Creatinine and potassium in abdominal fluid in cats and dogs

What it shows

These two codes are the same tests as in blood, only measured in the fluid from a body cavity — most often the abdomen:

• Effusion creatinine — creatinine is a waste product that the kidneys concentrate in urine, so urine contains far more of it than blood. When urine leaks into the abdomen, creatinine in the fluid stays well above the blood level.
• Effusion potassium — urine is also rich in potassium, so leaked urine raises potassium in the abdominal fluid.

A single number from the fluid means little on its own. What matters is the ratio of the fluid value to the blood value measured at the same time. Urea is less useful for this: it is a small molecule that quickly evens out between fluid and blood.

Higher than reference — what it may be a sign of

A fluid value clearly above the blood value supports a suspicion of uroabdomen — urine in the abdomen, for example from a torn bladder, ureter or urethra. Blunt and road-traffic trauma are a common cause in both cats and dogs; blockage of the urethra and injury during procedures on the urinary tract are others.

• Creatinine — in leaked urine it is usually more than twice the blood value, often two to four times higher. In a study in dogs, a fluid-to-blood creatinine ratio above 2:1 was 100% specific and 86% sensitive for uroabdomen.
• Potassium — in the same study in dogs, a fluid-to-blood potassium ratio above 1.4:1 was 100% sensitive and specific.

These cut-offs support the suspicion rather than replace imaging; the vet may confirm a leak with a contrast X-ray of the bladder and urethra. In a male dog, sperm seen in the fluid also points to urine.

Lower than reference

Creatinine and potassium in the fluid close to the blood values speak against urine in the abdomen, but a creatinine ratio below 2:1 does not fully rule it out: in the dog study its sensitivity was 86%, not 100%. A value lower than in blood has no special meaning. If the suspicion remains, the vet relies on imaging and on repeat tests.

Look at together with

• Blood creatinine, urea and potassium — needed for the ratio itself. Urine absorbed from the abdomen raises them in blood too; high blood potassium develops over 24 to 48 hours.
• Protein and cell count of the fluid — urine irritates the lining of the abdomen, so the fluid may look like a transudate or an exudate.
• Fluid cytology — to look for bacteria and other causes of the effusion.

Units and conversion

Effusion creatinine is written in µmol/L or mg/dL, as in blood: 1 mg/dL = 88.4 µmol/L. Effusion potassium is written in mmol/L. The ratio has no unit, but it only works when the fluid and the blood value are in the same unit — for example 400 µmol/L in the fluid against 100 µmol/L in blood is 4:1. In the app each row keeps the unit printed on the form.

Reference range

The reference range is on your lab’s form, but for fluid from a body cavity there is usually no “normal” interval. The lab or the vet compares the fluid with the blood taken at the same time; the ratios above are guides from studies, not a lab’s reference, and the vet reads them together with the animal’s history and imaging.

Cats and dogs

Sources

1. eClinPath (Cornell University) — Peritoneal fluid
2. Marvel S. Uroabdomen: Approach and Management. Today’s Veterinary Practice, 2022
3. Robakiewicz P, Halfacree Z. Urinary tract trauma in cats: stabilisation, diagnosis and management. J Feline Med Surg, 2023
4. Uroabdomen (Proceedings). dvm360