Fractional excretion and urine electrolytes in cats and dogs
Fractional excretion (FE) shows what share of a substance that the kidneys filtered from blood ended up in urine instead of being taken back. It is calculated from a urine sample and a blood sample taken at the same time, with creatinine in both to correct for how concentrated the urine is. These lines appear on a form when the vet looks more closely at how the kidney tubules handle salts.
What it shows
The kidneys filter a large volume of blood every day and then take back most of the useful substances — sodium, chloride, potassium, calcium and phosphorus — in the tubules. A urine concentration on its own is hard to read, because it rises and falls with how concentrated the urine is. Fractional excretion (FE) solves this by comparing the substance with creatinine, which the kidneys filter and hardly take back. Put simply, FE shows what share of the filtered amount ended up in urine instead of being reabsorbed.
The calculation is: FE (%) = (substance in urine ÷ substance in serum) × (creatinine in serum ÷ creatinine in urine) × 100. It needs a urine sample and a blood sample taken at about the same time; a single spot urine sample is commonly used. A low FE means the kidneys hold on to the substance; a high FE means more of it is lost in urine.
The lines on a form:
- FENa — fractional excretion of sodium, in %.
- FEP — fractional excretion of phosphorus, in %.
- Urine creatinine (uCrea) — the reference substance in the formula; it also serves as the denominator of ratios such as UPC.
- Urine sodium, potassium, chloride, calcium and phosphorus (uNa, uK, uCl, uCa, uP) — plain concentrations in urine, the raw material for FE.
Higher than reference — what it may be a sign of
- Damage to the kidney tubules — in dogs with acute kidney injury, FE of sodium, chloride, potassium, calcium and magnesium was higher when the kidney tissue itself was injured than when the injury came from poor blood supply and responded to fluids: median FENa was about 2.4% versus 0.24%.
- Chronic kidney disease — in a dog study, FE of sodium, potassium, chloride, calcium and phosphorus rose with the IRIS stage.
- Fanconi syndrome — a tubular disorder in which FE of glucose, sodium, potassium, phosphorus and bicarbonate is raised while blood levels are normal.
- Phosphorus and parathyroid hormone — PTH increases the excretion of phosphate by the kidneys.
- Fluids given before sampling — they can shift FE values, so the vet takes them into account.
Lower than reference
A low FENa fits kidneys that are saving sodium and water, for example when blood flow to the kidneys is reduced. In the dog study, however, FENa in fluid-responsive injury did not differ from that of healthy dogs, so a low value does not settle the cause on its own.
Look at together with
- Creatinine in blood — part of the formula and the main kidney marker.
- Sodium, potassium, chloride, calcium and phosphorus in blood — the serum half of each FE.
- Urine specific gravity — explains why plain urine concentrations rise and fall.
- PTH — when phosphorus handling is the question.
Units and conversion
FENa and FEP are percentages (%): the share of the filtered amount that reached urine. Urine creatinine is reported in mmol/L or mg/dL; 1 mg/dL equals 0.0884 mmol/L. Urine sodium, potassium, chloride, calcium and phosphorus are reported in mmol/L. For the calculation, each substance has to be in the same unit in urine and in serum — mixing mmol/L and mg/dL gives a wrong result. In the app each row keeps the unit printed on the form.
Reference range
The reference range is on your lab’s form; for FE many forms give none, and published values come from studies rather than from one standard. In the dog acute kidney injury study, FENa above 0.90% best separated injury of the kidney tissue from fluid-responsive injury — a research cut-off, not a reference range. Because serum creatinine is part of the formula, FE in dogs with raised creatinine is interpreted with caution.
Cats and dogs
The principle is the same in cats, but most published FE data come from dogs, and the dog figures above are not transferred to cats. Fanconi-type tubular disorders are uncommon in cats; one described acquired cause is a reaction to a particular anticancer medicine. If FE is measured in a cat, the vet reads it against the laboratory’s own values.
Most of what is known about FE in small animals comes from dogs, from studies of acute and chronic kidney disease. In Basenji dogs Fanconi syndrome is a heritable disorder, and it also occurs uncommonly in other breeds; raised FE of several substances helps document it.
Add a lab form — Pet Tracker Space recognises the lines, keeps the form’s units and draws each indicator over time with your lab’s reference band.
Sources
- Troìa R et al. Fractional excretion of electrolytes in volume-responsive and intrinsic acute kidney injury in dogs: diagnostic and prognostic implications. J Vet Intern Med, 2018 (PMC)
- Selin AK et al. Urinary cystatin C, glucose, urea, and electrolytes in dogs at various stages of chronic kidney disease. J Vet Intern Med, 2025 (PMC)
- Merck Veterinary Manual — Renal tubular defects in dogs and cats
- eClinPath (Cornell University) — Phosphate
- eClinPath (Cornell University) — Calculations: urine tests
This page explains what a lab value means. It is not a diagnosis and not a substitute for your veterinarian — only they can interpret results for your pet.