Differentiate between acute and chronic renal failure
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"acute kidney injury" AND "chronic kidney disease" AND differentiation
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Acute Renal Failure (AKI) vs. Chronic Renal Failure (CKD) - A Detailed Comparison
1. Definitions
| Parameter | Acute Kidney Injury (AKI) | Chronic Kidney Disease (CKD) / Chronic Renal Failure |
|---|---|---|
| Definition | Abrupt reduction in kidney function within hours to days | Structural or functional kidney abnormality persisting >3 months |
| Functional criteria | ↑ serum creatinine ≥0.3 mg/dL within 48 h, OR ↑ SCr ≥50% within 7 days, OR oliguria <0.5 mL/kg/h for >6 h | GFR <60 mL/min/1.73 m² for >3 months, OR markers of kidney damage (proteinuria, imaging abnormality) >3 months |
| Duration | ≤1 week (AKI); up to 3 months = Acute Kidney Disease (AKD) | >3 months |
| Reversibility | Potentially reversible with prompt treatment | Largely irreversible; progressive |
(Source: Brenner and Rector's The Kidney, Table 19.1 - KDIGO 2012 criteria)
2. Aetiology / Causes
Acute Renal Failure - 3 categories:
Prerenal (most common, ~55-60%) - reduced perfusion to an otherwise intact kidney:
- Volume depletion (GI losses, burns, haemorrhage, third-space shifts)
- Reduced cardiac output (CHF, cardiogenic shock)
- Sepsis, hepatorenal syndrome
- Renal artery stenosis
Intrarenal (intrinsic, ~35-40%) - direct parenchymal damage:
- Acute tubular necrosis (ATN) - ischaemia or nephrotoxins (aminoglycosides, IV contrast, NSAIDs, cisplatin)
- Glomerulonephritis (all causes)
- Acute interstitial nephritis (drug-induced: anticonvulsants, antibiotics; idiopathic)
- Vascular: HUS, TTP, renal artery/vein thrombosis
- Endogenous toxins: haemoglobin (haemolysis), myoglobin (rhabdomyolysis), uric acid (tumour lysis)
Postrenal (~5-10%) - obstruction anywhere from the renal pelvis to the urethra:
- Bilateral ureteral obstruction
- Obstructed single kidney
- Urethral obstruction (BPH, stones, strictures)
(Campbell Walsh Wein Urology, Box 21.4)
Chronic Renal Failure - common causes:
- Diabetic nephropathy (most common worldwide)
- Hypertensive nephrosclerosis
- Chronic glomerulonephritis (IgA nephropathy, FSGS)
- Polycystic kidney disease (ADPKD/ARPKD)
- Chronic tubulointerstitial disease (analgesic nephropathy, reflux nephropathy)
- Renovascular disease
- Obstructive uropathy (long-standing)
3. Pathophysiology
| Feature | AKI | CKD |
|---|---|---|
| Primary mechanism | Abrupt fall in GFR from decreased perfusion, tubular necrosis, or obstruction | Progressive nephron loss with compensatory hyperfiltration in remaining nephrons |
| Tubular function | Damaged in intrinsic AKI (↑ FENa >3%); intact in prerenal (FENa <1%) | Impaired tubular transport and concentrating ability |
| Electrolyte disruption | Acute: hyperkalaemia, hyponatraemia, metabolic acidosis, hyperphosphataemia, hypocalcaemia | Chronic accumulation: same electrolyte derangements, plus renal osteodystrophy (↓ active vitamin D, secondary hyperPTH) |
| Kidney size (imaging) | Normal or enlarged (swollen) | Typically small, echogenic, bilaterally shrunken kidneys |
4. Classification / Staging
AKI - AKIN Staging (Tietz Textbook of Laboratory Medicine, Table 49.9):
| AKIN Stage | Serum Creatinine Criteria | Urine Output Criteria |
|---|---|---|
| 1 | ↑ ≥0.3 mg/dL or ↑ 150-200% from baseline | <0.5 mL/kg/h for >6 h |
| 2 | ↑ >200-300% from baseline | <0.5 mL/kg/h for >12 h |
| 3 | ↑ >300% from baseline, or ≥4 mg/dL, or initiation of RRT | <0.3 mL/kg/h for >24 h, or anuria ≥12 h |
CKD - KDIGO GFR Staging (G categories) + Albuminuria (A categories):
| Stage | GFR (mL/min/1.73 m²) | Description |
|---|---|---|
| G1 | ≥90 | Normal or high (with kidney damage marker) |
| G2 | 60-89 | Mildly decreased |
| G3a | 45-59 | Mildly to moderately decreased |
| G3b | 30-44 | Moderately to severely decreased |
| G4 | 15-29 | Severely decreased |
| G5 | <15 | Kidney failure (ESKD) - requires KRT |
Albuminuria categories: A1 (<30 mg/g, normal to mildly increased), A2 (30-300 mg/g, moderately increased), A3 (>300 mg/g, severely increased / nephrotic range)
(Brenner and Rector's The Kidney, Table 19.1)
5. Clinical Features
| Feature | AKI | CKD |
|---|---|---|
| Onset | Sudden (hours to days) | Insidious (months to years) |
| Urine output | Oliguria (<400 mL/day) or anuria; occasionally non-oliguric | Variable; may be polyuric early (↓ concentrating ability); oliguria late |
| Haematuria / casts | May be present (especially in intrinsic AKI) | Less common unless active disease |
| Hypertension | May develop acutely | Very common (~75% at later stages) |
| Anaemia | Mild or absent (acute) | Present and progressive (↓ erythropoietin); often normocytic normochromic |
| Bone disease | Absent | Renal osteodystrophy (osteomalacia, osteitis fibrosa cystica) - secondary hyperparathyroidism |
| Uraemia | Develops rapidly if severe | Develops gradually; symptoms of fatigue, anorexia, nausea, vomiting, pruritus, peripheral neuropathy, encephalopathy |
| Pericarditis | Rare (severe/prolonged AKI) | Can occur in advanced uraemia |
| Nails/Skin | Unremarkable acutely | Half-and-half nails (Lindsay's nails), pallor, sallow skin, bruising |
(Goldman-Cecil Medicine, Chapter 100 & 116)
6. Laboratory / Diagnostic Differences
| Investigation | AKI | CKD |
|---|---|---|
| Serum creatinine | Rapid rise over days; rate of change helps gauge acuity | Elevated, but rises slowly over months/years |
| BUN | Rapidly elevated | Chronically elevated (uraemia) |
| Haemoglobin | Normal or near-normal early | Low (normocytic anaemia) |
| PTH | Normal | Elevated (secondary hyperparathyroidism) |
| Calcium/Phosphate | Acute hypocalcaemia, hyperphosphataemia | Chronic: low Ca, high PO₄, ↓ active Vit D |
| FENa | <1% prerenal; >3% intrinsic ATN | Not specifically useful for CKD staging |
| Urine sediment | Muddy brown granular casts (ATN), red cell casts (GN) | Waxy/broad casts (from dilated tubules in late CKD) |
| Kidney ultrasound | Normal to enlarged kidneys, normal echogenicity | Small, hyperechoic (echogenic) kidneys bilaterally; loss of corticomedullary differentiation |
| Anaemia markers | Normal EPO | Low EPO; ↑ EPO resistance |
7. Distinguishing AKI from CKD at the Bedside
A key practical point from Goldman-Cecil Medicine: "The change in serum creatinine over time indicates the tempo of the renal disease and can distinguish acute injury from chronic kidney disease." A rapidly rising creatinine with no prior history of CKD favours AKI. Prior lab records showing a chronically elevated creatinine, small kidneys on ultrasound, anaemia, hyperphosphataemia, and secondary hyperparathyroidism all point toward CKD.
A complicating scenario is Acute-on-Chronic Kidney Disease - AKI superimposed on CKD - which is important to recognise because patients with CKD have a substantially higher annual AKI incidence than the general population.
8. Complications
| Complication | AKI | CKD |
|---|---|---|
| Hyperkalaemia | Acute, life-threatening | Chronic, managed with diet/drugs |
| Metabolic acidosis | Acute, anion-gap | Chronic, normal anion-gap (type 4 RTA pattern) |
| Pulmonary oedema | Fluid overload in oliguric phase | Progressive; worsens with progression |
| Cardiovascular disease | Less emphasized acutely | Major cause of death in CKD |
| Mineral/bone disorder | Absent | CKD-MBD (cardinal feature) |
| Infections | High risk in ICU/AKI | Increased susceptibility |
| Neurological | Encephalopathy in severe AKI | Uraemic encephalopathy, peripheral neuropathy |
9. Management Principles
| Aspect | AKI | CKD |
|---|---|---|
| Primary goal | Remove precipitant, restore renal perfusion, prevent further injury | Slow progression, manage complications, prepare for RRT |
| Fluid management | Vigorous IV fluids for prerenal (isotonic crystalloids); avoid in intrinsic/oliguric AKI | Restrict fluid and sodium in advanced stages |
| Nephrotoxin avoidance | Immediate cessation of NSAIDs, aminoglycosides, contrast | Ongoing vigilance; dose-adjust renally cleared drugs |
| Electrolyte correction | Urgent: hyperkalemia (calcium gluconate, insulin/dextrose, kayexalate, dialysis) | Chronic: phosphate binders, potassium restriction, sodium bicarbonate |
| BP control | Treat if severely elevated | Target BP <130/80; ACEi/ARB preferred (reduce proteinuria, slow progression); beware hyperK with RAAS blockade in advanced CKD |
| Anaemia | Transfuse if symptomatic | ESA (erythropoietin-stimulating agents) + IV iron |
| Bone disease | Not applicable acutely | Vitamin D analogues, phosphate binders, cinacalcet |
| Nutrition | High-protein support in critically ill | Protein restriction (0.6-0.8 g/kg/day in non-dialysis CKD) to slow progression |
| Renal replacement therapy (RRT) | Indicated for: refractory hyperK, severe acidosis, fluid overload, uraemia, toxin removal | Initiate dialysis (haemodialysis/PD) or transplant at ESKD (G5); timing based on symptoms + GFR |
| SGLT2 inhibitors | Not used acutely | Now first-line nephroprotective agents in CKD with proteinuria (reduce progression) |
| Monitoring | Hourly urine output, daily creatinine/electrolytes | eGFR and albuminuria every 3-12 months depending on stage |
10. Prognosis
| AKI | CKD | |
|---|---|---|
| Reversibility | Potentially fully reversible if treated promptly; delay may cause ATN progressing to cortical necrosis | Irreversible; progressive toward ESKD |
| Mortality | Significant in ICU-acquired AKI; COVID-19-associated AKI with RRT carried particularly poor prognosis | Primarily from cardiovascular disease; 5-year mortality at dialysis ~20-25%/year |
| AKI -> CKD transition | Repeated or severe AKI episodes accelerate progression to CKD | Pre-existing CKD greatly increases AKI risk (creates a bidirectional, vicious cycle) |
Summary Table
| Feature | AKI | CKD |
|---|---|---|
| Onset | Hours to days | Months to years |
| Duration | ≤7 days (AKI), up to 3 months (AKD) | >3 months |
| Reversibility | Yes (if treated) | No (progressive) |
| Kidney size | Normal/enlarged | Small, echogenic |
| Anaemia | Absent/mild | Present (↓ EPO) |
| Bone disease | Absent | Present (CKD-MBD) |
| PTH | Normal | Elevated |
| Creatinine trend | Acute rise | Chronic plateau or slow rise |
| Urine casts | Granular/RBC casts | Broad waxy casts |
| FENa | <1% (prerenal) / >3% (intrinsic) | Not diagnostic |
| Primary treatment | Remove cause, restore perfusion | Slow progression, manage complications |
Sources:
- Brenner and Rector's The Kidney, 2-Volume Set (KDIGO 2012 criteria, Table 19.1)
- Goldman-Cecil Medicine, Chapters 100 & 116
- Campbell Walsh Wein Urology, 3-Volume Set, Chapter 21
- Tietz Textbook of Laboratory Medicine, 7th ed., Table 49.9 (AKIN staging)
- Frameworks for Internal Medicine, Chapter on AKI categories
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