Sodium and the Na/K ratio in cats and dogs

What it shows

According to eClinPath, sodium is a primary determinant of how concentrated the blood is (osmolality) and of the volume of fluid outside cells. A high or low sodium usually means a change in water relative to sodium, not necessarily a change in the total amount of sodium in the body.

The Na/K ratio is simply sodium divided by potassium. It is printed because one disease, Addison’s disease (hypoadrenocorticism), typically lowers sodium and raises potassium at the same time: without the hormone aldosterone the kidneys lose sodium and keep potassium. A low ratio is a reason for the vet to think about Addison’s disease — but other conditions can lower it too.

Higher than reference — what it may be a sign of

• Water lost faster than sodium: vomiting and diarrhea, heavy urination in chronic kidney disease or some acute kidney injuries, fever or overheating, panting.
• Not enough water: no access to water, inability to drink, central diabetes insipidus in dogs.
• Too much sodium: salt intake, hypertonic fluids, rarely an excess of aldosterone.
• Artefact: water evaporating from an unsealed tube.

eClinPath notes that high sodium is less common than low sodium in sick animals.

Lower than reference

• Loss of salt-rich fluid: lack of aldosterone (Addison’s disease), diuretics; fluid collecting in body cavities — a ruptured or blocked urinary tract, peritonitis, repeated draining of chest fluid.
• Water held back: heart failure, liver disease, nephrotic syndrome, advanced kidney failure; drinking very large amounts (psychogenic polydipsia in large dogs).
• High blood glucose pulls water into the blood and dilutes sodium.
• Artefact (pseudohyponatremia): marked lipemia or very high protein with some analyzer methods.

eClinPath adds that dogs and cats with vomiting or diarrhea are less likely to have low sodium unless there are other losses or marked fluid depletion.

Look at together with

• Potassium — with sodium it gives the Na/K ratio; both together point towards or away from Addison’s disease.
• Chloride — normally moves in step with sodium; when it does not, the vet looks at acid-base balance.
• Urea, creatinine and urine specific gravity — hydration and kidney function; VCA notes kidney values may rise in Addison’s disease.
• Glucose, triglycerides and total protein — to recognise dilution and artefacts.

Units and conversion

Sodium is reported in mmol/L or mEq/L; for sodium the two are the same number. The Na/K ratio has no unit: it is calculated from sodium and potassium measured in the same units. In the app each row keeps the unit printed on the form.

Reference range

The reference range is on your lab’s form. For the ratio, the Merck Veterinary Manual states that Na/K ratios below 27 suggest Addison’s disease but are not pathognomonic, and eClinPath calls the ratio neither sensitive nor specific. VCA adds that the same electrolyte changes may be seen with problems in other organs. The diagnosis is confirmed by an ACTH stimulation test, not by the ratio. A ratio inside the range does not rule the disease out either: in the atypical form, sodium and potassium are usually normal.

Cats and dogs

Sources

1. eClinPath (Cornell University) — Sodium
2. eClinPath (Cornell University) — Potassium
3. Addison Disease (Hypoadrenocorticism) in Animals. Merck Veterinary Manual
4. VCA Animal Hospitals — Testing and Monitoring in Addison’s Disease
5. VCA Animal Hospitals — Serum Electrolytes
6. Whitbread TJ. Clinical Biochemistry. Merck Veterinary Manual
7. eClinPath (Cornell University) — Phosphate