From the Guidelines
To distinguish between post-renal and intrarenal acute kidney injury (AKI), a thorough history, physical examination, and laboratory analysis of blood and urine, including urinalysis, urine electrolytes, and serum biomarkers, are essential, as recommended by the most recent guidelines 1. When evaluating a patient with suspected AKI, it is crucial to start with a detailed history and physical examination to identify potential causes of renal failure. Laboratory analysis of blood, including serum creatinine, blood urea nitrogen, complete blood count, and differential, and urine, including microscopy for casts and epithelial cells, chemistry, and biomarkers, is also vital 1.
Key Diagnostic Tests
- Urinalysis to identify muddy brown casts, renal tubular epithelial cells, and proteinuria, which are more commonly found in intrarenal AKI
- Urine electrolytes, such as fractional excretion of sodium (FENa), to differentiate between pre-renal and intrarenal causes
- Renal ultrasound to identify hydronephrosis and urinary tract obstruction, which are characteristic of post-renal AKI
- Serum biomarkers, like neutrophil gelatinase-associated lipocalin (NGAL) and kidney injury molecule-1 (KIM-1), which rise earlier in intrarenal AKI
- Urine sediment examination to identify different patterns, such as bland sediment in post-renal AKI versus cellular casts in intrarenal AKI
Additional Diagnostic Tools
If uncertainty persists after initial testing, a CT scan or MRI may provide more detailed structural information, while renal biopsy remains the gold standard for definitively diagnosing intrarenal pathology when the cause remains unclear despite other investigations 1.
Importance of Early Diagnosis
Early diagnosis and differentiation between post-renal and intrarenal AKI are critical, as they have distinct treatments and outcomes, and delayed diagnosis can lead to increased morbidity and mortality 1.
From the Research
Diagnostic Testing for Acute Kidney Injury (AKI)
To distinguish between post-renal and intrarenal AKI, several diagnostic tests can be utilized:
- Laboratory work-up, including measurement of serum creatinine level, complete blood count, urinalysis, and fractional excretion of sodium 2
- Ultrasonography of the kidneys to rule out obstruction, particularly in older men 2
- Renal imaging to assess for any postrenal causes of AKI 3
- Renal biopsy in some instances to determine the underlying cause of AKI 3, 4
Classification of AKI
AKI can be classified into three general categories:
- Prerenal causes, which are related to decreased renal perfusion 5, 3, 6
- Intrarenal causes, which are related to direct injury to the kidneys, such as acute tubular necrosis caused by ischemia or nephrotoxicity 6
- Postrenal causes, which are related to obstruction of the urinary tract 5, 3, 2, 4
Diagnostic Evaluation
The diagnostic evaluation of AKI involves:
- A thorough history and physical examination to identify the underlying cause 5, 3, 2, 4
- Laboratory tests, such as serum creatinine level and urinalysis, to assess kidney function 5, 3, 2, 4
- Imaging studies, such as ultrasonography, to rule out obstruction 2
- Identification of risk factors, such as older age, sepsis, hypovolemia/shock, cardiac surgery, infusion of contrast agents, diabetes mellitus, preexisting chronic kidney disease, cardiac failure, and liver failure 2, 4