Fractional excretion of sodium and urea
What are the fractional excretion of sodium and urea and how are they calculated?
The fractional excretion of sodium and urea are indices of tubular function that are useful in differentiating between prerenal azotemia and ATN, particularly in patients with oliguric AKI. In prerenal states, the tubular response of the kidney to decreased effective perfusion is to increase tubular sodium reabsorption. The fractional excretion of sodium (FE Na ), which is the percentage of the sodium filtered at the glomerulus that is excreted in the urine, will decrease from a normal value of slightly less than 1% and will be significantly less than 1%. In contrast, in many forms of ATN, impaired tubular function will result in elevated values of FE Na (>3%).
FE Na is calculated by dividing the urine sodium excretion by the filtered sodium load:
• The excreted Na + = U Na × V, where U Na is the urine sodium concentration and V is the urine volume
• The filtered Na + = P Na × GFR, where P Na is the plasma sodium concentration and GFR is the glomerular filtration rate
• The GFR can be estimated as the creatinine clearance, U creat × V / P creat , where U creat and P creat are the urine and plasma creatinine concentrations, respectively.
Although the FE Na is helpful in differentiating between prerenal azotemia and ATN, it is not completely reliable. Patients who develop ATN in the setting of true or effective volume depletion, such as with concurrent heart failure, cirrhosis, burn injury or sepsis, or as the result of contrast-induced nephropathy or rhabdomyolysis, may have a FE Na <1%. Conversely, the FE Na may be >1% in patients with prerenal azotemia who are on diuretics or who have underlying CKD. In these patients, the fractional excretion of urea (FE urea ) may be helpful in differentiating between prerenal azotemia and ATN. The FE urea is calculated in a fashion analogous to the FE Na as:
FE urea = (U urea / P urea )/ (U creat / P creat )
In prerenal azotemia FE urea is usually <35%; in ATN, values are typically >50%. Fractional excretion of urea may be artificially lowered in the setting of infections with urea-splitting organisms. It should also be recognized that the while these diagnostic indices are guides in the differentiation between prerenal and intrinsic AKI, their diagnostic performance is imperfect and they should not be used as the sole criteria for establishing the etiology of AKI.
