Acute Kidney Injury

Acute Kidney Injury (AKI) – Medical Student Study Guide

Definition

Acute Kidney Injury (AKI) is a sudden decline in kidney function over hours to days, leading to accumulation of nitrogenous waste (elevated urea and creatinine) and dysregulation of fluid, electrolyte, and acid–base balance. Modern definitions are based on changes in serum creatinine and urine output.

KDIGO diagnostic criteria for AKI (any one is sufficient):

• Increase in serum creatinine by ≥0.3 mg/dL (≥26.5 µmol/L) within 48 hours
• Increase in serum creatinine to ≥1.5 times baseline, known or presumed to have occurred within the prior 7 days
• Urine volume <0.5 mL/kg/h for 6 hours

Epidemiology

AKI is common in both community and hospital settings, with particularly high incidence in critically ill and postoperative patients. AKI frequently complicates sepsis, major surgery, and nephrotoxin exposure, and even transient episodes are associated with increased short- and long-term morbidity and mortality, including progression to chronic kidney disease (CKD). Large ICU datasets, such as MIMIC-IV, highlight sepsis-associated AKI as a major contributor to mortality in bacterial pneumonia and critical illness.^{[1], [2]}

Pathophysiology

AKI is classically categorized as prerenal, intrinsic (intrarenal), or postrenal, based on the primary site of dysfunction.

Prerenal AKI

Prerenal AKI results from decreased renal perfusion without initial structural kidney damage. Causes include true hypovolemia (hemorrhage, GI losses, over-diuresis), decreased effective circulating volume (heart failure, cirrhosis, sepsis), and renal vasoconstriction (NSAIDs, hepatorenal syndrome).

Reduced renal blood flow leads to:

• Activation of the renin–angiotensin–aldosterone system (RAAS), sympathetic nervous system, and ADH
• Increased sodium and water reabsorption (low urine Na⁺, concentrated urine, high BUN:Cr ratio)
• Initially reversible functional decline; prolonged ischemia can progress to intrinsic acute tubular necrosis (ATN)

Intrinsic (Intrarenal) AKI

Intrinsic AKI implies structural damage within the kidney. Major subtypes include acute tubular necrosis (ATN), acute interstitial nephritis, acute glomerulonephritis, and acute vascular lesions. The most common form in hospitalized patients is ischemic or nephrotoxic ATN, often seen in sepsis, major surgery, rhabdomyolysis, or after contrast and nephrotoxic drugs.

Key mechanisms in ATN:

• Tubular epithelial injury from ischemia, toxins, or inflammation causing cell death and loss of polarity
• Tubular obstruction from sloughed cells and casts, increasing intratubular pressure
• Back-leak of filtrate across damaged epithelium into the interstitium
• Microvascular dysfunction with vasoconstriction, endothelial injury, and congestion, perpetuating hypoxia

Sepsis-related AKI involves complex interactions between systemic inflammation, endothelial injury, microcirculatory dysfunction, and venous congestion; derangements in fluid resuscitation and venous overload further worsen renal perfusion and interstitial edema.^{[1], [3], [4]}

Postrenal AKI

Postrenal AKI is due to obstruction of urine flow anywhere from the renal pelvis to the urethra. It is often reversible if relieved promptly.

• Causes: prostatic hypertrophy or carcinoma, urethral stricture, bilateral ureteric obstruction (stones, tumors, retroperitoneal fibrosis), bladder outlet obstruction, or obstruction of a solitary kidney.
• Pathophysiology: obstruction increases intratubular pressure, decreasing GFR; prolonged obstruction leads to tubular atrophy, interstitial fibrosis, and permanent damage.

Clinical Presentation

Presentation varies from asymptomatic lab abnormalities to multisystem failure in critically ill patients.

General features:

• Oliguria (<400 mL/day) or anuria; or non-oliguric with normal/near-normal urine volume
• Rising serum creatinine and urea (BUN)
• Fatigue, malaise, nausea, vomiting, altered mental status from uremia in severe cases

Volume status-related findings:

• Hypovolemia/prerenal: tachycardia, hypotension, dry mucous membranes, decreased skin turgor, orthostatic hypotension
• Hypervolemia (ATN, advanced AKI): peripheral edema, pulmonary crackles, elevated JVP, weight gain

Electrolyte and acid–base manifestations:

• Hyperkalemia (muscle weakness, arrhythmias)
• Metabolic acidosis (tachypnea, Kussmaul respirations)
• Hyponatremia or hypernatremia depending on volume status and fluid management

Etiology-specific clues:

• Recent diarrhea/vomiting, hemorrhage, over-diuresis → prerenal
• Sepsis, major surgery, cardiogenic shock, nephrotoxic medications, iodinated contrast → ATN
• Rash, fever, eosinophilia, new drug exposure (e.g., antibiotics, PPIs, NSAIDs) → acute interstitial nephritis
• Hematuria, proteinuria, hypertension, edema, systemic symptoms (e.g., hematuria with RBC casts) → acute glomerulonephritis
• LUTS (urgency, hesitancy), enlarged prostate, flank pain, anuria, known stones → postrenal obstruction

Diagnosis

Diagnosis is based on meeting AKI criteria and determining the underlying cause through history, examination, laboratory tests, and imaging.

History and Physical Examination

• Baseline kidney function, timing and rate of creatinine rise
• Fluid losses, bleeding, recent illnesses (sepsis, cardiac events), surgery, or trauma
• Medication review: NSAIDs, ACEi/ARBs, diuretics, aminoglycosides, vancomycin, amphotericin B, contrast, calcineurin inhibitors, chemotherapy
• Systemic diseases: diabetes, hypertension, autoimmune disease, malignancy
• Volume assessment: blood pressure, JVP, edema, mucous membranes, capillary refill
• Abdominal, pelvic, and rectal exam for bladder distension, prostate enlargement, flank tenderness

Laboratory Evaluation

Serum studies:

• Creatinine, urea (BUN), electrolytes (K⁺, Na⁺, Cl^-), bicarbonate
• Calcium, phosphate, magnesium in more prolonged or severe cases
• Complete blood count, LFTs, CRP; blood cultures if sepsis suspected
• CK, LDH, uric acid, myoglobin if rhabdomyolysis or tumor lysis suspected
• Autoimmune panel (ANA, ANCA, anti-GBM, complement levels) if glomerulonephritis or vasculitis suspected

Urinalysis (key for differentiating etiologies):

• Dipstick for protein, blood, leukocyte esterase, nitrites
• Microscopy for casts and cells:
• Prerenal: bland urine sediment, hyaline casts
• ATN: muddy brown granular casts, renal tubular epithelial cells
• Glomerulonephritis: dysmorphic RBCs, RBC casts, proteinuria
• Interstitial nephritis: WBCs, WBC casts, eosinophiluria, mild proteinuria

Urine chemistries (especially if oliguric):

• Urine sodium, urine creatinine, fractional excretion of sodium (FENa), and/or urea (FEUrea)
• Prerenal AKI: typically low FENa (<1%) and FEUrea (<35%), reflecting sodium and urea avid reabsorption
• ATN: FENa usually >2%, FEUrea >50% (exceptions in sepsis, contrast nephropathy, and recent diuretic use)

Imaging

Renal ultrasound is the first-line imaging modality to assess kidney size and echogenicity and to look for hydronephrosis in suspected postrenal AKI. It is noninvasive and quickly identifies obstructive uropathy. Additional imaging (CT, MRI) is considered if ultrasound is inconclusive and suspicion remains high.

Staging of AKI (KDIGO)

• Stage 1: 1.5–1.9 × baseline creatinine or ≥0.3 mg/dL rise; urine output <0.5 mL/kg/h for 6–12 h
• Stage 2: 2.0–2.9 × baseline creatinine; urine output <0.5 mL/kg/h for ≥12 h
• Stage 3: 3.0 × baseline, or creatinine ≥4.0 mg/dL, or initiation of renal replacement therapy; urine output <0.3 mL/kg/h for ≥24 h or anuria for ≥12 h

Management

Management of AKI focuses on identifying and reversing the underlying cause, optimizing hemodynamics, avoiding further injury, and treating complications. Consider AKI in the broader context of the patient’s illness (e.g., sepsis, heart failure, postoperative state) and apply a phase-adapted approach to fluid and hemodynamic therapy, particularly in septic shock.^{[3], [4], [5]}

General Principles

• Identify and treat reversible causes early: hypovolemia, sepsis, obstruction, nephrotoxins
• Monitor urine output (hourly in critically ill), daily weights, vital signs, and volume status
• Adjust all medications for current GFR; avoid or minimize nephrotoxins
• Correct electrolyte and acid–base disorders, particularly hyperkalemia and acidosis

Prerenal AKI Management

• Volume resuscitation with isotonic crystalloids (e.g., balanced solutions) in hypovolemia; reassess frequently to avoid fluid overload
• Hemodynamic optimization: treat heart failure, optimize blood pressure and cardiac output; in septic shock, combine appropriate antibiotics, source control, and vasopressors as needed
• Withhold nephrotoxic agents and consider holding ACEi/ARBs and diuretics if contributing to hypoperfusion
• If corrected early, prerenal AKI is usually reversible with a rapid fall in creatinine

Intrinsic AKI Management

Acute Tubular Necrosis (ATN):

• No specific pharmacologic therapy; management is largely supportive
• Optimize perfusion and avoid further ischemic or toxic insults
• Careful fluid management: avoid both volume depletion and overload; consider dynamic assessment of venous congestion (e.g., ultrasound-based approaches) when tailoring derescuscitation in septic shock
• Treat underlying conditions (e.g., control sepsis, manage rhabdomyolysis with aggressive fluid and, if appropriate, alkalinization)

Acute Interstitial Nephritis:

• Stop offending drug immediately
• Consider corticosteroids in select cases after nephrology evaluation and often after biopsy confirmation

Acute Glomerulonephritis:

• Nephrology consultation and often kidney biopsy for diagnosis
• Immunosuppressive therapy (e.g., corticosteroids, cytotoxic agents, rituximab) tailored to the specific disease (e.g., ANCA-associated vasculitis, anti-GBM disease)
• Control of blood pressure and proteinuria when appropriate

Postrenal AKI Management

• Immediate relief of obstruction is crucial
• Insert Foley catheter for bladder outlet obstruction and monitor post-obstructive diuresis
• Urology involvement for ureteric stenting or percutaneous nephrostomy in upper tract obstruction
• Monitor fluids and electrolytes closely after decompression due to risk of large diuresis and volume depletion

Renal Replacement Therapy (RRT)

RRT (intermittent hemodialysis, continuous renal replacement therapy [CRRT]) is indicated for life-threatening complications of AKI or when conservative management fails.

Common indications (often summarized by “AEIOU”):

• Acid–base: severe metabolic acidosis refractory to medical therapy
• Electrolytes: refractory hyperkalemia or other severe derangements
• Intoxications: selected dialyzable toxins
• Overload: fluid overload causing respiratory compromise refractory to diuretics
• Uremia: symptomatic uremia (encephalopathy, pericarditis, seizures, severe nausea/vomiting)

The optimal timing of RRT initiation in critically ill AKI remains debated; recent evidence suggests that biomarker-guided or very early initiation has not consistently shown mortality benefit over careful clinical staging–based strategies. Decisions should integrate the severity of renal dysfunction, trend in parameters, and overall clinical status.^{[5], [2]}

Prevention of AKI

• Identify high-risk patients: CKD, diabetes, heart failure, advanced age, sepsis, major surgery, contrast exposure
• Optimize volume status and hemodynamics before and after surgery or contrast procedures
• Use lowest effective doses and shortest durations of nephrotoxic drugs; avoid combinations when possible
• Implement contrast nephropathy prevention protocols (adequate hydration, cautious contrast dosing, consider alternative imaging)
• In ICU patients, apply phase-adapted fluid strategies to avoid both under-resuscitation and venous congestion that exacerbates AKI.^{[3], [4]}

Prognosis and Long-Term Outcomes

AKI is associated with increased in-hospital mortality, especially when occurring with sepsis or multi-organ failure. Beyond the acute illness, survivors of AKI have higher risks of developing or accelerating CKD and end-stage kidney disease. Predictive models, including nomograms derived from large ICU datasets, show that AKI severity alongside sepsis and organ dysfunction is strongly linked to 28-day mortality in critically ill patients with bacterial pneumonia.^{[2], [1]}

Follow-up after AKI should include reassessment of kidney function, blood pressure control, and modification of CKD and cardiovascular risk factors.

Key Clinical Pearls for Exams and Practice

• Always categorize AKI as prerenal, intrinsic, or postrenal; this frames your diagnostic approach.
• Check for obstruction early with a bladder scan and renal ultrasound, especially in older men and patients with known urologic disease.
• Trend creatinine, urine output, and volume status closely; small changes can be clinically significant.
• Use urinalysis and sediment examination as a high-yield, low-cost tool to differentiate prerenal, ATN, glomerular, and interstitial causes.
• In sepsis, AKI is multifactorial: combine appropriate antibiotics and source control with careful, phase-adapted fluid and vasopressor management to protect the kidney.
• Avoid nephrotoxins whenever possible; adjust drug dosing to current GFR and monitor levels for narrow therapeutic index agents.
• Recognize red-flag features that suggest rapidly progressive glomerulonephritis (RPGN) and require urgent nephrology input (hematuria with RBC casts, rapid GFR decline, systemic vasculitis signs).
• Do not delay RRT when there are clear, refractory indications; timing based solely on creatinine without clinical context is not evidence supported.
• Every AKI episode is a risk factor for future CKD; plan follow-up to detect and manage long-term consequences.

Summary

Acute Kidney Injury is a common, clinically important syndrome characterized by a rapid decline in renal function. A systematic approach—recognizing KDIGO diagnostic criteria, differentiating prerenal, intrinsic, and postrenal causes, and applying targeted therapy to reverse insults, optimize hemodynamics, avoid nephrotoxins, and treat complications—improves outcomes. Medical students should be comfortable interpreting urine studies, evaluating volume status, and knowing the principal indications for renal replacement therapy, while appreciating AKI as both an acute emergency and a marker of long-term kidney and cardiovascular risk.