# Definition and Staging of Acute Kidney Injury — KDIGO 2012

## AKI Definition

AKI is defined as any of the following (Not Graded):

- Increase in serum creatinine (SCr) by **>= 0.3 mg/dL (>= 26.5 umol/L) within 48 hours**; or
- Increase in SCr to **>= 1.5 times baseline**, which is known or presumed to have occurred **within the prior 7 days**; or
- Urine volume **< 0.5 mL/kg/h for 6 hours**

### Baseline Creatinine

- When a baseline SCr is not available, if the patient has no history of CKD, a back-calculated value using the MDRD equation assuming a GFR of 75 mL/min/1.73 m2 may be considered as a surrogate baseline.
- A known baseline from the prior 7-12 months is preferred when available.

> **OpenMedicine Calculator:** `calculate_ckd_epi` — available via MCP for automated eGFR calculation to assist in baseline kidney function assessment.

> **OpenMedicine Calculator:** `calculate_cockcroft_gault` — available via MCP for creatinine clearance estimation relevant to drug dosing in AKI.

## AKI Staging

AKI should be staged for severity according to the following criteria (Not Graded):

| Stage | Serum Creatinine Criteria | Urine Output Criteria |
|---|---|---|
| **Stage 1** | 1.5-1.9 times baseline; OR increase >= 0.3 mg/dL (>= 26.5 umol/L) | < 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 (>= 353.6 umol/L); OR initiation of RRT; OR in patients < 18 years, decrease in eGFR to < 35 mL/min/1.73 m2 | < 0.3 mL/kg/h for >= 24 hours; OR anuria for >= 12 hours |

### Staging Rules

- Patients should be staged according to the criteria that place them in the **highest stage** (i.e., if creatinine places a patient in Stage 2 but urine output places them in Stage 3, they should be classified as Stage 3).
- Both SCr and urine output criteria should be applied.
- The KDIGO staging system harmonizes the prior RIFLE and AKIN classification systems.

## Risk Assessment

### Susceptibilities (Pre-existing Patient Factors)

The guideline identifies the following as patient-related susceptibilities that increase AKI risk (Not Graded):

- Dehydration or volume depletion
- Advanced age
- Female sex
- Black race
- Pre-existing CKD
- Chronic diseases (heart, lung, liver)
- Diabetes mellitus
- Cancer
- Anemia

### Exposures (Acute Insults)

Common exposures that may precipitate AKI include (Not Graded):

- Sepsis
- Critical illness
- Circulatory shock
- Burns
- Trauma
- Major surgery (especially cardiac surgery)
- Nephrotoxic drugs (NSAIDs, aminoglycosides, amphotericin B, radiocontrast agents)
- Poisonous plants and animals

## Evaluation and Management by Stage

The guideline recommends managing patients according to their AKI stage (Not Graded):

- **All patients at risk for AKI:** Monitor SCr and urine output; assess for changes in drug dosing and avoidance of nephrotoxins; consider diagnostic workup; consider non-invasive hemodynamic monitoring.
- **Stage 1:** All of the above, plus: check for reversible causes; if no improvement, consider invasive hemodynamic monitoring; avoid hyperglycemia.
- **Stage 2:** All of the above, plus: consider RRT.
- **Stage 3:** All of the above, plus: initiate RRT if indicated; avoid subclavian catheters for dialysis access.

## Limitations

- The KDIGO AKI staging system relies on changes in serum creatinine and urine output, both of which are imperfect markers. SCr may lag behind actual GFR changes by 24-48 hours and is influenced by muscle mass, hydration, and medications.
- Urine output criteria may be difficult to apply accurately outside of the ICU setting.
- The guideline acknowledges that novel biomarkers (e.g., NGAL, KIM-1, IL-18) may improve early detection but were not sufficiently validated at the time of publication for routine clinical use.
- Baseline SCr is frequently unknown, particularly in the emergency setting, introducing uncertainty into staging.
