What is the difference in serum creatinine levels between Acute Kidney Injury (AKI) and Chronic Kidney Disease (CKD)?

Distinguishing AKI from CKD Using Serum Creatinine in mol/L

The absolute creatinine value in mol/L cannot reliably distinguish AKI from CKD—you must assess the rate of change, baseline comparison, and clinical context. A creatinine of 265 μmol/L (3.0 mg/dL) could represent either severe AKI in someone with previously normal kidneys or moderate CKD that has been present for years 1, 2.

Key Diagnostic Principle: Rate of Change Over Time

The defining feature of AKI is a rapid rise in creatinine over 48 hours to 7 days, whereas CKD represents sustained elevation beyond 90 days 1, 2.

AKI Diagnostic Criteria (KDIGO)

• Stage 1: Creatinine rises to 1.5-1.9× baseline OR increases by ≥26.5 μmol/L (≥0.3 mg/dL) within 48 hours 1
• Stage 2: Creatinine rises to 2.0-2.9× baseline 1
• Stage 3: Creatinine rises to ≥3.0× baseline OR reaches ≥353.6 μmol/L (≥4.0 mg/dL) with acute rise OR requires dialysis 1

CKD Diagnostic Criteria

• Sustained kidney dysfunction for >90 days with GFR <60 mL/min/1.73m² or evidence of kidney damage regardless of GFR 1
• The absolute creatinine level defines severity but not whether it's acute or chronic 1

Critical Diagnostic Algorithm

Step 1: Search Exhaustively for Prior Creatinine Values

Using known creatinine values is always superior to imputation 1, 2. Look for any laboratory data from the past 7-365 days 1.

• If creatinine was normal 3-12 months ago and is now elevated → likely AKI or AKD 1, 2
• If creatinine has been elevated for >90 days → CKD by definition 1

Step 2: If No Prior Values Exist—Use Back-Calculation

The American Society of Nephrology recommends estimating baseline by back-calculating from an assumed eGFR of 75 mL/min/1.73m² 2. This works best in younger patients with likely preserved function but will overestimate AKI in populations with high CKD prevalence 1, 2.

Step 3: Assess Clinical Context for Timing

Features Favoring AKI/AKD:

• Recent acute illness, sepsis, hypotension, or nephrotoxin exposure within 7-90 days 2, 3
• Rapid symptom onset over hours to days 2
• Normal to enlarged kidneys on ultrasound with preserved cortical thickness 2

Features Favoring CKD:

• Long-standing hypertension, diabetes, or documented proteinuria 2
• Small kidneys (<9 cm) with increased echogenicity and cortical thinning on ultrasound 2
• Anemia, hyperparathyroidism, or metabolic bone disease 2

Understanding Acute Kidney Disease (AKD): The 7-90 Day Window

AKD represents the critical transition period between AKI and CKD, defined as kidney dysfunction persisting 7-90 days after an AKI-initiating event 1. If dysfunction persists beyond 90 days, it becomes CKD by definition 1.

AKD Staging Based on Creatinine

• Stage 0C: Creatinine <1.5× baseline but not back to baseline 1
• Stage 1: Creatinine 1.5-1.9× baseline 1
• Stage 2: Creatinine 2.0-2.9× baseline 1
• Stage 3: Creatinine ≥3.0× baseline or ≥353.6 μmol/L (≥4.0 mg/dL) or ongoing dialysis requirement 1

Critical Pitfall: Percentage Changes Are Misleading in CKD

Mathematical modeling demonstrates that the same absolute reduction in kidney function produces vastly different percentage creatinine increases depending on baseline function 4, 5.

• After 90% loss of kidney function, creatinine rises 246% in normal baseline but only 47% in Stage 4 CKD 4
• Absolute creatinine increases of 26.5 μmol/L (0.3 mg/dL) occur at nearly identical timeframes across all baseline kidney function levels 4
• This explains why KDIGO includes both percentage AND absolute criteria for Stage 1 AKI 1, 4

Practical Management Regardless of Classification

Immediate actions apply whether AKI or CKD:

• Discontinue all nephrotoxic agents immediately 1, 2
• Optimize volume status and hemodynamics 1, 2
• Order renal ultrasound to assess kidney size and exclude obstruction 2
• Check urinalysis for proteinuria, hematuria, and cellular casts 2
• Adjust all medication dosing for current kidney function 1

Follow-Up Strategy: The 90-Day Rule

Monitor kidney function serially for at least 90 days to definitively distinguish AKD from CKD progression 1, 2. Arrange nephrology follow-up within 7-14 days for any patient with creatinine elevation of unclear chronicity 2.

Common Diagnostic Errors to Avoid

• Never use eGFR equations designed for stable CKD to assess kidney function during acute changes—they are inaccurate when creatinine is rising or falling 2, 3
• Never assume recovery is complete when creatinine returns to baseline—subclinical injury and loss of renal reserve frequently persist 3
• Never discharge without a clear follow-up plan—the 7-90 day transition period is critical for preventing progression to CKD 2, 3
• Never rely solely on percentage creatinine increases in patients with known or suspected CKD—absolute changes are more reliable 4, 5