Acute kidney injury (KDIGO classification) is a clinical syndrome defined by a rapid deterioration of renal function occurring over hours to days, leading to the failure of the kidneys to excrete nitrogenous waste products, maintain fluid homeostasis, and regulate electrolyte and acid-base balance 2,5. As we walk the wards today, remember that AKI is rarely an isolated disease; rather, it is a common final pathway for numerous systemic insults—ranging from septic shock and major surgery to nephrotoxic drug exposure 4,6. The implementation of the Kidney Disease: Improving Global Outcomes (KDIGO) criteria has unified global definitions, allowing clinicians to stage severity accurately using standardized serum creatinine elevations and urine output thresholds 3.
1. Etiology & Risk Factors
- Prerenal Etiologies (Functional Renal Hypoperfusion): Hypovolaemia (haemorrhage, severe dehydration, gastrointestinal losses), systemic vasodilation (sepsis, anaphylaxis, cirrhosis/hepatorenal syndrome), and renal vasoconstriction (NSAIDs, calcineurin inhibitors, renal artery stenosis) 4,5.
- Intrarenal Etiologies (Intrinsic Parenchymal Damage): Acute tubular necrosis (ATN) resulting from prolonged ischaemia or nephrotoxins (aminoglycosides, iodinated contrast agents, myoglobin from rhabdomyolysis), acute interstitial nephritis (AIN caused by antibiotics like penicillins or proton pump inhibitors), and glomerular or vascular pathologies (glomerulonephritis, thrombotic microangiopathy) 1,4.
- Postrenal Etiologies (Urinary Tract Obstruction): Bilateral ureteric obstruction (malignancy, retroperitoneal fibrosis, calculi), bladder outlet obstruction (benign prostatic hyperplasia, neurogenic bladder, urethral stricture), or blocked indwelling urinary catheters 4,6.
- Patient-Specific Risk Factors: Advanced age, pre-existing chronic kidney disease (CKD), diabetes mellitus, cardiovascular disease, malignancy, and exposure to multiple concurrent nephrotoxic agents in intensive care settings 5,6.
2. Pathophysiology
The pathophysiology of AKI depends fundamentally on its anatomical classification, though cellular cross-talk between vascular endothelial cells and tubular epithelial cells is central to intrinsic injury 4,5. In prerenal states, decreased effective circulating volume triggers compensatory autoregulatory mechanisms—specifically, afferent arteriolar vasodilation mediated by prostaglandins and efferent arteriolar vasoconstriction mediated by angiotensin II—to preserve glomerular filtration rate (GFR). When these homeostatic mechanisms are overwhelmed by profound hypotension or disrupted by medications (such as NSAIDs blocking prostaglandins or ACE inhibitors dilating the efferent arteriole), GFR plummets 4.
In intrinsic acute tubular necrosis (ATN), the cellular cascade involves sustained ischaemia or direct tubular toxicity. This causes ATP depletion, disruption of actin cytoskeleton networks, loss of cell polarity, and detachment of tubular cells from the basement membrane. These sloughed cells, along with Tamm-Horsfall mucoprotein, form obstructing casts within the tubular lumen, raising intratubular pressure and further depressing net filtration. Simultaneously, local inflammation, oxidative stress, and reperfusion injury amplify renal tubular apoptosis and necrosis 4,5.
3. Clinical Presentation & Classic Clinical Signs
- History & Cardinal Symptoms: Patients may present asymptomatically with routine lab abnormalities, or report acute oliguria (urine output < 0.5 mL/kg/h) or anuria. In severe cases, symptoms of uraemia develop, including nausea, vomiting, metallic taste, lethargy, confusion, generalized pruritus, and fluid overload symptoms such as dyspnoea and orthopnoea 5,6.
- Physical Examination & Named Clinical Signs: Look for signs of volume status: dry mucous membranes, poor skin turgor, and postural hypotension in prerenal hypovolaemia; or peripheral oedema, raised jugular venous pressure (JVP), bilateral basal pulmonary crackles, and an S3 gallop in fluid-overloaded intrarenal or postrenal states. Palpate for a distended bladder or renal angle tenderness suggesting obstruction or pyelonephritis 4,6.
4. Diagnostic Workup & Interpretation (Laboratory, Imaging & ECG/Pathology)
| Diagnostic Modality / Test | Expected Clinical & Radiological / Laboratory Finding |
|---|---|
| Gold-Standard Investigation | Serial serum creatinine tracking combined with accurate hourly urine output measurement via indwelling urinary catheter; renal tract ultrasound to evaluate kidney size, echogenicity, and exclude obstruction 3,4. |
| Laboratory Panel | Rising urea and creatinine out of proportion (disproportionate BUN-to-creatinine ratio > 20:1 in prerenal disease), hyperkalaemia, metabolic acidosis with an increased anion gap, hyperphosphataemia, hypocalcaemia, and fractional excretion of sodium (FENa < 1% in prerenal, > 2% in ATN) 4,5. |
| Imaging / Radiology (X-ray, US, CT, MRI) | Urgent bedside renal ultrasound demonstrating hydronephrosis (indicative of postrenal obstruction), normal or enlarged kidneys in acute disease, or small, shrunken echogenic kidneys pointing toward chronic underlying kidney disease 4,6. |
| ECG / Histopathology / Procedural | ECG showing peaked T waves, prolonged PR interval, widened QRS complexes, and sine waves in hyperkalaemia; urinary microscopy revealing muddy brown granular casts and epithelial cells in ATN; renal biopsy if intrinsic glomerular or interstitial disease is suspected 1,4. |
5. Staging, Classification & Clinical Scoring Systems
| Criterion / Stage / Score Component | Clinical Threshold, Points & Management Implication |
|---|---|
| KDIGO Stage 1 | Serum creatinine 1.5–1.9 times baseline OR ≥ 26.5 µmol/L (0.3 mg/dL) absolute increase within 48 hours; Urine output < 0.5 mL/kg/h for 6–12 hours. Action: Review medications, optimize fluid status, monitor daily labs 3. |
| KDIGO Stage 2 | Serum creatinine 2.0–2.9 times baseline; Urine output < 0.5 mL/kg/h for ≥ 12 hours. Action: Active nephrology consultation, close haemodynamic monitoring, avoid all nephrotoxins 3. |
| KDIGO Stage 3 | Serum creatinine 3.0 times baseline OR rise to ≥ 353.6 µmol/L (4.0 mg/dL) with an acute rise of ≥ 0.5 mg/dL OR initiation of renal replacement therapy OR in patients < 18 years, drop in eGFR to < 35 mL/min/1.73m²; Urine output < 0.3 mL/kg/h for ≥ 24 hours or anuria for ≥ 12 hours. Action: Immediate ICU referral, urgent RRT evaluation 2,3. |
6. Differential Diagnosis
| Differential Condition | Distinguishing Clinical, Biochemical & Imaging Features |
|---|---|
| Prerenal Azotaemia | Reversible functional hypoperfusion; rapid response to fluid challenge; BUN:Cr ratio > 20:1; FENa < 1%; bland urinary sediment 4,5. |
| Acute Tubular Necrosis (ATN) | Structural tubular injury from ischaemia or toxins; persistent azotemia despite fluid resuscitation; muddy brown casts on microscopy; FENa > 2% 4,5. |
| Acute Interstitial Nephritis (AIN) | Allergic or inflammatory reaction to drugs (antibiotics, PPIs); may present with fever, maculopapular rash, and eosinophiluria; WBC casts on urinalysis 1,4. |
| Postrenal Obstruction | Mechanical blockage of urinary flow; sudden anuria or fluctuating oliguria; bilateral hydronephrosis confirmed on renal ultrasound 4,6. |
7. Management & Treatment Protocol
- Initial Stabilization & Resuscitation: Assess ABCDEs, establish secure IV access, insert a urinary catheter with a urometer for strict hourly urine output monitoring, and rapidly correct life-threatening electrolyte derangements 5,6.
- First-Line Medical / Pharmacological Therapy (with exact adult dosing):
- Nephrotoxin Cessation: Immediately stop NSAIDs, ACE inhibitors, ARBs, aminoglycosides, and hold iodinated contrast 1,2.
- Fluid Management: Administer targeted boluses of isotonic crystalloids (e.g., Plasmalyte or 0.9% NaCl, 250–500 mL IV over 30 minutes) only if clinical signs suggest fluid responsiveness or hypovolaemia. Avoid fluid overload in established oliguric AKI 2,5.
- Hyperkalaemia Management: If serum K+ > 6.5 mmol/L or ECG changes are present, administer IV Calcium Gluconate 10% (10 mL over 10 minutes) for membrane stabilization, followed by Soluble Insulin 10 units in 50 mL of 50% Dextrose IV over 15 minutes, and Nebulized Salbutamol 10-20 mg 2,6.
- Surgical / Interventional Indications: Urgent urological intervention (nephrostomy tube insertion or retrograde ureteric stent placement) for obstructed postrenal AKI; urgent catheterization for urinary retention; and emergency dialysis catheter insertion for initiation of renal replacement therapy (haemodialysis or continuous venovenous haemofiltration) 2,4.
8. Complications & Prognosis
- Acute Complications: Severe hyperkalaemia leading to ventricular arrhythmias, metabolic acidosis, pulmonary oedema secondary to fluid overload, uraemic encephalopathy, pericarditis, and bleeding diatheses due to platelet dysfunction 2,5.
- Long-Term Prognosis: While many patients achieve complete renal recovery, survivors of AKI face significantly elevated long-term risks of developing chronic kidney disease (CKD), end-stage kidney disease (ESKD), cardiovascular events, and premature all-cause mortality 2,4.
