AKI nursing reference: assessment, staging, and interventions

Acute kidney injury (AKI) is a sudden decline in kidney function that occurs over hours to days, affecting up to 15% of all hospitalized patients and more than 50% of ICU admissions. It carries a hospital mortality of 40–50% and represents one of the highest-stakes conditions in acute care nursing. The 2012 KDIGO (Kidney Disease: Improving Global Outcomes) guidelines provide the universal diagnostic and staging framework used in all clinical settings today.

This reference page is organized as a fast-access clinical resource: KDIGO staging criteria first, then the three AKI categories with comparison tables, then diagnostics, medical management, nursing interventions, and high-yield NCLEX scenarios. Use it alongside the nursing lab values cheat sheet and the electrolyte imbalances reference for complete renal coverage.

KDIGO AKI staging criteria

The KDIGO system defines AKI as any of the following: serum creatinine rise ≥0.3 mg/dL within 48 hours, serum creatinine ≥1.5× baseline within 7 days, or urine output <0.5 mL/kg/h for ≥6 hours. Once AKI is diagnosed, staging uses whichever criterion — creatinine or urine output — assigns the higher stage.

Key staging points:

• Stage 1 captures the 0.3 mg/dL absolute rise — this catches early AKI even when the percentage increase is small (e.g., creatinine 0.8 → 1.1 mg/dL within 48 hours meets Stage 1)
• Any patient requiring dialysis is automatically Stage 3, regardless of creatinine level
• When creatinine and urine output criteria disagree, assign the higher stage
• Urine output calculation requires the patient’s weight: for a 70 kg patient, the oliguria threshold is 35 mL/h

AKI classification: prerenal, intrinsic, and postrenal

Prerenal AKI

Prerenal AKI accounts for 60–70% of community-acquired cases. The kidneys themselves are healthy — they receive insufficient perfusion pressure to filter effectively. The intact tubules respond appropriately, reabsorbing sodium and water aggressively to compensate. This produces concentrated, low-sodium urine. If perfusion is restored within 24–72 hours, creatinine returns to baseline. Prolonged hypoperfusion causes ischemic tubular cell death, converting prerenal AKI to ATN.

Intrinsic AKI (ATN)

Acute tubular necrosis (ATN) is the most common form of intrinsic AKI (~45% of all cases). Damaged tubular cells cannot concentrate urine or reabsorb sodium — producing dilute, high-sodium urine with the pathognomonic muddy brown granular casts. ATN progresses through four phases:

1. Onset: initial insult (ischemia, nephrotoxin)
2. Oliguric phase: urine output <400 mL/day for 1–3 weeks; highest risk for hyperkalemia, metabolic acidosis, and fluid overload
3. Diuretic phase: urine output rises to 3–6 L/day as tubules recover function but cannot yet concentrate; risk shifts to dehydration and hypokalemia
4. Recovery phase: gradual GFR restoration over weeks to months; some patients have permanent GFR reduction

Other intrinsic causes: acute interstitial nephritis (AIN — drug-induced, typically penicillins, NSAIDs, PPIs; presents with eosinophiluria, WBC casts, fever, rash), acute glomerulonephritis (RBC casts), renal vascular injury (HUS/TTP, renal artery thrombosis).

Postrenal AKI

Obstruction downstream from the kidneys raises collecting-system pressure, opposing GFR. Bilateral obstruction (or unilateral in a single-kidney patient) is required to raise serum creatinine. Once relieved, recovery is typically rapid. After relief of chronic obstruction, a post-obstructive diuresis may occur — monitor closely for volume depletion.

Clinical presentation

Fluid and urine output changes

• Oliguria: urine output <0.5 mL/kg/h (KDIGO threshold) or <400 mL/day — the earliest clinical indicator of AKI
• Anuria: urine output <100 mL/day — severe; suggests Stage 3 or complete obstruction
• Fluid overload: peripheral edema, pulmonary crackles, orthopnea, hypertension, JVD, weight gain (1 kg ≈ 1 L fluid)
• Tea-colored or brown urine: myoglobinuria in rhabdomyolysis, or hemoglobinuria; signals pigment nephropathy
• Foamy urine: significant proteinuria indicating glomerular damage

Electrolyte disturbances

Hyperkalemia is the most immediately life-threatening complication of AKI. Potassium rises because impaired GFR reduces renal excretion, and cellular injury releases intracellular potassium. EKG changes progress in sequence: tall peaked T waves → PR prolongation → widened QRS → sine-wave pattern → ventricular fibrillation. Any potassium above 6.0 mEq/L with EKG changes is a medical emergency.

Hyponatremia can develop from water retention proportionally exceeding sodium retention, or from administration of hypotonic fluids. Monitor serum sodium daily and correct cautiously (rapid correction risks osmotic demyelination syndrome).

Metabolic acidosis develops because impaired kidneys cannot excrete hydrogen ions or regenerate bicarbonate. Patients compensate with Kussmaul respirations (deep, labored breathing). Severe acidosis (pH <7.1) depresses myocardial contractility.

Hyperphosphatemia arises from reduced renal phosphorus excretion. Elevated phosphorus binds calcium, producing hypocalcemia — monitor for Chvostek’s sign (facial twitch with CN VII tap) and Trousseau’s sign (carpopedal spasm with BP cuff inflation).

BUN and creatinine rise

BUN and creatinine accumulate as GFR falls. A creatinine of 2.0 mg/dL represents approximately 50% reduction in GFR from normal. BUN rises faster than creatinine in prerenal AKI (ratio >20:1) because intact tubules reabsorb urea along with water; in ATN the rise is proportional (ratio 10–15:1).

Uremic symptoms

As BUN exceeds 80–100 mg/dL, uremic toxins affect multiple organ systems: nausea, vomiting, metallic taste, anorexia, fatigue, confusion, asterixis (flapping hand tremor with wrists extended), pericardial friction rub (uremic pericarditis — a dialysis emergency), pruritus from phosphate crystal deposition, and platelet dysfunction causing bleeding tendency.

Diagnostics

Laboratory panel

FENa formula: FENa (%) = (urine Na × plasma Cr) / (plasma Na × urine Cr) × 100

FENa caveats: Results are unreliable in patients receiving loop diuretics (diuretics force natriuresis regardless of tubular function). Important exceptions where FENa may be <1% despite intrinsic AKI: contrast-induced nephropathy, myoglobinuria (rhabdomyolysis), early obstructive AKI, and acute glomerulonephritis.

Imaging

Renal ultrasound is the first-line imaging study for AKI. It evaluates kidney size (normal or enlarged = AKI; small/shrunken = CKD), identifies hydronephrosis (post-renal obstruction), and rules out structural lesions. A post-void residual bladder scan is performed if lower urinary tract obstruction (BPH, stricture) is suspected.

Renal biopsy

Indicated when the diagnosis remains unclear after clinical evaluation and imaging — particularly when intrinsic causes other than ATN are suspected (glomerulonephritis, vasculitis, thrombotic microangiopathy). Contraindicated with a single kidney, uncorrected coagulopathy, or severe hypertension. Post-procedure monitoring: bedrest, serial vital signs, urine color checks, CBC at 4–6 hours.

Medical management

Prerenal AKI: fluid resuscitation

Isotonic crystalloid (0.9% normal saline or lactated Ringer’s) is administered IV to restore renal perfusion. Monitor urine output response — improvement within hours suggests prerenal etiology. Fluid challenges in heart failure must be given cautiously to avoid precipitating pulmonary edema; in these patients, small boluses (250–500 mL) with reassessment are preferred over large-volume resuscitation.

Intrinsic/oliguric AKI: fluid restriction and diuretics

Patients with established ATN and fluid overload require restriction (typically prior day’s output + 500 mL for insensible losses) and loop diuretics. Furosemide (Lasix) is first-line; may be given as IV bolus or continuous infusion. Furosemide does not change the course of AKI or reduce the need for dialysis, but it facilitates fluid management and may prevent the need for emergent dialysis in mild-moderate overload.

Contrast-induced AKI: prevention protocol

Contrast-induced AKI typically peaks 48–72 hours post-contrast and resolves within 5–7 days in most patients. Prevention:

• IV isotonic saline: 1 mL/kg/h for 6–12 hours before and after contrast administration
• Hold nephrotoxins before the procedure: NSAIDs, aminoglycosides
• Hold metformin 48 hours after contrast (risk of lactic acidosis if AKI develops)
• Minimize contrast volume; use iso-osmolar agents in high-risk patients

Rhabdomyolysis AKI: aggressive hydration

Myoglobin released from damaged muscle is directly nephrotoxic, causing pigment nephropathy and tubular obstruction. Management:

• Aggressive IV fluid resuscitation: 250–500 mL/hr isotonic saline
• Urine output goal: 200–300 mL/hr (to flush myoglobin)
• Monitor urine color — tea or cola color indicates myoglobinuria; clears as myoglobin is flushed
• Serial CK, potassium, and creatinine monitoring
• Urinary alkalinization (sodium bicarbonate in IV fluids) may reduce myoglobin precipitation in tubules, though evidence is mixed

Dialysis indications: AEIOU mnemonic

Renal replacement therapy (RRT) is initiated when conservative management is insufficient. The AEIOU indications apply regardless of creatinine level — dialysis is a clinical decision, not a lab threshold decision.

• A — Acidosis: pH <7.1 refractory to bicarbonate therapy
• E — Electrolytes: life-threatening hyperkalemia unresponsive to medical management
• I — Intoxication/Ingestion: dialyzable toxins — methanol, ethylene glycol, lithium, salicylates
• O — Overload: pulmonary edema or severe fluid overload unresponsive to IV diuretics
• U — Uremia: symptomatic uremia — encephalopathy, pericarditis (uremic pericarditis is an absolute indication), platelet dysfunction with bleeding

Nursing assessment and interventions

Priority assessments

Hourly intake and output (I&O) is the most important nursing action in AKI. Urine output <0.5 mL/kg/h for 6 hours meets KDIGO Stage 1 criteria. Calculate the threshold by weight: a 70 kg patient must produce >35 mL/h to stay below the Stage 1 threshold. Document every drain, emesis, and insensible loss source.

Daily weights are performed at the same time with the same scale and same clothing. A 1 kg weight gain = 1 L fluid retained. Sudden weight gain signals fluid accumulation; sudden weight loss during the diuretic phase signals dehydration risk.

Fluid status assessment each shift:

• Auscultate lungs: crackles indicate pulmonary edema
• Inspect for peripheral edema: grade 1+ (slight) to 4+ (deep, non-pitting)
• Assess JVD with patient at 45 degrees
• Check vital signs: hypertension (fluid overload) vs hypotension/tachycardia (dehydration in prerenal AKI)
• Skin turgor and mucous membranes for hydration status

Electrolyte monitoring and hyperkalemia management

Monitor potassium with each lab draw. When potassium exceeds 5.5 mEq/L, perform a 12-lead EKG to assess for cardiac changes and notify the provider.

Hyperkalemia management sequence:

1. Calcium gluconate IV — cardiac membrane stabilization, onset within minutes; does NOT lower potassium
2. Insulin (regular) + dextrose IV — shifts K+ intracellularly, lowers serum K+ by 0.5–1.5 mEq/L within 20–30 minutes
3. Sodium bicarbonate IV — shifts K+ intracellularly (especially effective in metabolic acidosis)
4. Sodium polystyrene sulfonate (Kayexalate) or patiromer — GI potassium removal over hours
5. Dialysis — definitive removal when refractory or when other AEIOU indications coexist

Review the electrolyte imbalances reference for full hyperkalemia management detail.

Nephrotoxin avoidance

Review the medication administration record at each shift and flag:

• Aminoglycosides (gentamicin, tobramycin, amikacin) — monitor trough levels; dose adjustments required in AKI
• NSAIDs (ibuprofen, ketorolac, naproxen) — reduce prostaglandin-mediated afferent arteriolar dilation; worsen prerenal AKI
• ACE inhibitors and ARBs — reduce efferent arteriolar tone; decrease GFR in states of low renal perfusion (hold in acute hypoperfusion)
• IV contrast — hold for 48 hours post-exposure in high-risk patients; monitor creatinine at 48–72 hours
• Metformin — hold when creatinine rises or IV contrast given (lactic acidosis risk)
• Amphotericin B — directly nephrotoxic; use lipid formulations when possible

Collaborate with pharmacy for dose adjustments on renally cleared medications (antibiotics, anticoagulants, digoxin, opioids).

Skin and fluid assessment

Fluid overload causes skin to become taut, shiny, and fragile over edematous areas. Pressure injury risk increases substantially. Reposition every 2 hours, use pressure-redistributing mattresses, protect bony prominences, and apply barrier cream to intertriginous areas. Uremia also causes skin dryness and pruritus from phosphate crystal deposition — use emollient lotions and avoid scratching education.

Diet and fluid modifications

• Fluid restriction (if oliguric/fluid-overloaded): calculate allowance based on prior day output + 500 mL; distribute across meals and medication administrations
• Potassium restriction: avoid bananas, oranges, tomatoes, potatoes, salt substitutes
• Phosphorus restriction: limit dairy, nuts, cola drinks, processed meats; administer phosphate binders (calcium carbonate, sevelamer) with meals
• Sodium restriction: <2 g/day to reduce fluid retention
• Protein: moderate intake to prevent catabolism without excessive BUN production; nutrition consult for guidance in ICU patients

AKI vs CKD: key distinctions

Key pearl: A renal ultrasound showing bilaterally small kidneys strongly supports CKD, while normal or enlarged kidneys are more consistent with an acute process. Baseline creatinine records are invaluable — AKI requires a change from baseline; CKD is a chronic, stable elevation. For full CKD management, see the CKD and ESRD nursing reference.

NCLEX scenarios

Scenario 1: prerenal AKI from dehydration

A 68-year-old patient is admitted from a long-term care facility with three days of vomiting and diarrhea. Vital signs: BP 88/54, HR 118, dry mucous membranes. Labs: creatinine 2.4 mg/dL (baseline 0.9 mg/dL), BUN 72 mg/dL, urine sodium 14 mEq/L, urine specific gravity 1.028, FENa 0.6%.

Which intervention takes priority?

IV isotonic fluid resuscitation. The elevated BUN:creatinine ratio (30:1), low urine sodium, concentrated urine, and low FENa all point to prerenal AKI from volume depletion. Healthy tubules are aggressively retaining sodium and water. The priority is restoring renal perfusion — fluids, not restriction. Monitor for urine output improvement within 6–12 hours; persistent oliguria after adequate resuscitation suggests progression to ATN.

Scenario 2: rhabdomyolysis AKI

A 25-year-old is brought to the ED after being found unresponsive at home following suspected drug overdose. He has been on a hard floor for an estimated 18 hours. Labs: CK 42,000 U/L, creatinine 3.1 mg/dL, potassium 6.2 mEq/L. Urine is tea-colored.

What is the nursing priority?

Aggressive IV fluid resuscitation to flush myoglobin from the renal tubules. The goal urine output is 200–300 mL/hr to clear the pigment. Tea-colored urine confirms myoglobinuria. Potassium of 6.2 mEq/L requires an immediate EKG and hyperkalemia management (calcium gluconate first for cardiac stabilization). Serial CK and creatinine monitoring every 4–6 hours. Position change every 2 hours given immobility duration. Watch for compartment syndrome in crush injury patients.

Scenario 3: contrast-induced AKI

A 72-year-old patient with hypertension and diabetes has a CT angiogram with IV contrast. Creatinine before the procedure was 1.3 mg/dL. At 48 hours post-procedure, creatinine is 1.9 mg/dL; at 72 hours it is 2.2 mg/dL.

What nursing actions are indicated?

This presentation — creatinine peak at 48–72 hours post-contrast — is the classic timing of contrast-induced AKI. Nursing actions: hold nephrotoxins (NSAIDs, metformin if not already held), maintain IV fluid hydration as ordered, monitor creatinine daily until trending downward, monitor urine output and restrict nephrotoxins in subsequent care. Teach the patient that metformin is withheld 48 hours after contrast. Most cases resolve within 5–7 days without requiring dialysis.

Scenario 4: hyperkalemia management in AKI

A patient with ATN from aminoglycoside nephrotoxicity has potassium 6.8 mEq/L. The EKG shows widened QRS complexes and peaked T waves. The provider orders: calcium gluconate IV, regular insulin with D50W IV, sodium bicarbonate IV, and sodium polystyrene sulfonate oral.

In what order do the nurse administer these?

1. Calcium gluconate first — immediate cardiac membrane stabilization given EKG changes (widened QRS). Acts within minutes. Does not lower potassium — it protects the heart while other agents work.
2. Insulin with D50W — shifts potassium intracellularly within 20–30 minutes; lowers serum K+ by 0.5–1.5 mEq/L.
3. Sodium bicarbonate — shifts potassium intracellularly by correcting acidosis; adjunct to insulin.
4. Sodium polystyrene sulfonate — removes potassium from the body via GI excretion over hours; the slowest-acting of the four.

Scenario 5: dialysis indications in AKI (AEIOU)

A patient with Stage 3 AKI has the following: pH 7.06, bicarbonate 10 mEq/L, potassium 7.2 mEq/L despite two rounds of insulin/dextrose and sodium polystyrene sulfonate, urine output <20 mL over the past 12 hours, and a new pericardial friction rub on auscultation.

What action does the nurse anticipate?

Emergent renal replacement therapy. This patient meets multiple AEIOU indications simultaneously: severe metabolic acidosis (pH 7.06 < 7.1), refractory hyperkalemia (7.2 mEq/L despite medical management), anuria (Stage 3 urine output criterion), and uremic pericarditis (absolute indication — pericardial friction rub). The nurse should notify nephrology immediately, prepare the patient for vascular access, and document all current hemodynamic status. Uremic pericarditis carries a risk of cardiac tamponade — urgent dialysis is the only definitive treatment.

Scenario 6: AKI vs CKD differentiation

A 55-year-old patient with type 2 diabetes presents with creatinine 4.8 mg/dL. The chart shows a creatinine of 4.2 mg/dL from two years ago. Renal ultrasound shows bilateral kidneys measuring 8 cm (normal 9–12 cm). Hemoglobin is 9.1 g/dL.

Is this AKI, CKD, or AKI-on-CKD?

This presentation is most consistent with CKD, with possible AKI-on-CKD if a specific precipitant exists. Evidence for CKD: bilaterally small kidneys on ultrasound, normocytic anemia (decreased erythropoietin), and chronically elevated creatinine from records two years prior. The current creatinine is only modestly elevated above the prior value, making an acute-on-chronic process possible — a new precipitant (infection, dehydration, contrast exposure, new nephrotoxin) could explain the incremental rise. Management: identify and correct any acute precipitant, avoid nephrotoxins, and assess for CKD complications. See the CKD and ESRD nursing reference for full CKD management.

Common confusions

FENa <1% does NOT always mean prerenal. Several intrinsic causes can produce a low FENa: contrast-induced AKI, myoglobinuria (rhabdomyolysis), acute glomerulonephritis, and early obstruction. FENa must always be interpreted in clinical context. In patients receiving diuretics, FENa is unreliable — use FEUrea <35% to identify prerenal etiology instead.

Oliguria vs anuria thresholds:

• Oliguria = urine output <400 mL/day (or <0.5 mL/kg/h)
• Severe oliguria = <100–200 mL/day
• Anuria = <100 mL/day (KDIGO Stage 3 criterion is <50 mL over 12 hours or complete anuria)
• Anuria is more commonly postrenal (obstruction) or end-stage ATN; true anuria from prerenal AKI is rare

Calcium gluconate does not lower potassium. It stabilizes the cardiac membrane by raising the action potential threshold. The question “which intervention lowers potassium the fastest?” — the answer is insulin with dextrose (intracellular shift, onset 20–30 min) or dialysis (for immediate removal). Calcium gluconate is given first in emergencies but it is not a potassium-lowering agent.

BUN:Cr ratio >20:1 can occur without AKI. High-protein diet, GI bleeding, and catabolic states all elevate BUN disproportionately without necessarily indicating prerenal kidney injury. Use the ratio in context: the patient must have rising creatinine (meeting AKI criteria) for the ratio to be interpretively useful.

Diuretic phase of ATN is a high-risk period. Nursing students often focus on the oliguric phase as the danger zone, but the diuretic phase (urine output 3–6 L/day) carries its own risks: severe volume depletion and hypokalemia. Replace fluids and electrolytes as ordered and monitor for orthostatic hypotension, muscle cramps, and dysrhythmias.

References

• KDIGO Clinical Practice Guideline for Acute Kidney Injury. Kidney International Supplements. 2012;2(1):1–138.
• Acute Kidney Injury. StatPearls [Internet]. National Library of Medicine. NCBI Bookshelf NBK441896.
• Acute Kidney Injury (Nursing). StatPearls. NCBI Bookshelf NBK568593.
• Rhabdomyolysis. StatPearls. NCBI Bookshelf NBK448168.
• Contrast-Induced Nephropathy. StatPearls. NCBI Bookshelf NBK448066.
• Kellum JA, Lameire N, KDIGO AKI Guideline Work Group. Diagnosis, evaluation, and management of acute kidney injury. Critical Care. 2013;17:1–15.