What tests should I order for a patient with Acute Kidney Injury (AKI) imposed on Chronic Kidney Disease (CKD)?

Diagnostic Tests for Acute Kidney Injury Superimposed on Chronic Kidney Disease

For patients with AKI superimposed on CKD, a comprehensive diagnostic evaluation should include serum creatinine, eGFR calculation, urinalysis, and electrolyte panel as core tests, with additional biomarkers and imaging studies based on clinical presentation. 1

Core Laboratory Tests

• Serum creatinine and blood urea nitrogen (BUN) should be measured to assess kidney function and determine the severity of AKI 1
• Estimated GFR (eGFR) should be calculated using the 2009 CKD-EPI equation rather than relying on serum creatinine concentration alone 2
• Complete electrolyte panel including sodium, potassium, calcium, phosphorus, and magnesium to identify electrolyte imbalances that may accompany renal dysfunction 1, 2
• Urinalysis with microscopy to evaluate for casts, cells, and crystals that may indicate the underlying cause of AKI 1
• Urine albumin-to-creatinine ratio (ACR) to quantify proteinuria and assess glomerular damage 1, 2

Specialized Tests for Differential Diagnosis

• Fractional excretion of sodium (FENa) and renal failure index (RFI) to differentiate between prerenal and intrarenal causes of AKI, with high specificity for prerenal AKI 3
• Urine sodium concentration (UNa) and urine specific gravity (USG) which have high specificity (>85%) for prerenal AKI 3
• Urine osmolality to assess concentrating ability of the kidneys 3
• Urine to plasma creatinine ratio (UCr/PCr) to evaluate tubular function 3

Biomarkers for Risk Stratification

• Cystatin C should be considered as a confirmatory test when eGFR based on serum creatinine may be less accurate, particularly in patients with muscle wasting or malnutrition 2
• Novel biomarkers such as NGAL, KIM-1, and IL-18 may be considered in specialized settings to detect kidney damage even before changes in serum creatinine, though these are not yet routinely recommended 1, 4

Imaging Studies

• Renal ultrasound should be performed to assess kidney structure, rule out obstruction, and evaluate for evidence of chronic changes such as small echogenic kidneys 1
• Doppler examination of renal vessels should be considered to exclude renovascular causes, particularly in patients with risk factors for vascular disease 1

Additional Considerations for CKD Monitoring

• Parathyroid hormone (PTH) and phosphorus levels should be measured to assess for mineral metabolism disorders, especially in patients with advanced CKD 2, 5
• Repeat measurements of serum creatinine, eGFR, and urine ACR should be performed at least annually in patients with moderate-to-severe CKD 1

Special Considerations

• Interpretation of laboratory values requires comparison to baseline values, as AKI superimposed on CKD may present with smaller absolute increases in serum creatinine 2
• Medication review is essential to identify and discontinue nephrotoxic agents that may be contributing to AKI 1, 6
• The presence of underlying CKD or sepsis poses additional challenges in differential diagnosis, as these conditions alter both baseline biomarker excretion and biomarker performance 4

Follow-up Testing

• After an episode of AKI in a CKD patient, regular monitoring of kidney function is essential as these patients are at increased risk for CKD progression 5, 7
• Monitoring frequency should be guided by the severity of AKI, baseline CKD stage, and rate of kidney function recovery 2

Remember that a single abnormal test result is insufficient for diagnosis, and persistence of abnormalities for >3 months is required to diagnose progression of CKD following an AKI episode 2.