Acute kidney injury (AKI) is a sudden decline in kidney function that develops over hours to days. It affects up to 10–15% of all hospitalized patients and over 50% of ICU patients, making it one of the most common conditions nursing students will encounter on medical-surgical and critical care floors. Hospital mortality for patients with AKI ranges from 40–50%, and at least 12–15% of survivors require permanent dialysis. Understanding the pathophysiology, staging criteria, and nursing priorities for AKI is essential for both clinical practice and the NCLEX.
This reference covers the full scope of AKI: how each type damages the kidney, KDIGO staging criteria with specific lab thresholds, laboratory interpretation including the FENa calculation, priority nursing assessments and interventions, dialysis indications using the AEIOU mnemonic, and high-yield NCLEX review points. Use it alongside the nursing lab values cheat sheet and electrolyte imbalances reference for a complete renal reference library.
| Quick reference | Detail |
|---|---|
| Definition | Sudden decrease in renal function causing waste buildup, fluid overload, and electrolyte imbalances |
| KDIGO diagnostic criteria | Cr rise ≥0.3 mg/dL within 48 h, OR Cr ≥1.5× baseline within 7 days, OR urine output <0.5 mL/kg/h for 6+ h |
| Three categories | Pre-renal (decreased perfusion), intrinsic (parenchymal damage), post-renal (obstruction) |
| Key labs | BUN, creatinine, BUN:Cr ratio, FENa, potassium, phosphorus, calcium, ABGs |
| Priority nursing action | Strict I&O monitoring — urine output is the earliest bedside indicator of renal function change |
| Dialysis indications | AEIOU: Acidosis, Electrolytes, Intoxication, Overload, Uremia |
Pathophysiology: how AKI damages the kidney
All three categories of AKI share a common endpoint — a decline in glomerular filtration rate (GFR) — but they reach it through different mechanisms. Understanding these mechanisms is the key to distinguishing AKI types on an exam and at the bedside.
The glomerular filtration rate
GFR measures the volume of plasma filtered by the glomeruli per minute. A normal GFR is approximately 90–120 mL/min. When GFR falls, creatinine and BUN accumulate in the blood because the kidneys cannot clear them. Urine output drops because less filtrate is being produced. The clinical result is azotemia (elevated nitrogenous waste) and, if severe, uremia (symptomatic toxin accumulation).
Pre-renal mechanism
Pre-renal AKI accounts for approximately 60–70% of community-acquired cases and results from decreased blood flow to the kidneys. The nephrons themselves are intact — they simply do not receive enough perfusion pressure to filter effectively. Because the tubular cells are healthy and functioning, they respond appropriately by reabsorbing sodium and water aggressively. This is why pre-renal AKI produces concentrated urine with low sodium content — the kidney is doing exactly what it should do when perfusion drops.
If the perfusion deficit is corrected quickly (within 24–72 hours), creatinine returns to baseline. If hypoperfusion persists, ischemic damage to the tubular cells develops, and the injury converts to intrinsic AKI (specifically acute tubular necrosis). This progression is why early recognition and fluid resuscitation matter so much.
Intrinsic renal mechanism
Intrinsic AKI involves direct damage to the kidney parenchyma — the glomeruli, tubules, interstitium, or vasculature. Acute tubular necrosis (ATN) is the most common form, accounting for approximately 45% of all AKI cases. ATN develops when tubular epithelial cells are injured by prolonged ischemia (from unresolved pre-renal injury) or direct nephrotoxins (aminoglycosides, contrast dye, myoglobin from rhabdomyolysis).
Damaged tubular cells slough off and obstruct the tubular lumen, causing back-leak of filtrate into the interstitium. The tubules lose their ability to concentrate urine or reabsorb sodium, which is why intrinsic AKI produces dilute urine with elevated sodium — the opposite of pre-renal findings. ATN progresses through four phases: onset (initial insult), oliguric phase (urine output less than 400 mL/day, lasting 1–3 weeks), diuretic phase (urine output 3–6 L/day as tubules begin recovering but cannot yet concentrate), and recovery phase (gradual return to normal function over months).
Other forms of intrinsic AKI include acute interstitial nephritis (often drug-induced — penicillins, NSAIDs, proton pump inhibitors), acute glomerulonephritis (post-streptococcal, lupus nephritis), and renal vascular injury (renal artery thrombosis, HUS/TTP).
Post-renal mechanism
Post-renal AKI results from urinary tract obstruction that increases pressure behind the blockage. As urine backs up, hydrostatic pressure in the collecting system rises and opposes glomerular filtration. If obstruction is bilateral (or unilateral in a patient with a single kidney), GFR declines within hours. Common causes include kidney stones, benign prostatic hyperplasia, tumors compressing the ureters, and blood clots.
Post-renal AKI is often the most reversible category — once the obstruction is relieved (catheterization, stent placement, nephrostomy), kidney function frequently recovers rapidly.
AKI classification: pre-renal, intrinsic, and post-renal
The table below compares the three AKI categories across causes, key lab findings, and clinical features. This comparison is heavily tested on the NCLEX and forms the basis for clinical decision-making about AKI etiology.
| Feature | Pre-renal | Intrinsic (ATN) | Post-renal |
|---|---|---|---|
| Mechanism | Decreased renal perfusion | Direct parenchymal damage | Urinary tract obstruction |
| Common causes | Hypovolemia, hemorrhage, heart failure, sepsis, burns, dehydration | ATN (ischemic or nephrotoxic), AIN (drug-induced), glomerulonephritis, rhabdomyolysis | BPH, kidney stones, tumors, blood clots, strictures |
| BUN:Cr ratio | >20:1 | 10–15:1 | Variable (often >20:1 early) |
| FENa | <1% | >2% | Variable |
| Urine sodium | <20 mEq/L | >40 mEq/L | Variable |
| Urine specific gravity | >1.020 (concentrated) | <1.010 (dilute, fixed) | Variable |
| Urine osmolality | >500 mOsm/kg | <350 mOsm/kg | Variable |
| Urine sediment | Bland (hyaline casts) | Muddy brown granular casts (ATN), WBC casts (AIN), RBC casts (GN) | Normal or crystals |
| Response to fluids | Cr improves within 24–72 h | No improvement | Improves after obstruction relieved |
Clinical pearl: The gold standard for differentiating pre-renal AKI from intrinsic disease is the response to adequate fluid resuscitation. If creatinine trends back toward baseline within 24–72 hours of volume correction, the injury was pre-renal. If it does not improve, the injury has progressed to ATN.
KDIGO staging criteria
The Kidney Disease: Improving Global Outcomes (KDIGO) classification is the current standard for staging AKI severity. It replaced the earlier RIFLE and AKIN systems and uses two criteria — serum creatinine and urine output — to assign a stage. The higher stage from either criterion is used.
| Stage | Serum creatinine criteria | Urine output criteria | Clinical implication |
|---|---|---|---|
| Stage 1 | 1.5–1.9× baseline OR ≥0.3 mg/dL increase within 48 h | <0.5 mL/kg/h for 6–12 h | Mild injury — optimize perfusion, discontinue nephrotoxins, monitor closely |
| Stage 2 | 2.0–2.9× baseline | <0.5 mL/kg/h for ≥12 h | Moderate injury — nephrology consult recommended, prepare for possible RRT |
| Stage 3 | ≥3.0× baseline OR ≥4.0 mg/dL OR initiation of RRT | <0.3 mL/kg/h for ≥24 h OR anuria for ≥12 h | Severe injury — high mortality risk, RRT often indicated |
Key points for NCLEX:
- Stage 1 includes the 0.3 mg/dL absolute rise criterion — this catches early AKI even when the percentage increase is small (for example, a creatinine rising from 0.8 to 1.1 mg/dL within 48 hours meets Stage 1 criteria)
- Urine output criteria require accurate I&O documentation and the patient’s weight for the mL/kg/h calculation
- Stage 3 includes any patient who requires renal replacement therapy, regardless of creatinine level
- A patient can meet different stages by creatinine and urine output — always assign the higher stage
Laboratory interpretation
Interpreting renal labs requires understanding what each value tells you about kidney function and how patterns of results help identify the AKI type. Review these values alongside the nursing lab values cheat sheet for normal ranges.
BUN and creatinine
BUN (blood urea nitrogen): Normal 6–20 mg/dL. Urea is a byproduct of protein metabolism filtered by the glomeruli. BUN rises in AKI but is less specific than creatinine because it is also affected by protein intake, GI bleeding, catabolic states, and dehydration.
Creatinine: Normal 0.6–1.2 mg/dL. Creatinine is a byproduct of muscle metabolism and is filtered at a relatively constant rate. It is the primary marker for GFR estimation. A creatinine of 2.0 mg/dL represents an approximate 50% reduction in GFR from normal.
BUN:creatinine ratio
The ratio helps differentiate pre-renal from intrinsic AKI:
- Greater than 20:1 — suggests pre-renal etiology. BUN is disproportionately elevated because the intact tubules reabsorb urea along with sodium and water in response to hypoperfusion. Creatinine rises more slowly because it is not reabsorbed.
- 10–15:1 (normal ratio) — suggests intrinsic AKI. Both BUN and creatinine rise proportionally because the damaged tubules cannot preferentially reabsorb urea.
Fractional excretion of sodium (FENa)
FENa is the percentage of filtered sodium that is excreted in the urine. It is one of the most useful calculations for differentiating pre-renal from intrinsic AKI.
Formula:
FENa = (urine Na × plasma Cr) / (plasma Na × urine Cr) × 100
Interpretation:
- FENa less than 1% — pre-renal AKI. The healthy tubules are reabsorbing nearly all filtered sodium in an attempt to conserve volume.
- FENa greater than 2% — intrinsic AKI (ATN). The damaged tubules cannot reabsorb sodium effectively, so more sodium spills into the urine.
- FENa 1–2% — indeterminate. Could represent early ATN or partially treated pre-renal injury.
Important caveat: FENa is unreliable in patients receiving diuretics, because diuretics force sodium excretion regardless of tubular function. In diuretic-treated patients, use the fractional excretion of urea (FEUrea) instead — a FEUrea less than 35% suggests pre-renal, while greater than 50% suggests intrinsic disease.
Other key labs in AKI
- Potassium: Rises in AKI because the kidneys cannot excrete potassium. Hyperkalemia is the most immediately life-threatening electrolyte abnormality — monitor for tall peaked T waves on EKG and cardiac arrhythmias.
- Phosphorus: Rises (kidneys normally excrete phosphorus). Elevated phosphorus binds calcium, contributing to hypocalcemia.
- Calcium: Falls due to phosphorus binding and reduced renal conversion of vitamin D to its active form (calcitriol). Monitor for Chvostek sign and Trousseau sign.
- Metabolic acidosis: The kidneys cannot excrete hydrogen ions or regenerate bicarbonate. Check ABGs — expect pH less than 7.35 with low bicarbonate.
- GFR (estimated): Calculated from creatinine, age, sex, and race. Normal is 90–120 mL/min. In AKI, GFR drops rapidly.
Nursing assessment
AKI assessment focuses on three domains: fluid balance, urinary changes, and systemic effects of waste accumulation. A thorough head-to-toe assessment should be performed each shift, with particular attention to the renal-specific findings below.
Fluid balance monitoring
- Strict I&O every hour in acute settings. Urine output less than 0.5 mL/kg/h for 6 hours meets KDIGO criteria for AKI. Calculate the threshold for your patient — for a 70 kg patient, that is less than 35 mL/h.
- Daily weights at the same time, same scale, same clothing. A weight gain of 1 kg equals approximately 1 liter of fluid retained.
- Assess for fluid overload: auscultate lungs for crackles, check for peripheral edema (pitting scale 1–4+), monitor JVD, note orthopnea or dyspnea. Compare with vital signs trends — watch for hypertension from volume overload.
- Assess for dehydration in pre-renal AKI: poor skin turgor, dry mucous membranes, orthostatic hypotension, tachycardia. These patients need fluid resuscitation — restriction would worsen their injury.
Urinary changes
- Color and clarity: dark, concentrated urine in pre-renal AKI; tea-colored or brown urine may suggest myoglobinuria (rhabdomyolysis) or hemoglobinuria
- Foamy urine: suggests significant proteinuria (glomerular damage)
- Urine output pattern: track the phase — oliguric (less than 400 mL/day), anuric (less than 100 mL/day), or diuretic (3–6 L/day during ATN recovery)
Systemic effects of uremia
As waste products accumulate, assess for uremic symptoms: nausea and vomiting, metallic taste, confusion or decreased LOC, asterixis (flapping tremor), pericardial friction rub (uremic pericarditis — a medical emergency and indication for emergent dialysis), and pruritus from phosphorus crystal deposition in the skin.
Nursing interventions
Nursing interventions in AKI are guided by the underlying cause and the severity of injury. The overarching goals are to restore kidney perfusion when possible, prevent further damage, manage complications, and support recovery.
Fluid management
- Pre-renal AKI: Administer IV isotonic fluids (0.9% normal saline) as ordered to restore renal perfusion. Monitor for fluid responsiveness — urine output should improve within hours if the injury is pre-renal. Fluid challenges must be given carefully in patients with heart failure to avoid precipitating pulmonary edema. Collaborate with the provider on fluid volume and rate. Review the heart failure reference for cardiorenal syndrome considerations.
- Intrinsic and post-renal AKI with fluid overload: Restrict fluids as ordered (typically previous day’s output plus 500 mL for insensible losses). Administer loop diuretics (furosemide) as prescribed. Weigh daily and trend fluid balance.
Nephrotoxin avoidance
Review the medication list and flag nephrotoxic agents — aminoglycosides (gentamicin, tobramycin), NSAIDs, ACE inhibitors/ARBs (in acute hypoperfusion states), IV contrast dye, and amphotericin B. Check the drug classifications reference for a complete list. Hold or dose-adjust medications cleared by the kidneys. Monitor drug trough levels for nephrotoxic antibiotics.
Contrast-induced nephropathy prevention
For patients receiving IV contrast for imaging:
- Ensure adequate pre-procedure hydration (IV normal saline, typically 1 mL/kg/h for 6–12 hours before and after the procedure)
- Hold metformin for 48 hours after contrast administration (risk of lactic acidosis if AKI develops)
- Use the lowest effective contrast volume
- Monitor creatinine at 48–72 hours post-procedure
Electrolyte management
- Hyperkalemia (potassium greater than 5.0 mEq/L): Administer calcium gluconate for cardiac membrane stabilization (does not lower potassium — protects the heart). Give insulin with dextrose to shift potassium intracellularly. Administer sodium polystyrene sulfonate (Kayexalate) or patiromer for GI potassium removal. Restrict dietary potassium. For refractory hyperkalemia, prepare for emergent dialysis. Review the electrolyte imbalances reference for detailed management.
- Metabolic acidosis: Administer sodium bicarbonate as ordered for pH less than 7.2. Monitor ABGs serially.
- Hyperphosphatemia and hypocalcemia: Administer phosphate binders (calcium carbonate, sevelamer) with meals. Restrict dietary phosphorus. Do not administer IV calcium unless the patient is symptomatic (tetany, seizure).
Dietary modifications
- Restrict potassium (avoid bananas, oranges, tomatoes, potatoes, salt substitutes)
- Restrict phosphorus (avoid dairy, nuts, colas, processed meats)
- Restrict sodium (less than 2 g/day) to reduce fluid retention
- Moderate protein intake — enough to prevent catabolism but not excessive (increases BUN production)
Renal replacement therapy indications: the AEIOU mnemonic
When conservative management fails, renal replacement therapy (dialysis) is indicated. The AEIOU mnemonic identifies the five major indications:
- A — Acidosis: Severe metabolic acidosis (pH less than 7.1) refractory to bicarbonate therapy
- E — Electrolytes: Life-threatening hyperkalemia unresponsive to medical management
- I — Intoxication: Toxic ingestion of a dialyzable substance (methanol, ethylene glycol, lithium, salicylates)
- O — Overload: Pulmonary edema or severe fluid overload unresponsive to diuretics
- U — Uremia: Symptomatic uremia — encephalopathy, pericarditis (uremic pericarditis is an absolute indication), bleeding diathesis
AKI vs CKD: key differences
AKI and chronic kidney disease (CKD) can coexist, and differentiating them has important implications for treatment and prognosis. The table below highlights the key distinctions.
| Feature | AKI | CKD |
|---|---|---|
| Onset | Hours to days | Months to years |
| Reversibility | Often reversible if caught early | Irreversible — progressive decline |
| Kidney size on ultrasound | Normal or enlarged | Small, shrunken (bilateral) |
| Hemoglobin/hematocrit | Usually normal (unless concurrent illness) | Anemia — decreased erythropoietin production |
| Bone disease | Absent | Renal osteodystrophy from chronic vitamin D and calcium-phosphorus imbalances |
| Creatinine trend | Rapid rise from known baseline | Elevated chronically, may fluctuate slowly |
| Urine output pattern | Oliguria → possible diuretic phase → recovery | Initially normal → progressive decline → ESRD |
| Dialysis | Temporary (if needed) | Permanent once ESRD is reached (GFR <15) |
Clinical pearl: A renal ultrasound showing small kidneys bilaterally strongly suggests CKD rather than AKI. Normal-sized or enlarged kidneys are more consistent with an acute process. Baseline creatinine values from prior records are invaluable for determining whether the current elevation is acute, chronic, or acute-on-chronic.
Complications of AKI
AKI can produce life-threatening complications that require rapid nursing recognition and intervention.
Hyperkalemia is the most immediately dangerous complication. Potassium rises because the kidneys cannot excrete it, and cellular injury releases additional intracellular potassium. EKG changes progress from tall peaked T waves to widened QRS to sine wave pattern to cardiac arrest. Any potassium above 6.0 mEq/L with EKG changes is an emergency.
Metabolic acidosis develops because the kidneys cannot excrete hydrogen ions or regenerate bicarbonate. The patient compensates with Kussmaul respirations (deep, rapid breathing). Severe acidosis (pH less than 7.1) depresses myocardial contractility and reduces the threshold for arrhythmias. Monitor ABG results for worsening acidemia.
Fluid overload and pulmonary edema occur when the kidneys cannot excrete excess volume. Assess for acute dyspnea, orthopnea, crackles on auscultation, pink frothy sputum (alveolar edema), and hypertension. This may require emergent dialysis if unresponsive to IV furosemide.
Uremia (BUN typically greater than 100 mg/dL) causes a constellation of symptoms affecting nearly every organ system: encephalopathy (confusion, seizures), pericarditis (friction rub — listen during assessment), platelet dysfunction (bleeding tendency), nausea and anorexia, and peripheral neuropathy. Uremic pericarditis specifically is an absolute indication for emergent dialysis because of the risk of cardiac tamponade.
Acute respiratory distress syndrome (ARDS) can develop secondary to fluid overload and systemic inflammation in severe AKI. The kidney-lung crosstalk in critical illness means AKI significantly increases the risk of respiratory failure — and vice versa.
Infection risk increases in AKI patients due to uremia-induced immune dysfunction. Sepsis is both a leading cause and a major complication of AKI, creating a dangerous bidirectional cycle in critically ill patients.
NCLEX tips
- FENa less than 1% = pre-renal; FENa greater than 2% = intrinsic (ATN). This is one of the most commonly tested lab distinctions. Remember: healthy tubules hold onto sodium (low FENa), damaged tubules waste sodium (high FENa).
- BUN:Cr ratio greater than 20:1 suggests pre-renal AKI. Both values rise, but BUN rises disproportionately because intact tubules reabsorb urea along with water.
- KDIGO Stage 1 includes the absolute 0.3 mg/dL creatinine rise within 48 hours. You do not need to know the patient’s baseline to diagnose AKI using this criterion.
- Muddy brown granular casts on urinalysis = ATN. This is the classic urine sediment finding for acute tubular necrosis. RBC casts suggest glomerulonephritis. WBC casts suggest interstitial nephritis or pyelonephritis.
- The AEIOU mnemonic for dialysis indications: Acidosis, Electrolytes (hyperkalemia), Intoxication, Overload (fluid), Uremia. If a question stem describes a patient with AKI and any of these refractory findings, the answer is dialysis.
- Uremic pericarditis is an absolute indication for emergent dialysis. If you hear a pericardial friction rub in an AKI patient, think dialysis immediately.
- Pre-renal AKI is confirmed by response to fluids. If creatinine improves within 24–72 hours of adequate volume resuscitation, the injury was pre-renal. No improvement suggests progression to ATN.
- Hold nephrotoxic medications in AKI. Aminoglycosides, NSAIDs, and ACE inhibitors/ARBs should be discontinued or dose-adjusted. If a question asks which medication to hold in a patient with rising creatinine, look for these drug classes.
- Calcium gluconate does not lower potassium — it stabilizes the cardiac membrane. Insulin with dextrose is the intervention that shifts potassium intracellularly. Do not confuse the two mechanisms.
- Monitor for the diuretic phase of ATN (urine output 3–6 L/day). During this recovery phase, the patient is at risk for dehydration and hypokalemia — the opposite of the oliguric phase. Replace fluids and electrolytes as ordered.
- Contrast-induced nephropathy prevention = IV hydration before and after. Pre-procedure IV normal saline is the primary preventive measure. Hold metformin for 48 hours after contrast.
- AKI vs CKD: check kidney size on ultrasound. Small kidneys = CKD. Normal or large kidneys = AKI. Also check for anemia — chronic kidney disease causes anemia from decreased erythropoietin, while AKI typically does not.
References
- Acute Kidney Injury. StatPearls [Internet]. National Library of Medicine. NCBI Bookshelf NBK441896.
- Acute Kidney Injury (Nursing). StatPearls. NCBI Bookshelf NBK568593.
- Prerenal Kidney Failure. StatPearls. NCBI Bookshelf NBK560678.
- KDIGO Clinical Practice Guideline for Acute Kidney Injury. Kidney International Supplements. 2012;2(1):1–138.
- Bellomo R, Ronco C, Kellum JA, et al. Acute renal failure — definition, outcome measures, animal models, fluid therapy and information technology needs. Critical Care. 2004;8(4):R204–R212 (RIFLE criteria).