What labs to order for a patient with Acute Kidney Injury (AKI) on Chronic Kidney Disease (CKD) workup?

Laboratory Workup for AKI on CKD

For a patient with AKI superimposed on CKD, order serial serum creatinine, complete metabolic panel, urinalysis with microscopy, urine albumin-to-creatinine ratio, and renal ultrasound as the core initial workup. 1, 2

Essential Blood Tests

Immediate Priority Labs

• Serial serum creatinine measurements to establish trajectory and confirm AKI diagnosis (≥0.3 mg/dL increase within 48 hours or ≥1.5 times baseline within 7 days) 1, 3
• Complete metabolic panel including sodium, potassium, calcium, chloride, phosphorus, and magnesium to evaluate electrolyte disturbances 2
• Blood urea nitrogen (BUN) with BUN-to-creatinine ratio calculation 2, 4
• Cystatin C as a confirmatory test when creatinine-based eGFR may be inaccurate, particularly in patients with altered muscle mass or chronic conditions 2

Additional Blood Work

• Complete blood count to assess for anemia (common in CKD and can worsen with AKI) 2
• Phosphorus and parathyroid hormone (PTH) for mineral metabolism assessment, especially important in the CKD population 5

Critical Urine Studies

First-Line Urine Tests

• Urinalysis with microscopic examination to detect cells, casts, and crystals that differentiate causes of AKI 2, 3

  • Bland/normal sediment suggests prerenal physiology 6, 3
  • Muddy brown casts indicate acute tubular necrosis 6
  • Red blood cell casts suggest glomerulonephritis 2

• Urine albumin-to-creatinine ratio (ACR) from untimed spot urine sample as the preferred method for proteinuria assessment 2, 5

Diagnostic Urine Biochemistry

• Spot urine sodium concentration and fractional excretion of sodium (FENa) to differentiate prerenal from intrinsic causes 2, 6, 4, 7

  • FENa <1% with urine sodium <10 mEq/L strongly suggests prerenal physiology 6, 3
  • FENa >1% indicates tubular injury or acute tubular necrosis 6, 3
  • Important caveat: FENa has limitations in sepsis and patients on diuretics 6

• Fractional excretion of urea (FEUrea) as complementary parameter when FENa is unreliable (e.g., diuretic use), with <28% suggesting prerenal physiology 6

• Urine specific gravity (USG) and renal failure index (RFI) provide high specificity (>85%) for prerenal AKI and are not confounded by medications or comorbidities 7

Mandatory Imaging

• Renal ultrasound to evaluate kidney size, echogenicity, and rule out obstruction 1, 2, 6, 3

  • Small kidneys suggest underlying CKD 2
  • Normal-sized kidneys support acute process 3
  • Hydronephrosis indicates postrenal obstruction 2

• Unenhanced CT abdomen/pelvis if ultrasound shows hydronephrosis to characterize level and cause of obstruction 2

  • Critical caveat: Avoid iodinated contrast in AKI unless absolutely necessary 2, 3

Monitoring Strategy

Acute Phase (First 48-72 Hours)

• Serial creatinine every 24-48 hours to assess trajectory and response to interventions 1, 3
• Daily urine output monitoring as oliguria (<0.5 mL/kg/h for 6 hours) defines AKI staging 1, 3
• Daily electrolytes until stabilized, particularly potassium and bicarbonate 2

Follow-Up Assessment (3 Months Post-AKI)

• Repeat creatinine and eGFR calculation to assess for resolution versus progression to CKD 1, 2, 3
• Repeat urine albumin-to-creatinine ratio to evaluate for persistent kidney damage 2, 5
• Persistence of abnormalities >3 months is required for CKD diagnosis 2

Tests to AVOID or Use Selectively

Low-Yield Tests in Routine AKI Workup

• Autoimmune serologies (ANA, ANCA, anti-GBM) have extremely low diagnostic yield unless clinical suspicion for glomerulonephritis exists 8
• Complement levels should be reserved for cases with active urine sediment suggesting glomerular disease 8
• Kidney biopsy is not routine but consider if diagnosis remains unclear after initial workup or if glomerulonephritis is suspected 8

Common Pitfall

Many diagnostic tests ordered for AKI have low clinical utility even when abnormal 8. Focus on the core workup above rather than reflexively ordering extensive autoimmune panels unless specific clinical features suggest systemic disease 8.

Interpretation Framework for Risk Stratification

Stage AKI Severity 1

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

Classify Underlying CKD by eGFR and Albuminuria 2

• GFR categories: G1 (≥90), G2 (60-89), G3a (45-59), G3b (30-44), G4 (15-29), G5 (<15 mL/min/1.73m²)
• Albuminuria categories: A1 (<30 mg/g), A2 (30-300 mg/g), A3 (>300 mg/g)

Key Diagnostic Consideration

Interpreting labs in AKI-on-CKD requires comparison to baseline values 2. A creatinine of 2.0 mg/dL may represent severe AKI in someone with baseline 0.8 mg/dL but minimal change in someone with baseline CKD and creatinine of 1.8 mg/dL 1.