What diagnostic studies are needed for Acute Kidney Injury (AKI)?

Diagnostic Studies for Acute Kidney Injury (AKI)

Laboratory Studies (Essential First-Line)

The initial diagnostic workup for AKI requires serum creatinine measurement to establish diagnosis and staging, urinalysis with microscopy to differentiate causes, and basic metabolic panel to assess complications. 1

Core Laboratory Tests

• Serum creatinine and BUN: Measure to diagnose AKI (increase ≥0.3 mg/dL within 48 hours or ≥50% rise within 7 days) and calculate estimated GFR using CKD-EPI equation 1, 2
• Complete metabolic panel: Assess electrolytes (sodium, potassium, calcium, chloride, phosphorus, magnesium) to detect life-threatening complications like hyperkalemia 1, 2
• Complete blood count: Evaluate for anemia, infection, or systemic illness 1, 3

Urine Studies

• Urinalysis with microscopy: Examine for proteinuria (>500 mg/day), hematuria (>50 RBCs per high-power field), cellular casts, and crystals to differentiate structural kidney injury from functional causes 1, 2
• Urine sodium and fractional excretion of sodium (FENa): Calculate to distinguish prerenal AKI (FENa <1%, urine sodium <10 mEq/L) from intrinsic renal damage (FENa >1%) 1, 4, 3
• Fractional excretion of urea (FEUrea): Use when FENa is unreliable (recent diuretic use); FEUrea <28.16% suggests hepatorenal syndrome with 75% sensitivity and 83% specificity 1, 4
• Urine albumin-to-creatinine ratio: Quantify proteinuria to assess for glomerular disease 2

Important caveat: FENa has significant limitations in sepsis and cirrhosis—in cirrhotic patients, FENa <1% has 100% sensitivity but only 14% specificity for prerenal causes 1. The traditional prerenal/intrinsic/postrenal classification can be misleading, as "prerenal" is often misinterpreted as "hypovolemic," leading to inappropriate fluid administration 1, 4.

Imaging Studies

Renal ultrasound is the mandatory first-line imaging study for all AKI patients to assess kidney size, echogenicity, and exclude obstruction. 1, 4, 2

Ultrasound (First-Line Imaging)

• Renal ultrasound with Doppler: Perform to evaluate kidney size (small echogenic kidneys suggest chronic kidney disease), detect hydronephrosis, assess cortical thickness, and rule out renal artery stenosis 1, 4, 2
• Bladder ultrasound: Check for urinary retention and postrenal obstruction 1, 4

Key interpretation: Normal kidney size suggests AKI rather than chronic kidney disease, though this does not differentiate between prerenal and intrinsic causes 1, 2. Ultrasound is particularly essential in older men to exclude obstruction 3.

Advanced Imaging (Selective Use Only)

• Non-contrast CT abdomen/pelvis: Reserve for cases where ultrasound is non-diagnostic due to body habitus, or to characterize hydronephrosis and identify urinary tract calculi (most sensitive modality for stones) 1
• CT with IV contrast: Absolutely contraindicated in AKI diagnosis due to nephrotoxicity risk; only consider in established dialysis patients without residual renal function 1
• MRA abdomen/pelvis: Consider only when high suspicion exists for renovascular causes (renal artery stenosis, thrombosis, arterial injury) that cannot be adequately assessed by ultrasound 1

Critical pitfall: Avoid iodinated contrast media exposure in AKI patients—this is a nephrotoxic medication that worsens kidney injury 1, 5.

Specialized Studies (Context-Dependent)

For Cirrhosis Patients with AKI

• Diagnostic paracentesis: Mandatory to exclude spontaneous bacterial peritonitis (most common cause of hepatorenal syndrome-AKI) 1, 5
• Blood and urine cultures: Obtain before empirical antibiotics when infection is suspected 1
• Chest radiography: Evaluate for pneumonia as precipitating factor 1

When Glomerular Disease Suspected

• Renal biopsy: Indicated when urinalysis shows proteinuria >500 mg/day, dysmorphic RBCs, red blood cell casts, or unexplained AKI with active sediment 2

Monitoring Parameters

• Serial serum creatinine: Measure every 4-6 hours initially to assess AKI progression and stage (Stage 1: 1.5-1.99× baseline; Stage 2: 2.0-2.99× baseline; Stage 3: ≥3.0× baseline or ≥4 mg/dL with acute rise ≥0.3 mg/dL) 1, 5
• Urine output: Document hourly; oliguria defined as <0.5 mL/kg/hour for >6 hours (Stage 1) 1

Physical Examination Findings to Document

• Volume status assessment: Check for jugular venous distension, peripheral edema, pulmonary crackles (suggests volume overload), or orthostatic hypotension (suggests hypovolemia) 4, 6
• Abdominal examination: Palpate for bladder distension, masses, or ascites 1, 4
• Skin examination: Look for rashes indicating systemic illness (vasculitis, atheroembolic disease) 3

The physical examination alone has significant limitations for differentiating prerenal from intrinsic AKI, especially when both conditions coexist—always combine clinical assessment with laboratory and imaging data. 4