What is the appropriate workup for acute kidney injury (AKI) in a hospitalized patient?

Workup of Acute Kidney Injury in Hospitalized Patients

Begin with immediate measurement of serum creatinine, complete blood count, urinalysis with microscopy, and fractional excretion of sodium (FENa), followed by renal ultrasonography in most patients to rule out obstruction. 1

Initial Diagnostic Classification

The workup should systematically classify AKI as prerenal, intrinsic renal, or postrenal through a structured approach 1:

History and Medication Review

• Identify all nephrotoxic medications including NSAIDs, aminoglycosides, vancomycin, and ACE inhibitors/ARBs that require immediate discontinuation 1, 2
• Document recent contrast exposure within the preceding 48-72 hours, as contrast-induced AKI is preventable with proper recognition 3
• Assess for systemic illnesses causing poor renal perfusion (sepsis, heart failure, cirrhosis) or direct renal injury (vasculitis, glomerulonephritis) 1

Physical Examination Priorities

• Evaluate intravascular volume status through jugular venous pressure, mucous membranes, skin turgor, and orthostatic vital signs to distinguish prerenal from intrinsic causes 1
• Examine for skin rashes suggesting systemic vasculitis or atheroembolic disease 1
• Assess for bladder distention indicating possible urinary obstruction 1

Laboratory Workup

Essential Initial Tests

• Serum creatinine with comparison to baseline - AKI is defined as ≥0.3 mg/dL increase within 48 hours or ≥1.5× baseline within 7 days 4
• Urinalysis with microscopy - Look for:
  • Muddy brown casts (acute tubular necrosis)
  • Red blood cell casts (glomerulonephritis)
  • White blood cell casts (acute interstitial nephritis)
  • Eosinophils (allergic interstitial nephritis) 1
• Fractional excretion of sodium (FENa) - FENa <1% suggests prerenal azotemia, while >2% indicates intrinsic renal disease 1
• Complete blood count - Assess for anemia (hemolysis, chronic disease) and thrombocytopenia (thrombotic microangiopathy) 1

Imaging

• Renal ultrasonography should be performed in most patients, particularly older men, to exclude obstructive uropathy 1
• Ultrasound also evaluates kidney size (small kidneys suggest chronic disease) and echogenicity 1

Electronic Alert Systems

Hospitals should implement automated KDIGO-based electronic alerts that identify AKI through delta checks showing absolute creatinine rise ≥0.3 mg/dL within 48 hours 3. These alerts have successfully reduced nephrotoxic medication use and prevented contrast-induced AKI 3.

Staging and Risk Stratification

Apply KDIGO staging criteria immediately 4:

• Stage 1: Creatinine 1.5-1.9× baseline or ≥0.3 mg/dL increase; urine output <0.5 mL/kg/hr for 6-12 hours
• Stage 2: Creatinine 2.0-2.9× baseline; urine output <0.5 mL/kg/hr for ≥12 hours
• Stage 3: Creatinine ≥3.0× baseline or ≥4.0 mg/dL or initiation of RRT; urine output <0.3 mL/kg/hr for ≥24 hours or anuria for ≥12 hours

Common Pitfalls to Avoid

• Do not assume transient AKI is benign - even rapidly reversible AKI with recovery within 3 days carries 15% hospital mortality versus 4% without AKI 3
• Recognize that acute-on-chronic kidney disease (ACKD) is particularly high-risk and warrants aggressive management even if percentage creatinine rises appear modest 3
• Avoid using imputed baseline creatinine when possible - this may overestimate kidney function and create false-positive AKI alerts, though some false positives are acceptable for safety 3
• Do not delay workup for obstruction in older men - postrenal causes are readily reversible if identified early 1

Hemodynamic Assessment

Use isotonic crystalloids rather than colloids (albumin or hydroxyethyl starches) for initial volume expansion in patients at risk for or with established AKI 3. The 6S Trial demonstrated that HES 130/0.42 increased mortality, need for renal replacement therapy, and bleeding compared to Ringer's acetate 3. Despite theoretical advantages, colloids show no mortality benefit and add unnecessary cost 3.