Subacute AKI (Acute Kidney Disease) Definition
Subacute AKI is now formally termed Acute Kidney Disease (AKD), defined as kidney dysfunction or damage persisting between 7 and 90 days after an acute kidney injury initiating event. 1
Diagnostic Criteria for AKD
AKD is diagnosed when any of the following criteria are met for a duration of 7–90 days after kidney injury onset: 1, 2
- Persistence of AKI criteria (serum creatinine ≥1.5× baseline OR ≥0.3 mg/dL increase OR urine output <0.5 mL/kg/h for ≥6 hours) 1, 2
- eGFR <60 mL/min/1.73 m² for less than 3 months 1, 3
- ≥35% decline in GFR from baseline 2
- >50% rise in serum creatinine from baseline 2
- Markers of kidney damage (proteinuria, abnormal urinalysis, imaging abnormalities) even without meeting functional AKI thresholds 1, 3
Temporal Framework: AKI → AKD → CKD
The KDIGO 2021 Consensus Conference harmonized definitions across the continuum of acute and chronic kidney disease: 1, 3
- 0–7 days = Acute Kidney Injury (AKI) 1, 3
- 7–90 days = Acute Kidney Disease (AKD) 1, 3, 2
- >90 days = Chronic Kidney Disease (CKD) 1, 3
AKI is a subset of AKD—all patients meeting AKI criteria automatically have AKD if dysfunction persists beyond 7 days, but AKD can also occur without preceding AKI when kidney injury develops more gradually. 1, 3
Subacute AKD Staging
The ADQI 16 Workgroup proposed a "Stage 0 Subacute AKD" category to capture patients in partial recovery who no longer meet AKI Stage 1 criteria but have not fully recovered: 1
- Stage 0C: Serum creatinine remains elevated above baseline but <1.5× baseline (partial functional recovery) 1, 4
- Stage 0B: Serum creatinine returned to baseline but ongoing kidney damage markers persist (proteinuria, loss of renal reserve, positive injury biomarkers) 1, 4
- Stage 0A: Creatinine normalized with no structural damage markers, but history of AKI confers long-term risk 1
Patients in Stage 0B/C represent the subacute AKD population most clinicians encounter—those with incomplete recovery who fall through the cracks between AKI and CKD definitions. 1
Why the AKD Concept Matters Clinically
AKD not associated with AKI is nearly 3 times more prevalent than AKI itself, yet often goes undetected because it doesn't meet the dramatic 48-hour or 7-day AKI thresholds. 3 For example:
- A patient whose creatinine rises slowly over 2 weeks (not meeting AKI criteria) still has AKD and requires intervention 3
- Community-acquired AKD frequently goes unrecognized because it lacks the acute presentation typical of hospital-acquired AKI 3
Patients with AKD without preceding AKI have an adjusted hazard ratio of 2.26 for the composite outcome of incident CKD, kidney failure, or death. 3 Those with AKD following AKI have even worse outcomes, with mortality rates of 47% versus 19% for CKD alone. 3
Clinical Implications for Management
The AKD framework allows you to capture and manage patients in the recovery or progression phase after AKI who still have ongoing kidney dysfunction: 3
- Monitor patients for 7–90 days after any AKI episode or acute kidney insult, even if creatinine appears to normalize 2, 4
- Don't rely on creatinine alone—check for proteinuria, loss of renal reserve, and structural damage markers 4
- Coordinate nephrology follow-up for any patient with persistent dysfunction beyond 7 days 4
- Avoid nephrotoxins throughout the entire 90-day AKD window, as vulnerability persists 4
Common Pitfalls
- Assuming recovery is complete when creatinine returns to baseline—subclinical injury and loss of reserve frequently persist, placing patients in Stage 0B AKD 1, 4
- Discharging patients without a clear follow-up plan—the 7–90 day window is critical for preventing progression to CKD 4
- Missing AKD entirely when kidney injury develops gradually over 2–3 weeks without meeting the 48-hour or 7-day AKI criteria 3
- Using eGFR equations designed for CKD to assess kidney function during AKI/AKD recovery—they are inaccurate in this dynamic setting 4