Acute Kidney Injury (AKI): Symptoms, Causes, Diagnosis, and Early Management

Acute Kidney Injury is a clinically important renal topic. AKI is present if any one of the following occurs:

What Is Acute Kidney Injury?

Acute Kidney Injury (AKI) is an abrupt decline in renal function causing impaired excretion of nitrogenous waste and disturbance of fluid, electrolyte, and acid-base homeostasis.

KDIGO Diagnostic Criteria

AKI is present if any one of the following occurs:

1. Increase in serum creatinine by ≥0.3 mg/dL (≥26.5 µmol/L) within 48 hours, or
2. Increase in serum creatinine to ≥1.5 times baseline within the prior 7 days, or
3. Urine volume <0.5 mL/kg/hour for 6 hours.

Urine output may become abnormal before serum creatinine rises.

Why Acute Kidney Injury Matters

• Define AKI using KDIGO criteria.
• Explain the anatomical and physiological basis of AKI.
• Classify AKI into pre-renal, intrinsic renal, and post-renal causes.
• Use urinalysis, urine electrolytes, creatinine trend, urine output, and imaging to localize the cause.

Definition and Diagnostic Criteria

Acute Kidney Injury (AKI) is an abrupt decline in renal function causing impaired excretion of nitrogenous waste and disturbance of fluid, electrolyte, and acid-base homeostasis.

KDIGO Diagnostic Criteria

AKI is present if any one of the following occurs:

1. Increase in serum creatinine by ≥0.3 mg/dL (≥26.5 µmol/L) within 48 hours, or
2. Increase in serum creatinine to ≥1.5 times baseline within the prior 7 days, or
3. Urine volume <0.5 mL/kg/hour for 6 hours.

Urine output may become abnormal before serum creatinine rises.

For a formal framework, see the KDIGO guidance.

Classification and Staging

A. Etiological classification

1. Pre-renal AKI
2. Intrinsic renal AKI
3. Post-renal AKI

B. Severity classification: KDIGO Staging

Common Causes of Acute Kidney Injury

1. Pre-renal causes

True volume depletion

• vomiting
• diarrhea
• hemorrhage
• burns
• excessive diuresis
• poor oral intake
• pancreatitis / third spacing

Reduced effective arterial blood volume

• heart failure
• cirrhosis
• nephrotic syndrome
• sepsis

Hemodynamic / renovascular causes

• bilateral renal artery stenosis
• ACE inhibitor / ARB effect in susceptible states
• NSAID-induced afferent vasoconstriction
• severe hypotension / shock

2. Intrinsic renal causes

a. Acute tubular necrosis (ATN)

• ischemia
• sepsis
• nephrotoxins:
• aminoglycosides
• radiocontrast
• amphotericin B
• cisplatin
• myoglobin
• hemoglobin

b. Acute interstitial nephritis (AIN)

• drugs:
• beta-lactams
• cephalosporins
• PPIs
• NSAIDs
• rifampicin
• infections
• autoimmune disorders

c. Glomerular diseases

• rapidly progressive glomerulonephritis
• lupus nephritis
• post-infectious GN
• IgA nephropathy
• anti-GBM disease

d. Vascular causes

• vasculitis
• thrombotic microangiopathy (TTP/HUS)
• malignant hypertension
• DIC
• atheroembolic disease
• renal vein thrombosis / renal artery occlusion

3. Post-renal causes

• benign prostatic hyperplasia
• prostate carcinoma
• urethral stricture
• neurogenic bladder
• bladder neck obstruction
• stones
• pelvic malignancy
• retroperitoneal fibrosis
• bilateral ureteric obstruction
• obstruction of a solitary functioning kidney

If you want to compare this with chronic kidney disease (CKD), you can also browse the Nephrology category.

Acute Kidney Injury Symptoms and Clinical Features

Symptoms

• oliguria or anuria
• dark urine
• hematuria
• frothy urine
• edema
• breathlessness
• nausea, vomiting
• lethargy
• confusion
• flank or loin pain
• fever / rash / arthralgia in immune or drug-related causes
• lower urinary tract symptoms in obstruction

Signs

• dehydration: tachycardia, postural hypotension, dry mucosa
• fluid overload: raised JVP, peripheral edema, basal crackles
• sepsis / shock signs
• bladder distension
• enlarged prostate on examination
• purpura / rash / vasculitic features
• hypertensive emergency
• altered sensorium in severe uremia

Diagnosis and Evaluation of Acute Kidney Injury: A Practical Approach

Stepwise bedside approach

1. Confirm AKI

• compare current creatinine with baseline
• assess urine output trend

1. Look for immediate threats

• hyperkalemia
• pulmonary edema
• severe metabolic acidosis
• uremic encephalopathy / pericarditis
• anuria / complete obstruction
• sepsis / shock

1. Classify mechanism

• pre-renal?
• intrinsic?
• post-renal?

1. Take focused history

• fluid losses
• sepsis
• nephrotoxic drugs
• ACEi/ARB/NSAID/diuretic exposure
• contrast exposure
• urinary symptoms
• rash, arthralgia, hemoptysis

1. Examine volume status and systemic clues
2. Order core investigations

• U&E, creatinine, bicarbonate, CBC, urinalysis, urine microscopy, ECG, ultrasound KUB

1. Treat cause + monitor dynamically

Rapid practical algorithm

“`mermaid

flowchart TD

A[Rising creatinine or oliguria] –> B[Confirm AKI using KDIGO]

B –> C[Check emergencies: K, edema, acidosis, uremia, anuria]

C –> D[History + exam + drug review]

D –> E[Urinalysis and microscopy]

E –> F[Ultrasound KUB to exclude obstruction]

F –> G{Likely category?}

G –>|Pre-renal| H[Restore perfusion / treat cause]

G –>|Intrinsic| I[Define subtype: ATN AIN GN Vascular]

G –>|Post-renal| J[Relieve obstruction urgently]

H –> K[Monitor creatinine, urine output, potassium]

I –> K

J –> K

K –> L{AEIOU indication?}

L –>|Yes| M[Urgent dialysis / RRT]

L –>|No| N[Continue supportive care]

“`

Bedside

• strict intake-output chart
• daily weight when appropriate
• urine dipstick
• ECG
• bladder scan if retention suspected

Blood tests

• serum creatinine and urea
• electrolytes: Na, K, Cl, HCO₃⁻
• calcium, phosphate, magnesium
• CBC
• CRP / ESR
• blood cultures if septic
• ABG / VBG if acidosis or critical illness
• CK if rhabdomyolysis suspected
• hemolysis screen / LDH if TMA suspected

Urine investigations

• dipstick: blood, protein, leukocytes, nitrite
• urine microscopy:
• muddy brown granular casts
• RBC casts
• WBC casts
• crystals
• urine sodium
• urine osmolality
• FeNa
• FeUrea when diuretics confound FeNa
• urine protein:creatinine ratio where relevant

Imaging

• Ultrasound KUB: core imaging to exclude obstruction
• CT KUB if stones strongly suspected
• Doppler if renovascular cause suspected

Immunological / special investigations

• ANA
• anti-dsDNA
• ANCA
• anti-GBM
• complements C3/C4
• hepatitis serology
• ASO / infection-related tests where relevant
• serum electrophoresis if myeloma suspected
• renal biopsy when indicated

For a patient-friendly overview, see the NIDDK page.

Interpretation Frameworks

1. Creatinine interpretation

• Creatinine may lag behind actual injury.
• A “normal” creatinine may still represent AKI in a frail patient with low baseline muscle mass.
• Always compare with baseline and trend.

2. Urine output interpretation

• Oliguria is an early warning sign.
• Non-oliguric AKI can occur, especially in nephrotoxic ATN.

3. Urinalysis / microscopy pattern recognition

4. Pre-renal vs ATN framework

5. Post-renal clues

• LUTS, bladder distension, enlarged prostate
• anuria or fluctuating oliguria
• hydronephrosis on ultrasound
• rising creatinine relieved after decompression

6. Acid-base interpretation in AKI

AKI commonly causes metabolic acidosis due to impaired hydrogen ion and ammonium excretion. Severe acidosis worsens hemodynamic instability and hyperkalemia.

7. Potassium interpretation

Hyperkalemia is the most immediately lethal biochemical complication of AKI.

ECG changes in hyperkalemia

• peaked T waves
• PR prolongation
• QRS widening
• sine-wave pattern
• bradyarrhythmias / VF / asystole

Differential Diagnosis

• CKD
• AKI on CKD
• obstructive uropathy with partial retention
• pseudo-creatinine rise (rare)
• hepatorenal syndrome
• cardiorenal syndrome

AKI vs CKD

\*Exceptions include diabetes, amyloidosis, and polycystic kidney disease.

Management of Acute Kidney Injury

Core principles

Management always has two components:

1. Stabilize the patient
2. Treat the cause

A. Immediate stabilization

• ABC assessment
• cardiac monitoring if hyperkalemia suspected
• IV access and urgent blood tests
• strict urine output monitoring
• stop ongoing renal insults

B. Drug review immediately

Hold or review, depending on context:

• NSAIDs
• ACE inhibitors
• ARBs
• diuretics in hypovolemia
• metformin in significant unstable AKI
• aminoglycosides and other nephrotoxins
• contrast exposure unless essential

C. Fluid management

If hypovolemic

• use isotonic crystalloid
• reassess frequently:
• pulse
• BP
• JVP
• crackles
• urine output
• mental state

If overloaded

• fluid restriction
• oxygen if needed
• loop diuretic if responsive and appropriate
• dialysis if refractory pulmonary edema

Giving repeated fluid boluses without reassessment is a common exam and ward mistake.

D. Cause-specific treatment

Pre-renal AKI

• replace fluid losses
• treat sepsis early
• treat bleeding
• improve effective circulation in heart failure or cirrhosis
• remove precipitating drugs

ATN

• supportive care
• avoid further nephrotoxins
• optimize perfusion pressure
• treat sepsis / ischemia / toxin source
• no magic drug reverses established ATN

AIN

• stop offending drug
• supportive care
• consider steroids in selected cases with specialist input

GN / vasculitis

• urgent nephrology review
• immunological work-up
• biopsy when indicated
• steroids / cyclophosphamide / rituximab / plasmapheresis depending on cause

Post-renal AKI

• bladder catheterization for lower tract obstruction
• nephrostomy or ureteric stenting for upper tract obstruction if needed
• urology referral
• monitor for post-obstructive diuresis

E. Electrolyte management

Hyperkalemia

If severe or with ECG changes:

1. IV calcium gluconate to stabilize myocardium
2. Insulin + dextrose
3. Nebulized salbutamol
4. Sodium bicarbonate if significant acidosis
5. Remove potassium:

• diuretics if appropriate
• potassium binders in selected scenarios
• dialysis if refractory / severe

Sodium disorders

Manage according to symptoms, severity, and chronicity.

Phosphate / calcium problems

Treat clinically significant abnormalities and consider RRT if severe.

F. Acid-base management

• correct underlying cause
• bicarbonate may be considered in selected severe metabolic acidosis
• dialysis if acidosis is severe or refractory

G. Nutrition and supportive care

• avoid excess potassium and phosphate intake if elevated
• provide adequate calories
• adjust protein strategy according to clinical state
• VTE and pressure area prevention in immobile patients

H. Renal Replacement Therapy (RRT)

Dialysis indications: AEIOU

• Acidosis refractory to medical therapy
• Electrolyte disturbance, especially refractory hyperkalemia
• Ingestions / intoxications with dialyzable toxins
• Overload causing refractory pulmonary edema
• Uremic complications: encephalopathy, pericarditis, severe symptomatic uremia

Dialysis is initiated for indications, not simply because creatinine is “very high.”

Complications of Acute Kidney Injury

• hyperkalemia
• metabolic acidosis
• pulmonary edema
• hypertension / volume overload
• uremic encephalopathy
• uremic pericarditis
• infection
• progression to CKD
• death in severe systemic illness

FCPS/MRCP Exam Pearls and Clinical Boxes

Frequently Asked Questions About Acute Kidney Injury

Can acute kidney injury improve or be stabilized?

That depends on the cause and the stage of disease. Early recognition, risk-factor control, and prompt treatment of complications often improve outcomes and may slow progression substantially.

When should acute kidney injury become urgent?

Urgent escalation is needed when severe complications, rapid deterioration, marked biochemical abnormalities, or major cardiorespiratory compromise appear.

Conclusion

AKI is a common and potentially reversible emergency syndrome. The safest exam and clinical approach is: recognize it early, classify the mechanism, look for immediately dangerous complications, review drugs, assess volume, exclude obstruction, interpret urine findings intelligently, and dialyze when AEIOU criteria are met. A structured bedside approach, early recognition of complications, and appropriate nephrology follow-up remain central to safe care.

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Medical disclaimer: This article is for medical education and professional awareness. Clinical decisions should always be individualized according to the patient’s condition, local protocols, and specialist advice when necessary.