Signs of Acute Kidney Injury
AKI is primarily diagnosed through laboratory criteria rather than clinical signs, as it often presents asymptomatically, but when present, oliguria and volume overload are the cardinal clinical manifestations. 1
Laboratory Diagnostic Criteria (Primary Signs)
The KDIGO criteria define AKI through three measurable parameters 2, 3:
- Serum creatinine increase ≥0.3 mg/dL within 48 hours 2, 3
- Serum creatinine increase to ≥1.5 times baseline within 7 days 2, 3
- Urine output <0.5 mL/kg/hour for ≥6 consecutive hours 2, 3
Any single criterion is sufficient for diagnosis—you do not need all three. 3 Notably, even small creatinine increases of ≥0.3 mg/dL are independently associated with approximately fourfold increase in hospital mortality, which is why this threshold was incorporated into diagnostic criteria. 3, 1
Clinical Signs and Symptoms
Urinary Changes
- Oliguria (urine output <0.5 mL/kg/hour for ≥6 hours) is a cardinal sign, though AKI can occur with normal urine output 1
- Anuria (complete cessation of urine output) indicates severe AKI 2
Volume Status Changes
- Peripheral edema 1
- Pulmonary edema 1
- Jugular venous distension 1
- Signs of volume depletion in prerenal AKI (hypotension, tachycardia, decreased skin turgor) 4
Systemic Manifestations
AKI often presents without specific symptoms, making laboratory monitoring essential 1. When symptoms occur, they reflect:
- Uremic symptoms (in advanced AKI): altered mental status, nausea, vomiting, fatigue 4, 5
- Electrolyte disturbances: hyperkalemia manifestations (cardiac arrhythmias, muscle weakness) 4, 5
- Metabolic acidosis: Kussmaul respirations in severe cases 4
AKI Staging Based on Signs
The severity is classified into three stages based on the most severe criterion met 3, 1:
Stage 1:
- Creatinine 1.5-1.9 times baseline OR increase ≥0.3 mg/dL 3, 1
- Urine output <0.5 mL/kg/hour for 6-12 hours 3, 1
Stage 2:
Stage 3:
- Creatinine ≥3.0 times baseline OR increase to ≥4.0 mg/dL 3, 1
- Urine output <0.3 mL/kg/hour for ≥24 hours OR anuria for ≥12 hours 3, 1
- Initiation of renal replacement therapy 3, 1
Additional Laboratory Findings
Beyond the diagnostic criteria, urinalysis findings help identify etiology 4, 5:
- Hematuria (>50 RBCs per high-power field): suggests glomerular disease 6
- Proteinuria (>500 mg/day): suggests glomerular disease 6
- Cellular casts (particularly renal tubular epithelial cell casts): suggests acute tubular necrosis 6
- Elevated blood urea nitrogen (BUN) 2, 4
Critical Pitfalls to Avoid
Do not rely solely on serum creatinine without considering urine output criteria, as this may miss cases of AKI. 3 However, urine output criteria have important limitations:
- In cirrhotic patients with ascites, urine output is unreliable because these patients are frequently oliguric with avid sodium retention yet may maintain relatively normal GFR 2, 6
- Diuretic therapy confounds urine output interpretation 2, 6
- In these populations, focus exclusively on serum creatinine changes 2, 6
Do not wait for creatinine to reach 1.5 mg/dL before diagnosing AKI, as this outdated threshold often indicates GFR has already fallen to approximately 30 mL/min. 6 Instead, monitor temporal changes in creatinine at 48-hour intervals to detect the 0.3 mg/dL threshold. 6
Failure to establish an accurate baseline creatinine can lead to misclassification. 3 Using known creatinine values is superior to imputation methods, as back-calculation from an estimated GFR of 75 mL/min/1.73 m² may overestimate AKI incidence in populations with high CKD prevalence. 3
Risk Factors That Should Prompt Monitoring
Patients at highest risk who warrant close monitoring for AKI signs include 1: