What is the difference between Acute Kidney Injury (AKI) and Acute Kidney Disease (AKD)?

Difference Between AKI and AKD

AKI (Acute Kidney Injury) is an abrupt decrease in kidney function occurring over 7 days or less, while AKD (Acute Kidney Disease) is a broader term that encompasses AKI plus any kidney dysfunction or damage lasting between 7 and 90 days—essentially, AKI is a subset of AKD. 1

Temporal Definitions

AKI represents the acute phase (≤7 days):

• Defined by rapid changes in serum creatinine: rise of ≥0.3 mg/dL within 48 hours, OR ≥50% increase from baseline within 7 days 2
• Alternatively diagnosed by urine output <0.5 mL/kg/h for ≥6 hours 2
• Focuses specifically on functional criteria (creatinine and urine output) without incorporating structural damage markers 1

AKD captures the subacute phase (7-90 days):

• Includes all patients meeting AKI criteria PLUS those with kidney abnormalities not severe enough to meet AKI thresholds 1
• Defined as acute or subacute kidney damage and/or loss of function for 7-90 days after an AKI initiating event 2
• Can occur with OR without preceding AKI 1
• If AKD persists beyond 90 days, it transitions to chronic kidney disease (CKD) 2

The Critical Conceptual Relationship

AKI is included within AKD, not separate from it:

• Think of AKD as the umbrella term capturing all kidney dysfunction <90 days duration with health implications 1
• This framework recognizes that patients can have significant kidney abnormalities (structural or functional) that don't meet the strict AKI criteria but still require clinical attention 1
• The term AKD was specifically created to fill the gap between AKI and CKD definitions 1

Clinical Significance and Outcomes

AKD without AKI is surprisingly common and dangerous:

• AKD not associated with AKI is nearly 3 times more prevalent than AKI itself 1
• Patients with AKD without AKI have an adjusted hazard ratio of 2.26 for the composite outcome of incident CKD, kidney failure, or death 1
• Patients with AKD post-AKI have even worse outcomes with an adjusted HR of 2.51 1

Both conditions carry significant mortality and morbidity risks:

• AKD in combination with CKD confers the highest risk of CKD progression and kidney failure 1
• CKD in combination with AKI confers the highest risk of death 1
• Even small increases in serum creatinine (≥0.3 mg/dL) in AKI are associated with approximately fourfold increase in hospital mortality 2

Practical Clinical Implications

Why this distinction matters in real practice:

• A patient whose creatinine rises slowly over 2 weeks (not meeting the 48-hour or 7-day AKI criteria) still has AKD and requires intervention 1
• Community-acquired AKD often goes undetected because it doesn't meet dramatic AKI criteria, yet has long-term health implications 1
• The AKD framework allows clinicians to capture and manage patients in the recovery or progression phase after AKI who still have ongoing kidney dysfunction 1

Common Pitfalls to Avoid

Don't assume normal creatinine means no kidney disease:

• Patients can have structural kidney damage (abnormal urinalysis, proteinuria, imaging findings) without meeting functional AKI criteria—this is still AKD 1
• Relying solely on serum creatinine without considering urine output criteria may miss AKI cases 2

Don't discharge AKI patients without recognizing AKD risk:

• Approximately 25% of AKI survivors develop AKD 3
• These patients need structured follow-up between 7-90 days to monitor for progression to CKD 3

Don't confuse the timeline:

• 0-7 days = AKI (if meets criteria)
• 7-90 days = AKD (includes persistent AKI, recovering AKI, or new dysfunction)

• 90 days = CKD 1, 2