0.3 mg/dL within 48 hours; a rise to ≥1.5× baseline known or presumed within 7 days; or urine output <0.5 mL/kg/h for ≥6 hours. Staging is by the same two axes — Stage 1 (1.5–1.9× baseline), Stage 2 (2.0–2.9×), Stage 3 (≥3× baseline, creatinine ≥4.0 mg/dL, or initiation of renal replacement therapy). Creatinine is a lagging marker: it rises only after GFR has already fallen, so a "normal" creatinine early in an ischemic insult is falsely reassuring.
Step 2 — Localize the lesion. Three tests answer most stems
- Urinalysis with microscopy (the highest-yield single test): muddy brown granular casts → ATN; dysmorphic RBCs and RBC casts → glomerulonephritis; WBC casts with sterile pyuria (± eosinophiluria) → AIN; bland sediment → prerenal or postrenal. Dipstick positive for blood with no RBCs on microscopy → myoglobinuria (rhabdomyolysis) or hemoglobinuria.
- Renal ultrasound with post-void residual (or bladder scan/catheterization): hydronephrosis or a large residual volume confirms obstruction. Obstruction must be excluded early because it is the most reversible category.
- Urine indices: FENa <1%, BUN/Cr >20:1, and urine osmolality >500 mOsm/kg support prerenal physiology; FENa >2% with isosthenuric urine (~300 mOsm/kg) supports ATN. Use FEUrea (<35% prerenal) when the patient is on a loop diuretic, which artifactually raises FENa.
Step 3 — Targeted serologies and biopsy: ANA/anti-dsDNA, ANCA, anti-GBM, complements, cryoglobulins, hepatitis serologies, and SPEP/free light chains when nephritic or nephrotic features are present. Renal biopsy is the gold standard and is reserved for AKI without an explanation, suspected rapidly progressive glomerulonephritis, or steroid decisions in AIN.
There is no drug that reverses established AKI; KDIGO 2012 management is supportive and cause-directed.
Immediate stabilization
- Screen for life-threatening complications first: hyperkalemia with ECG changes, refractory acidosis, and pulmonary edema. Hyperkalemia with ECG changes → IV calcium gluconate to stabilize the myocardium, then insulin with dextrose and a beta-2 agonist to shift potassium, then a removal strategy (loop diuretic, potassium binder, or dialysis).
- Relieve obstruction: bladder catheter for outlet obstruction; percutaneous nephrostomy or ureteral stenting for upper-tract obstruction. This is definitive therapy and requires no confirmation of etiology beyond imaging.
First-line for prerenal/ATN physiology
- Isotonic crystalloid (balanced solutions such as lactated Ringer's or normal saline); KDIGO recommends crystalloid over colloid, and avoiding hydroxyethyl starch. Volume expansion is also the mainstay for rhabdomyolysis, tumor lysis, and contrast prophylaxis.
- Vasopressors (norepinephrine) with fluids in vasomotor shock to restore renal perfusion pressure.
- Stop the insult: hold ACE inhibitors/ARBs, NSAIDs, aminoglycosides, and other nephrotoxins; renally dose all drugs; hold metformin (lactic acidosis risk) and SGLT2 inhibitors during acute illness.
Second-line and cause-specific
- Loop diuretics (furosemide): for volume overload only. KDIGO explicitly recommends against using diuretics to prevent or treat AKI or to "convert" oliguric to nonoliguric AKI.
- Glucocorticoids (prednisone) for drug-induced AIN after culprit withdrawal; glucocorticoids plus cyclophosphamide or rituximab for ANCA vasculitis, with plasma exchange for anti-GBM disease.
Renal replacement therapy — emergent indications (AEIOU): refractory Acidosis, Electrolyte derangement (hyperkalemia), Ingestions/dialyzable toxins, volume Overload refractory to diuretics, and Uremic complications (pericarditis, encephalopathy, bleeding).
Contraindicated/not recommended: low-dose "renal" dopamine, N-acetylcysteine and sodium bicarbonate for contrast prophylaxis (no benefit over saline), and nephrotoxin re-exposure. Per ACR guidance, gadolinium-based agents carry nephrogenic systemic fibrosis risk in severe AKI, though risk with group II agents is very low.
Emergencies
- Hyperkalemia: loss of distal potassium secretion plus transcellular shift in acidosis; signaled by peaked T waves, PR prolongation, QRS widening, and ultimately a sine-wave rhythm progressing to cardiac arrest. The most common lethal complication of AKI.
- Pulmonary edema/volume overload: impaired sodium and water excretion; hypoxemia, rales, and a rising oxygen requirement. Refractoriness to loop diuretics is a dialysis indication.
- Uremic pericarditis: retained uremic solutes inflame the pericardium; friction rub with pleuritic chest pain, and risk of hemorrhagic effusion and tamponade. An absolute indication for urgent dialysis — not for anticoagulation.
- Uremic encephalopathy: confusion, asterixis, seizures; also a dialysis indication.
Metabolic and hematologic
- High anion gap metabolic acidosis: failure to excrete sulfate, phosphate, and organic anions; drives hyperkalemia and Kussmaul breathing.
- Hyperphosphatemia with hypocalcemia: reduced phosphate excretion and impaired 1-alpha-hydroxylation of vitamin D; may produce tetany or a prolonged QT, and is severe in tumor lysis and rhabdomyolysis.
- Uremic platelet dysfunction: qualitative defect with a normal platelet count and prolonged bleeding time; mucosal bleeding. Desmopressin is the temporizing agent.
- Anemia: reduced erythropoietin plus dilution and blood sampling.
Treatment-related
- Post-obstructive diuresis after relief of obstruction: washout of retained solute and tubular concentrating defect cause massive polyuria with hypovolemia, hypokalemia, and hypernatremia — replace fluids and monitor electrolytes closely.
- Drug accumulation: opioids, gabapentin, digoxin, and low-molecular-weight heparin accumulate with reduced clearance.
- Dialysis complications: intradialytic hypotension worsening ischemic injury, catheter-related bloodstream infection, and dialysis disequilibrium syndrome from rapid urea clearance and cerebral edema.
Long-term: incomplete tubular repair with maladaptive fibrosis means AKI is an independent risk factor for progression to CKD and ESRD; KDIGO recommends follow-up creatinine and albuminuria assessment after recovery.
- The three-test reflex: for any unexplained creatinine rise, the best next steps are urinalysis with microscopy, renal ultrasound (or bladder scan), and a volume assessment. Sediment localizes the lesion faster than any serology.
- Buzzword-to-diagnosis map: muddy brown granular casts → ATN; RBC casts with dysmorphic RBCs → glomerulonephritis; WBC casts with eosinophiluria → AIN; dipstick blood without RBCs → myoglobinuria; livedo reticularis, blue toe, eosinophilia, low complement after catheterization → cholesterol emboli.
- FENa has exceptions that examiners love: FENa can be <1% despite intrinsic injury in contrast nephropathy, rhabdomyolysis, early glomerulonephritis, and hepatorenal syndrome. If the patient is on a loop diuretic, use FEUrea (<35% = prerenal) instead.
- The one association tested: NSAID + ACE inhibitor/ARB + diuretic ("triple whammy") in a volume-depleted or elderly patient. Mechanism: loss of prostaglandin-mediated afferent dilation plus loss of angiotensin II–mediated efferent constriction abolishes both arms of autoregulation.
- Timing discriminates nephrotoxins: contrast-associated AKI peaks about 3–5 days after exposure and typically recovers; aminoglycoside AKI appears roughly a week in and is classically nonoliguric with hypokalemia and hypomagnesemia; AIN follows drug exposure by 1–2 weeks.
- Common distractor — furosemide: KDIGO recommends diuretics only for volume overload, never to prevent or treat AKI. Likewise, low-dose dopamine and N-acetylcysteine are wrong answers.
- **Dialysis indications (AEIOU)**: refractory acidosis, hyperkalemia, ingestions, overload, and uremic pericarditis or encephalopathy. A pericardial friction rub in a uremic patient means dialysis, not NSAIDs or anticoagulation.
- Never miss obstruction: it is a small fraction of cases but the most reversible. An anuric patient needs a bladder catheter and ultrasound before anything else.