From the Guidelines
The initial laboratory workup for acute kidney injury (AKI) should include a comprehensive metabolic panel (CMP), complete blood count (CBC), urinalysis with microscopy, and urine electrolytes. The CMP will provide serum creatinine, blood urea nitrogen (BUN), and electrolytes to assess kidney function and identify electrolyte abnormalities 1. A CBC can reveal anemia or signs of infection that may contribute to AKI. Urinalysis with microscopy is essential to evaluate for hematuria, proteinuria, casts, or crystals that may suggest the etiology of AKI. Urine electrolytes, including sodium, should be ordered to calculate the fractional excretion of sodium (FENa), which helps differentiate between prerenal, intrinsic, and postrenal causes.
Key Laboratory Tests
- Comprehensive metabolic panel (CMP) to assess kidney function and identify electrolyte abnormalities
- Complete blood count (CBC) to reveal anemia or signs of infection
- Urinalysis with microscopy to evaluate for hematuria, proteinuria, casts, or crystals
- Urine electrolytes, including sodium, to calculate the fractional excretion of sodium (FENa)
Additional targeted tests based on clinical suspicion include urine protein-to-creatinine ratio, serum and urine protein electrophoresis, complement levels, antinuclear antibodies, anti-neutrophil cytoplasmic antibodies, anti-glomerular basement membrane antibodies, and renal ultrasound to assess kidney size and rule out obstruction 1. These tests are crucial because they help determine the cause of AKI, which guides appropriate management and improves outcomes. Early identification of the underlying etiology allows for targeted interventions to prevent further kidney damage and potential progression to chronic kidney disease.
Importance of Early Identification
Early identification of the underlying etiology of AKI is critical to guide appropriate management and improve outcomes. The diagnosis of AKI should be made when the serum creatinine increases by ≥0.3 mg/dL within 48 hours or is ≥50% from baseline or when the urine output is reduced below 0.5 mL/kg/h for >6 hours 1.
From the Research
Labs to Work Up AKI
To work up acute kidney injury (AKI), several labs should be ordered to aid in diagnosis and management. These include:
- Serum creatinine level: This is a crucial test to assess kidney function and diagnose AKI 2, 3, 4.
- Complete blood count (CBC): This test can help identify underlying conditions that may be contributing to AKI, such as anemia or infection 2.
- Urinalysis: This test can provide information about the presence of blood, protein, or other abnormalities in the urine that may indicate kidney damage 2, 3, 5.
- Fractional excretion of sodium (FeNa): This test can help differentiate between prerenal and intrinsic AKI 2, 5.
- Fractional excretion of urea (FeU): This test can also be useful in evaluating the likelihood of AKI reversibility 5.
- Urine biochemical profile: This may include measurements of enzymes, proteins, and other biomarkers that can indicate kidney damage or dysfunction 3, 4, 5.
- Serum biomarkers: Such as cystatin C, neutrophil gelatinase-associated lipocalin, and uric acid, which can be used to detect AKI early and monitor its progression 4.
Additional Tests
In some cases, additional tests may be ordered to further evaluate AKI, including:
- Ultrasonography of the kidneys: To rule out obstruction or other structural abnormalities 2.
- Blood urea nitrogen (BUN) to creatinine ratio: Although its diagnostic performance is limited, it may still be used in some cases to distinguish between prerenal and intrinsic AKI 6.
- Other biomarkers: Such as kidney injury molecule 1, cytokines, and chemokines, which may be used to establish the duration and severity of injury, predict clinical outcome, and monitor response to treatment 4.