From the Guidelines
The fractional excretion of sodium (FENa) test is the most helpful test to differentiate the cause of acute kidney injury (AKI). This test measures the percentage of sodium filtered by the kidneys that is excreted in the urine, helping distinguish between prerenal, intrinsic, and postrenal causes of AKI. A FENa less than 1% typically indicates prerenal AKI (caused by decreased blood flow to the kidneys), while a value greater than 2% suggests intrinsic renal damage 1. To perform this test, both urine and serum samples are collected to measure sodium and creatinine levels. Other useful tests include urine microscopy (to look for casts, cells, or crystals), urine osmolality, and the BUN-to-creatinine ratio.
In patients taking diuretics, the fractional excretion of urea (FEUrea) may be more reliable as diuretics can artificially elevate the FENa 1. These tests are valuable because they provide immediate information to guide treatment decisions before more definitive tests like kidney biopsy can be performed, allowing clinicians to quickly determine if the kidney injury is due to volume depletion, intrinsic kidney damage, or urinary tract obstruction. The use of biomarkers, such as neutrophil gelatinase–associated lipocalin (NGAL) and cystatin C, can also help improve the diagnostic accuracy of AKI and recognize the different pathophysiological processes 1.
Some key points to consider when interpreting the results of these tests include:
- The clinical context in which the test is being performed
- The presence of any underlying kidney disease or other comorbidities
- The potential for false positives or false negatives
- The need for prompt treatment and follow-up to prevent further kidney damage.
Overall, the FENa test, in combination with other diagnostic tests and clinical information, can help clinicians quickly and accurately diagnose the cause of AKI and guide treatment decisions to improve patient outcomes.
From the Research
Diagnostic Tests for Acute Kidney Injury
The following tests help differentiate the cause of acute kidney injury:
- Urinalysis with microscopy to narrow the differential diagnosis 2
- Measurement of serum creatinine level to assess the increase in serum creatinine 3, 2, 4, 5, 6
- Fractional excretion of sodium to classify acute kidney injury as prerenal, intrinsic renal, or postrenal 5
- Ultrasonography of the kidneys to rule out obstruction, particularly in older men 5, 6
- Serum and urine electrolyte measurements to determine the underlying cause of acute kidney injury 6
- Complete blood count to identify systemic illnesses that might cause poor renal perfusion or directly impair renal function 5
Laboratory Evaluation
The initial laboratory evaluation for acute kidney injury includes:
- Measurement of serum creatinine level 3, 2, 4, 5, 6
- Complete blood count 5
- Urinalysis with microscopy 2
- Fractional excretion of sodium 5
- Serum and urine electrolyte measurements 6
Imaging Studies
Imaging studies, such as ultrasonography of the kidneys, are used to: