By The Pawline veterinary teamVeterinary-reviewed by Alastair Greenway, BVM&S MRCVS · 14 August 2026
Ionised calcium is the free, biologically active fraction of calcium — the part that actually does the work. Because total calcium is heavily influenced by albumin, ionised calcium is the measurement that confirms whether an abnormal total is real, and it's the one that matters clinically.
Calcium travels in the blood in three forms: bound to proteins (mostly albumin), complexed with other small molecules, and free — the ionised fraction. Only that free, ionised portion is biologically active: it is what nerves, muscle, the heart and the clotting system actually use. Roughly half of total calcium is ionised, and the rest is, functionally speaking, in storage or in transit.
Only the free, ionised fraction of calcium is biologically active — which is why it confirms whether an abnormal total calcium is real.
This is why ionised calcium exists as a separate test, and why it so often settles a question that total calcium leaves open.
Why total calcium can mislead
Because so much calcium rides on albumin, anything that changes albumin changes total calcium — without necessarily changing the active fraction at all:
A pet with low albumin (liver disease, protein-losing gut or kidney disease) will often show a low total calcium while its ionised calcium is perfectly normal. The animal is not calcium-deficient; there is simply less protein to carry calcium about.
Conversely, dehydration concentrating albumin can nudge total calcium up.
So a low total calcium in a low-albumin patient is frequently a false alarm, and ionised calcium is the test that reveals it. (Older "correction formulae" that adjusted total calcium for albumin have been shown to perform poorly in dogs and cats, which is why direct measurement of the ionised fraction is preferred.)
When ionised calcium is the answer
Ionised calcium is the measurement of record whenever calcium genuinely matters:
Confirming hypercalcaemia. A persistently high total calcium is confirmed with ionised calcium before the serious differentials — cancer-associated hypercalcaemia, primary hyperparathyroidism, kidney disease, Addison's disease, vitamin D excess — are pursued. A truly raised ionised calcium is the finding that drives that work-up, and it is usually measured alongside parathyroid hormone.
Confirming hypocalcaemia. Symptomatic low calcium (tremors, twitching, seizures — as in eclampsia in a nursing mother, or after thyroid/parathyroid surgery) is judged on the ionised fraction, since that is what determines whether nerves and muscle misbehave.
Monitoring patients on treatment for calcium disorders, and in critical care.
In chronic disease
In chronic kidney disease, calcium and phosphorus handling are disturbed together, and total calcium can be a poor guide to the active fraction. Following ionised calcium gives a truer picture of mineral status over time, and it is watched in patients being treated for calcium disorders to keep the active level in a safe range.
Cat vs dog
The principle is identical in both species, and in both, correction formulae are unreliable while direct ionised measurement is dependable. Species-specific reference intervals apply. In each, a confirmed, persistent ionised hypercalcaemia is a finding that earns a systematic search for its cause.
Cats add one entity of their own: idiopathic hypercalcaemia — a persistent, confirmed rise in ionised calcium with no cause found despite a full search. It is a recognised feline condition (long-haired cats are over-represented) and in referral series it sits among the leading causes of ionised hypercalcaemia alongside kidney disease and cancer, with the proportions varying between studies. It is a diagnosis of exclusion: the label is only earned after the hormonal, kidney and cancer-related causes have been looked for and not found, and management, guided by the vet, is often dietary.
About the reference range and sample handling
This page shows no range strip — Pawline draws a band only where it holds a verified interval, and ionised calcium intervals are analyser-specific. Handling matters unusually much here: the sample should be collected anaerobically and handled promptly, and the direction of the error is worth knowing. Exposure to air lets carbon dioxide escape, the sample's pH rises, more calcium binds to protein — and the measured ionised value falls. A poorly-handled sample therefore reads falsely low, which is why an unexpected low result on an ordinarily-collected sample is usually repeated with proper handling before it is believed. Read your value against the range and method on your own report.
For orientation only, one published example: Cornell University's veterinary laboratory runs ionised calcium on a dedicated blood-gas analyser (a Radiometer ABL-800) rather than the main chemistry panel, and reports roughly 1.16–1.40 mmol/L (≈4.6–5.6 mg/dL) in dogs and 1.11–1.38 mmol/L (≈4.4–5.5 mg/dL) in cats. That "different instrument for one analyte" point is itself the lesson — the figure is assay-specific, so your own report's range is what counts.
What to do next
If a total calcium is abnormal — particularly if albumin is also abnormal — the constructive next step with your vet is an ionised calcium to establish whether the active fraction is genuinely disturbed. If it is, that is the trigger for a focused work-up (often with parathyroid hormone) into the hormonal, kidney and cancer-related causes. If it is normal, an odd total calcium can often be set aside with confidence.
This page is educational and is not a diagnosis. Lab results are interpreted in the full clinical context by your veterinary surgeon; a single value rarely tells the whole story.
