Acute-on-Chronic Kidney Disease (AKI on CKD) - Consultant-Level Review

1. Definition and Conceptual Framework

"AKI can develop de novo in the setting of intact kidney function or can be superimposed on underlying chronic kidney disease (acute on chronic kidney injury), and the presence of underlying impaired kidney function has been shown to be one of the most important risk factors for the development of AKI." - Brenner and Rector's The Kidney, 2-Volume Set

2. Why CKD is a Major Risk Factor for AKI

• Diminished renal functional reserve - reduced nephron mass with less adaptive capacity
• Impaired salt and water conservation - predisposes to intravascular volume contraction
• Decreased activity of detoxification mechanisms
• Increased susceptibility to cytotoxic injury (tubular cells already under stress)
• Impaired clearance of nephrotoxins - longer exposure duration
• Associated macro- and microvascular disease - heightened ischemic risk

3. Diagnosis

3a. The Baseline Creatinine Problem

• Not available (no recent outpatient values)
• Ambiguous (which historical value to use - admission vs. most recent outpatient vs. 7-day prior)

1. Use the most recent stable outpatient creatinine (ideally within 3-6 months, pre-decompensation)
2. If unavailable, use the lowest recent inpatient value if the patient has been admitted recently
3. Back-calculate using MDRD or CKD-EPI to estimate the creatinine consistent with a GFR of 75 mL/min/1.73m² (used in epidemiologic studies but less reliable clinically)
4. Serial creatinine trending on admission helps identify the acuity

3b. KDIGO Criteria (applied to AKI on CKD)

3c. Urine Output Limitations in CKD

• Many CKD patients already have reduced, often fixed, urine output
• Tubular concentrating ability is impaired - they may maintain urine output even with severe GFR decline
• Nonoliguric AKI is common in CKD

3d. Distinguishing Causes of the Acute Component

• Volume depletion (diarrhea, vomiting, over-diuresis, poor intake)
• Reduced cardiac output (heart failure decompensation)
• Medications: NSAIDs (afferent arteriolar constriction), ACE-I/ARBs (efferent dilation - GFR-dependent kidneys particularly susceptible), contrast agents, aminoglycosides, calcineurin inhibitors
• Hepatorenal syndrome

• ATN (ischemic or nephrotoxic)
• Acute glomerulonephritis (crescentic GN, rapidly progressive GN - requires urgent workup with ANCA, anti-GBM, complements, ANA, anti-dsDNA)
• Acute interstitial nephritis (drug-induced - PPIs, beta-lactams, NSAIDs, antibiotics)
• Thrombotic microangiopathy
• Cholesterol emboli (after vascular procedures)
• Renovascular disease / bilateral renal artery stenosis (especially on ACE-I/ARB)

• Obstruction - particularly relevant in CKD patients with diabetic neuropathy (neurogenic bladder), prostate disease, or prior pelvic malignancy

3e. Diagnostic Workup

• Serial serum creatinine, BUN, electrolytes (K⁺, bicarb, phosphate, calcium)
• Urinalysis with microscopy: RBC casts (GN), WBC/granular casts (AIN/ATN), protein quantification
• Urine Na⁺, FENa (though unreliable on diuretics - use FEUrea instead), urine osmolality
• Spot urine albumin:creatinine ratio (to compare against prior values)
• Renal ultrasound: bilaterally small echogenic kidneys confirm chronicity; exclude obstruction; assess for hydronephrosis
• CBC, LFTs, coagulation, blood cultures if sepsis suspected

• Complement C3/C4, ANA, anti-dsDNA, ANCA (PR3/MPO), anti-GBM, cryoglobulins, serum protein electrophoresis (myeloma), hepatitis B/C serology, HIV
• Urine eosinophils (AIN - low sensitivity, ~25%)

• NGAL (urine and plasma), KIM-1, IL-18, L-FABP, TIMP-2 x IGFBP7 (NephroCheck) - help differentiate functional prerenal AKI from intrinsic tubular injury; can provide early diagnosis before creatinine rises; still being validated for routine clinical use
• Serum and urine cystatin C: less influenced by muscle mass than creatinine, more sensitive for mild GFR changes

• Indicated when the cause of the acute component is unclear and immunosuppressive therapy may be warranted (suspected rapidly progressive GN, vasculitis, lupus nephritis)
• Relative contraindications: uncorrected coagulopathy, single functioning kidney, uncontrolled hypertension

4. Management

4a. General Principles

4b. Hemodynamic Optimisation and Volume Management

• Volume depletion (most common scenario): Isotonic saline or balanced crystalloid (Plasmalyte/Hartmann's - preferred over normal saline to avoid hyperchloremic acidosis, which is poorly tolerated in already acidotic CKD patients)
• Avoid aggressive fluid resuscitation in CKD patients - they are prone to volume overload, pulmonary edema, and dilutional hyponatremia
• Target MAP >65 mmHg; use vasopressors (norepinephrine) if persistent hypotension despite adequate volume
• Daily weights, strict fluid balance, clinical assessment of volume status (JVP, lung auscultation, peripheral edema)
• Bladder catheterization for accurate urine output monitoring

4c. Nephrotoxin Avoidance and Medication Review

• HOLD: NSAIDs, ACE-I/ARBs (especially in volume-depleted state), aminoglycosides, contrast agents, metformin (lactic acidosis risk), SGLT-2 inhibitors (temporarily - ineffective and unsafe below GFR 30-45 depending on indication)
• Review and dose-adjust all renally-cleared drugs
• Use contrast-enhanced imaging only when essential; pre-hydrate if contrast is unavoidable; consider iso-osmolar contrast agents

4d. Electrolyte and Metabolic Management

• Hyperkalemia: Dietary restriction (K+ <2g/day), stop K⁺-sparing agents, sodium bicarbonate for acidosis, patiromer or sodium zirconium cyclosilicate (SZC/Lokelma) as oral K⁺ binders; calcium gluconate for cardiac protection if ECG changes; emergency dialysis if K⁺ >6.5 and refractory
• Metabolic acidosis: Sodium bicarbonate supplementation if bicarb <15-18 mEq/L (benefits in CKD include slowing progression); oral bicarbonate or IV sodium bicarbonate; dialysis if severe
• Hyperphosphatemia: Dietary restriction, phosphate binders (calcium carbonate, sevelamer, lanthanum carbonate)
• Fluid overload: Loop diuretics (furosemide) - but note: high doses may be needed in CKD due to impaired tubular secretion; if oliguria/anuria, diuretics may be ineffective
• Anemia: Correct iron deficiency; hold ESAs during acute illness

4e. Treating the Underlying Cause

• Sepsis: Source control, appropriate antibiotics (renally adjusted), hemodynamic support
• Contrast nephropathy: Volume expansion peri-procedure
• Obstruction: Urgent catheterization or nephrostomy; urology referral
• Rapidly progressive GN/vasculitis: High-dose corticosteroids, cyclophosphamide (or rituximab for ANCA vasculitis); plasmapheresis for anti-GBM disease
• AIN: Identify and withdraw offending drug; consider corticosteroids (prednisone 1 mg/kg/day for 2-4 weeks, then taper) if no recovery within 1-2 weeks of drug withdrawal
• Myeloma cast nephropathy: Chemotherapy, hydration, avoid contrast/nephrotoxins; bisphosphonates cautiously
• Heart failure: Decongestive therapy (loop diuretics or ultrafiltration), cardiorenal optimization

4f. Renal Replacement Therapy (RRT) in AKI on CKD

• Acidosis: Severe metabolic acidosis (pH <7.1) refractory to bicarbonate
• Electrolytes: Hyperkalemia K⁺ >6.5 mEq/L refractory to medical management
• Intoxication: Dialyzable toxins
• Overload: Pulmonary edema/volume overload unresponsive to diuretics
• Uremia: Uremic encephalopathy, pericarditis, platelet dysfunction/bleeding

• Intermittent hemodialysis (IHD): Preferred if hemodynamically stable
• Continuous renal replacement therapy (CRRT): Preferred in hemodynamically unstable patients or with cerebral edema
• Peritoneal dialysis: Less commonly used in acute setting, but an option
• Unique concern in AKI on CKD: The patient may NOT recover to their previous CKD baseline - a proportion will transition to a new, lower plateau (new CKD stage) or become dialysis-dependent. Advance planning for long-term RRT should begin during the acute admission

5. Why Differentiating AKI on CKD from Pure AKI Matters

5a. Prognosis is Significantly Worse

• AKI on CKD carries higher rates of non-recovery, progression to ESRD, and death than AKI in previously healthy kidneys
• A 2009 study (cited in Brenner & Rector) showed significantly higher non-recovery of kidney function and mortality after acute-on-chronic renal failure vs. pure AKI
• A 2012 study (Comprehensive Clinical Nephrology) showed that acute-on-chronic kidney injury at hospital discharge is associated with long-term dialysis dependence
• Pre-existing CKD is one of the strongest independent risk factors for AKI progression to ESRD

5b. Baseline Reference Matters for Staging

• Staging with the wrong baseline (assuming it's a normal kidney) will overestimate AKI severity if the patient had elevated creatinine at baseline for another reason, or underestimate if the "normal" creatinine masks true CKD (e.g., elderly sarcopenic patient with a creatinine of 0.8 mg/dL but GFR of 45)
• Misclassification affects triage, RRT decision-making, and prognosis counseling

5c. Drug Dosing and Nephrotoxin Management

• A patient with CKD eGFR 25 already on ACE-I who develops AKI needs immediate ACE-I hold - failure to recognize the CKD context may delay this
• Dose adjustments for renally cleared drugs must use the true premorbid GFR, not the acutely worsened level (which would over-restrict dosing once AKI recovers)
• Contrast agents, aminoglycosides, NSAIDs, and calcineurin inhibitors all carry greater toxicity risk in the CKD context

5d. Recovery Expectations are Different

• In pure AKI, full return to baseline is expected in the majority with reversible causes
• In AKI on CKD, partial recovery only is the more likely outcome - creatinine typically does not return to the pre-AKI CKD baseline
• This distinction is essential for:
  • Counseling patients and families
  • Timing decisions about RRT
  • Planning for renal transplant listing
  • Avoiding inappropriate withdrawal of long-term supportive CKD therapies (ESAs, bicarbonate, phosphate binders) too early

5e. Different Follow-up Strategy

• AKI on CKD requires nephrology follow-up within 2-4 weeks of discharge to reassess trajectory, re-establish creatinine baseline, and resume appropriate CKD medications
• Long-term CKD management (RAAS blockade for proteinuria, SGLT-2 inhibitors, GLP-1 agonists where applicable) may need to be re-titrated at the new baseline
• Compared with pure AKI patients, there is a greater and more immediate risk of progression to ESRD, requiring earlier planning for AV fistula creation or peritoneal dialysis catheter placement

5f. Distinct Etiology Considerations

• CKD patients have specific precipitants not typically seen in pure AKI:
  • Renovascular disease with bilateral renal artery stenosis (particularly unmasked by ACE-I/ARB initiation)
  • Superimposed crescentic GN on IgA nephropathy, lupus nephritis, or MPGN
  • Cholesterol embolism post-angiography
  • Progressive urinary obstruction (particularly in diabetic nephropathy with neuropathy)

6. Summary: AKI vs. CKD vs. AKI on CKD at a Glance

• Brenner and Rector's The Kidney, 2-Volume Set - Chapter 29 (AKI Definition, Staging, AKD framework, AKI on CKD mechanisms)
• Comprehensive Clinical Nephrology, 7th Edition - KDIGO AKI criteria and acute-on-chronic outcomes
• KDIGO 2012 AKI Clinical Practice Guideline - the primary consensus framework
• CKD Prognosis Consortium Meta-Analysis, JAMA 2023 (PMID 37787795) - eGFR, albuminuria and adverse outcomes in CKD
• SGLT-2 inhibitors in CKD - BMJ Practice Guideline 2024 (PMID 39353639) - updated guidance on SGLT-2i in CKD management