Fractional excretion of sodium (FENa): a hospital kidney test explained

The kidneys filter large amounts of sodium and reabsorb almost all of it. FENa expresses the fraction of filtered sodium that ends up in urine. When blood flow to the kidneys is low, as in dehydration or poor heart output, the kidneys hold on to sodium and little appears in urine. When the kidney tubules are damaged, they cannot hold on to sodium effectively and more is lost.

Creatinine serves as the reference because, as Lab Tests Online UK explains, it is a waste product made by muscles at a steady rate and filtered out by healthy kidneys. FENa is calculated from four numbers measured at the same time: sodium and creatinine in blood, and sodium and creatinine in a spot urine sample. According to a 2022 editorial in the Clinical Journal of the American Society of Nephrology, it is simple to obtain yet complex to interpret, and it can be calculated at any moment without measuring urine volume.

FENa is requested in hospital when kidney function has dropped rapidly, an acute kidney injury, to help judge whether the cause is mainly reduced blood flow (prerenal azotaemia) or damage to the tubules (acute tubular necrosis). A cut-off of less than 1 per cent has been used to define the sodium-avid state of prerenal azotaemia, whereas a value above 1 per cent indicates tubular damage and intrinsic kidney injury, as the editorial explains.

This distinction matters because prerenal states often reverse with fluids or treatment of the heart or blood pressure, whereas established tubular injury needs supportive care while the kidney recovers.

The editorial summarises a meta-analysis in which pooled sensitivity and specificity of the 1 per cent cut-off were 95 per cent and 91 per cent in studies of oliguric patients without chronic kidney disease or diuretic use, but fell to 83 per cent and 66 per cent when such patients were included. The authors conclude FENa is most useful in oliguric patients without chronic kidney disease and not on diuretics.

Because FENa depends on how much sodium is filtered, it can be misleading when kidney filtration is relatively preserved, in chronic kidney disease where filtered sodium is already low, and in injuries that spare tubular function such as glomerulonephritis, interstitial nephritis or vasculitis, or those causing vasoconstriction, such as contrast. Pigment injuries like rhabdomyolysis can mimic a prerenal state.

The fractional excretion of urea is an alternative that is less affected by diuretics but is not widely used. The editorial argues that FENa is purely a marker for renal perfusion and tubular sodium handling and cannot fundamentally differentiate prerenal from renal causes, which lie on a spectrum, and that the reversibility of kidney injury depends more on the cause of reduced perfusion than on the label. Biomarkers have shown promise, but none has a consensus for routine risk assessment.

In practice, clinicians combine FENa with the clinical picture, urine output, urine microscopy and the trajectory of creatinine rather than relying on it alone.

You are unlikely to request FENa yourself. If it appears in notes, it usually means the team is working out why kidney function dropped. Treatment follows the cause: stopping or adjusting medicines, giving fluids carefully, treating infection or blockage, and monitoring blood tests and urine output.

Seek urgent help if you pass very little urine, feel very drowsy or confused, or are vomiting repeatedly and cannot keep fluids down; call NHS 111, or 999 in an emergency.