4.1 Acute Kidney Injury & Chronic Kidney Disease
Key Takeaways
- KDIGO criteria defines Acute Kidney Injury (AKI) as an increase in serum creatinine by ≥0.3 mg/dL within 48 hours or ≥1.5 times baseline within 7 days, or urine output <0.5 mL/kg/h for 6 hours.
- Prerenal azotemia is distinguished from intrinsic Acute Tubular Necrosis (ATN) by Fractional Excretion of Sodium (FENA <1% in prerenal, >2% in ATN) and Urine Sodium (<20 mEq/L in prerenal, >40 mEq/L in ATN).
- Chronic Kidney Disease (CKD) is defined by GFR <60 mL/min/1.73 m² or structural/functional markers of kidney damage present for >3 months, staged I-V based on GFR and A1-A3 based on albuminuria.
- CKD Mineral and Bone Disorder (CKD-MBD) arises from 1-alpha-hydroxylase deficiency and phosphate retention, producing secondary hyperparathyroidism, hypocalcemia, hyperphosphatemia, and osteitis fibrosa cystica.
- Life-threatening complications requiring emergent hemodialysis follow the 'AEIOU' mnemonic: Acidosis (pH <7.1), Electrolytes (K+ >6.5 mEq/L), Intoxications (SLIME), Overload (refractory pulmonary edema), and Uremia (pericarditis/encephalopathy).
Acute Kidney Injury (AKI) & Chronic Kidney Disease (CKD)
Renal medicine forms a core pillar of the General Medicine paper in UPSC CMS. A mastery of AKI diagnostic criteria, laboratory indices for etiological differentiation, CKD staging, and uremic emergency management is essential for clinical and exam success.
1. Acute Kidney Injury (AKI): Diagnostic Criteria & Staging
Acute Kidney Injury is defined as an abrupt decline in renal function occurring over hours to days, leading to the accumulation of nitrogenous waste products (azotemia) and deregulation of fluid, electrolyte, and acid-base homeostasis.
KDIGO Clinical Definition of AKI
According to the KDIGO (Kidney Disease: Improving Global Outcomes) guidelines, AKI is diagnosed when any one of the following criteria is met:
- Increase in serum creatinine by ≥0.3 mg/dL (≥26.5 μmol/L) within 48 hours.
- Increase in serum creatinine to ≥1.5 times baseline, which is known or presumed to have occurred within the prior 7 days.
- Urine volume <0.5 mL/kg/h for 6 consecutive hours.
| KDIGO Stage | Serum Creatinine Criteria | Urine Output Criteria |
|---|---|---|
| Stage 1 | 1.5–1.9 times baseline OR ≥0.3 mg/dL rise | <0.5 mL/kg/h for 6–12 hours |
| Stage 2 | 2.0–2.9 times baseline | <0.5 mL/kg/h for ≥12 hours |
| Stage 3 | 3.0 times baseline OR Increase to ≥4.0 mg/dL OR Initiation of RRT | <0.3 mL/kg/h for ≥24 hours OR Anuria for ≥12 hours |
2. Etiological Classification & Laboratory Differentiation
AKI is categorized into three anatomical categories: Prerenal Azotemia (55–60%), Intrinsic Renal (35–40%), and Postrenal Obstructive (5–10%).
Etiologies
- Prerenal: Intravascular volume depletion (hemorrhage, diarrhea, burns), decreased effective arterial blood volume (heart failure, hepatic cirrhosis, nephrotic syndrome), or altered intrarenal hemodynamics (NSAIDs constrict afferent arterioles; ACE inhibitors / ARBs dilate efferent arterioles).
- Intrinsic Renal:
- Acute Tubular Necrosis (ATN): Ischemic (prolonged prerenal insult, shock) or Nephrotoxic (Aminoglycosides like gentamicin, Amphotericin B, Cisplatin, Radiocontrast media, Rhabdomyolysis with myoglobinuria).
- Acute Interstitial Nephritis (AIN): Drug-induced hypersensitivity (Penicillins, Cephalosporins, Sulfonamides, NSAIDs, PPIs). Triad of Fever, Maculopapular Rash, and Eosinophilia/Eosinophiluria with WBC casts in urine.
- Acute Glomerulonephritis: Dysmorphic RBCs, RBC casts, and proteinuria.
- Postrenal: Urinary tract obstruction (BPH, neurogenic bladder, bilateral ureteral calculi, retroperitoneal fibrosis).
Prerenal Azotemia vs. Intrinsic Acute Tubular Necrosis (ATN)
Distinguishing prerenal azotemia from intrinsic ATN is a classic UPSC CMS question topic. In prerenal disease, tubular reabsorptive capacity remains intact, whereas in ATN, damaged tubular epithelial cells lose the ability to concentrate urine or reabsorb sodium.
| Diagnostic Parameter | Prerenal Azotemia | Intrinsic ATN |
|---|---|---|
| Fractional Excretion of Sodium (FENa) | <1% | >2% |
| Fractional Excretion of Urea (FEUrea) | <35% | >50% |
| Urine Sodium (UNa) | <20 mEq/L | >40 mEq/L |
| Urine Osmolality | >500 mOsm/kg H₂O | <350 mOsm/kg H₂O (Isosthenuria) |
| BUN / Serum Creatinine Ratio | >20 : 1 | 10–15 : 1 |
| Urine Specific Gravity | >1.020 | ~1.010 |
| Urinary Sediment | Hyaline casts (normal) | Muddy brown granular casts, tubular epithelial cell casts |
Exam Tip / Pitfall: FENa may be <1% despite intrinsic renal injury in Contrast-induced nephropathy, Acute Glomerulonephritis, Rhabdomyolysis, and Hepatorenal syndrome. In patients taking diuretics, FEUrea (<35% for prerenal) is more reliable than FENa.
3. Chronic Kidney Disease (CKD): Staging & Pathophysiology
Chronic Kidney Disease (CKD) is defined as abnormalities of kidney structure or function, present for >3 months, with implications for health. Criteria include either GFR <60 mL/min/1.73 m² or evidence of kidney damage (albuminuria, urinary sediment abnormalities, renal histological or imaging defects).
Etiology in India
- Diabetic Nephropathy (Most common cause ~30–40%)
- Hypertensive Nephrosclerosis (~20%)
- Chronic Glomerulonephritis (~15%)
- Autosomal Dominant Polycystic Kidney Disease (ADPKD)
KDIGO Staging Framework
GFR Categories (G1–G5)
- G1: GFR ≥90 mL/min/1.73 m² (Normal or high, with structural damage)
- G2: GFR 60–89 mL/min/1.73 m² (Mildly decreased)
- G3a: GFR 45–59 mL/min/1.73 m² (Mildly to moderately decreased)
- G3b: GFR 30–44 mL/min/1.73 m² (Moderately to severely decreased)
- G4: GFR 15–29 mL/min/1.73 m² (Severely decreased)
- G5: GFR <15 mL/min/1.73 m² (Kidney Failure / End-Stage Renal Disease)
Persistent Albuminuria Categories (A1–A3)
- A1: Urine Albumin-to-Creatinine Ratio (UACR) <30 mg/g (<3 mg/mmol) — Normal to mildly increased
- A2: UACR 30–300 mg/g (3–30 mg/mmol) — Moderately increased (Microalbuminuria)
- A3: UACR >300 mg/g (>30 mg/mmol) — Severely increased (Macroalbuminuria)
4. Complications of CKD & Uremic Syndrome
A. CKD Mineral and Bone Disorder (CKD-MBD)
As GFR drops below 30–45 mL/min/1.73 m², phosphate excretion decreases, and renal tissue loss impairs 1-alpha-hydroxylase activity (reducing conversion of 25-OH-Vit D to active 1,25-(OH)₂ Vitamin D₃ / calcitriol).
- Pathophysiologic Cascade: Phosphate retention + Calcitriol deficiency → Hypocalcemia → Stimulation of parathyroid glands → Secondary Hyperparathyroidism.
- FGF-23 (Fibroblast Growth Factor 23): Rises early in CKD to promote phosphaturia, but inhibits 1-alpha-hydroxylase.
- Bone Disease (Renal Osteodystrophy): Osteitis fibrosa cystica (high-turnover bone disease driven by PTH, characterized by subperiosteal bone resorption, brown tumors, and "rugger-jersey spine" on X-ray).
- Management: Dietary phosphate restriction, Phosphate binders (Sevelamer, Calcium acetate), Calcitriol / Vitamin D analogs, and Calcimimetics (Cinacalcet).
B. Hematologic & Cardiovascular Complications
- Anemia of CKD: Normocytic normochromic anemia primarily due to reduced production of Erythropoietin (EPO) by renal peritubular interstitial cells. Target Hemoglobin on EPO therapy: 10.0–11.5 g/dL (avoiding >12 g/dL due to stroke/thrombotic risks). Always check iron stores prior to EPO (Target Ferritin >100–200 ng/mL, TSAT >20%).
- Uremic Bleeding: Qualitative platelet dysfunction due to uremic toxins (prolonged bleeding time with normal PT and aPTT). Treated with DDAVP (Desmopressin), cryoprecipitate, or conjugated estrogens.
- Cardiovascular Disease: The leading cause of death in CKD patients (ischemic heart disease, left ventricular hypertrophy, accelerated vascular calcification, pericarditis).
5. Uremic Emergencies & Indications for Dialysis
Emergent renal replacement therapy (Hemodialysis) is indicated when conservative medical management fails to control life-threatening uremic or electrolyte complications.
UPSC CMS High-Yield Mnemonic: "AEIOU"
- A – Acidosis: Refractory metabolic acidosis with pH <7.10 or HCO₃⁻ <10 mEq/L.
- E – Electrolytes: Severe, refractory hyperkalemia (Serum K⁺ >6.5 mEq/L or ECG changes like peaked T waves/QRS widening).
- I – Intoxications: Acute poisoning with dialyzable toxins (SLIME: Salicylates, Lithium, Isopropanol, Methanol, Ethylene glycol).
- O – Overload: Refractory volume overload / Acute pulmonary edema unresponsive to high-dose IV diuretics.
- U – Uremia: Manifestations of uremic organ dysfunction (Uremic Pericarditis, Uremic Encephalopathy, Uremic Neuropathy, or Uremic Bleeding).
A 55-year-old male with severe dehydration due to acute gastroenteritis presents with a serum creatinine of 2.8 mg/dL (baseline 0.9 mg/dL). Urinalysis shows hyaline casts. Which set of laboratory findings is most characteristic of his underlying condition?
According to the KDIGO clinical classification, a patient with a persistent GFR of 22 mL/min/1.73 m² and a urine albumin-to-creatinine ratio (UACR) of 180 mg/g is correctly staged as:
Which pathophysiologic sequence correctly describes the development of CKD Mineral and Bone Disorder (CKD-MBD)?
A 62-year-old chronic kidney disease patient on hemodialysis presents to the emergency room with friction rub, chest pain, confusion, and a potassium of 5.8 mEq/L. Which of the following is an absolute indication for emergent hemodialysis in this patient?