From the Guidelines
The patient's Acute Kidney Injury (AKI) with a Fractional Excretion of Sodium (FeNa) of 4% strongly indicates intrinsic renal tubular damage, specifically pointing to a problem in the tubules. The patient's hospital course was complicated by AKI with a FeNa of 4%, which suggests that the kidneys are unable to appropriately reabsorb sodium in the tubules, leading to excessive sodium excretion despite decreased kidney function.
- A FeNa greater than 1% in the setting of AKI suggests that the kidneys are unable to appropriately reabsorb sodium in the tubules, leading to excessive sodium excretion despite decreased kidney function, as noted in a recent study 1.
- This pattern is characteristic of tubular dysfunction, particularly acute tubular necrosis (ATN), which can result from nephrotoxic medications, ischemic injury, or other insults to the tubular epithelium.
- In contrast, pre-renal causes of AKI (which would affect the renal arterioles) typically present with a FeNa less than 1% as the tubules remain functional and avidly reabsorb sodium in response to decreased perfusion, as discussed in the context of patients with cirrhosis 1.
- Glomerular disorders generally don't cause such high FeNa values unless they're severe enough to cause secondary tubular damage.
- The high FeNa value of 4% definitively points to a tubular problem rather than a primary glomerular or vascular issue, and the use of Fractional Excretion of Urea (FEUrea) may also be helpful in the differential diagnosis, as noted in a small retrospective study 1.
From the Research
Fractional Excretion of Sodium (FeNa) and Acute Kidney Injury (AKI)
- A FeNa of 4% is elevated, suggesting acute tubular necrosis (ATN) as the likely cause of AKI, as a FeNa greater than 3% is typically seen in ATN 2.
- However, it is essential to consider other factors and diagnostic tools, as the sole reliance on FeNa to exclude ATN should be abandoned, and urine microscopy should be pursued for AKI diagnosis 3.
Diagnostic Considerations
- The presence of muddy brown granular casts (MBGC) in urine microscopy is highly suggestive of ATN, and their presence can be used to diagnose ATN, even with a low FeNa [<1%) 3.
- Urine microscopy with examination of the urine sediment examination provides useful diagnostic information about the histology of the kidneys and can be used to differentiate between pre-renal AKI and ATN 4.
- The clinical utility of urine microscopy in the differential diagnosis and prediction of outcome in AKI may be increased by using a simple urinary scoring system based on the number of renal tubular epithelial cells and renal tubular epithelial cell/granular casts 4.
AKI Phenotypes and FeNa
- FeNa can be used to differentiate AKI phenotypes, such as ATN-AKI, hepatorenal syndrome (HRS-AKI), and prerenal AKI, in decompensated cirrhosis 5.
- A lowered cut-off value of FeNa can make the diagnosis of ATN-AKI easier and faster, and thus obviate the need for extensive workup in a significant proportion of patients 5.