A calcium result that comes back just above the reference interval is one of the most common surprises on a senior panel, and it is also one of the easiest to mishandle. In a cat, the number that decides the case is ionized calcium: above 1.4 mmol/L on a properly handled sample means real hypercalcemia, above 1.8 mmol/L is an emergency, and the cause is usually one of 2 things, lymphoma or an overactive parathyroid gland.
What counts as high: 1.4 mmol/L ionized
About 55-60% of calcium in blood is ionized, roughly 30% rides on albumin, and the rest is bound to phosphate and citrate. Ionized calcium is the fraction the body actually uses, so it is the one to measure. Typical reference intervals are 1.20-1.40 mmol/L ionized, 8.0-10.5 mg/dL total (2.0-2.6 mmol/L), and different laboratories set slightly different limits, so read the interval printed on your own report. Values between 1.40 and 1.50 mmol/L with no symptoms are common in cats over 10 and often repeat as normal 2-4 weeks later, which is why one high result alone is not a diagnosis.
Why total calcium above 11 mg/dL misleads
Total calcium moves with albumin. A dehydrated cat can show 11.5 mg/dL total calcium that is entirely protein-driven, while a cat with low albumin can sit at 9.8 mg/dL total and still be ionized-hypercalcemic. Some labs report an albumin-corrected total, but the correction formula was validated in dogs and is unreliable in cats. Sample handling matters just as much: ionized calcium needs a heparin tube, minimal air exposure, and a run within 1-2 hours, or the reading drifts up as CO2 escapes. A value taken from a clotted or 6-hour-old sample should be repeated before anyone books a workup.
The two causes that change the plan
Lymphoma is the malignancy most often behind high calcium in cats, and it raises calcium through PTHrP released by tumour cells rather than through the parathyroid. Expect enlarged lymph nodes, weight loss, or an abdominal mass, and expect the calcium to fall within 1-2 weeks of starting prednisolone. Primary hyperparathyroidism is the opposite picture: a benign adenoma on 1 gland, usually in a cat 8-12 years old, often found by accident, with calcium creeping up over months and no other signs. Idiopathic hypercalcemia fills the gap in younger seniors, roughly 2-8 years old, and is a diagnosis of exclusion.
Test order: ionized calcium before anything else
Order the tests in the sequence below and you spend 200-400 dollars once instead of 3 times. Start with ionized calcium on a fresh sample, add total calcium, albumin, phosphorus, creatinine, and urine specific gravity, then add PTH and PTHrP only after hypercalcemia is confirmed. Chest radiographs in 2 views and an abdominal ultrasound belong in the same week, because a lung mass or an enlarged liver changes the differential before any endocrine result comes back.
| Test | Typical reference | Reading that flags disease | What it separates |
|---|---|---|---|
| Ionized calcium | 1.20-1.40 mmol/L | above 1.40 mmol/L | true hypercalcemia |
| Total calcium | 8.0-10.5 mg/dL | above 11.0 mg/dL | screening only |
| Phosphorus | 3.1-7.5 mg/dL | below 3.0 mg/dL | favours hyperparathyroidism |
| Creatinine | 0.8-2.4 mg/dL | above 2.4 mg/dL | kidney cause or damage |
| PTH | 10-60 pg/mL | above 40 pg/mL with high calcium | primary hyperparathyroidism |
| PTHrP | under 1.5 pmol/L | above 1.5 pmol/L | paraneoplastic cause |
Reference intervals differ slightly between laboratories and analysers; always compare against the range printed on the report you were handed.
Why PTH and PTHrP come second
PTH should drop to the low end or below detection when calcium is high. A PTH in the middle or upper half of the reference interval next to a calcium of 1.5 mmol/L is therefore abnormal, because a healthy gland would be silent. That is the signature of primary hyperparathyroidism, and neck ultrasound or scintigraphy then looks for the single enlarged gland. PTHrP is the mirror test: it should be undetectable, and a raised PTHrP with a suppressed PTH points to a tumour. Running both at once on a cat with 1.45 mmol/L and no clinical signs is how owners end up with 2 ambiguous results and no decision.
Lowering calcium without guessing the cause
An ionized calcium above 1.8 mmol/L, or total calcium above 14 mg/dL, needs same-day care: 0.9% saline to rehydrate, then furosemide once the cat is hydrated, then a bisphosphonate if calcium stays high after 24-48 hours. Corticosteroids lower calcium and shrink lymphoma, but they blur a lymph node aspirate, so the aspirate comes first unless the cat is unstable. For confirmed primary hyperparathyroidism the options are surgery on the affected gland or ultrasound-guided ablation, and both are decided with a surgeon rather than in the consult room. Feeding a renal formula at 1.0-1.2% calcium on a dry-matter basis lowers intake without drugs.
Signs that mean the kidneys are already involved
High calcium damages the kidney tubules and soft tissues long before the cat looks ill. Watch for urine specific gravity under 1.020, drinking above 60 mL/kg a day, vomiting 2-3 times a week, constipation that needs 3 days or more between stools, and weakness when jumping onto a 60 cm surface. Calcium oxalate stones form in this setting, so any cat with a history of bladder stones and a high calcium deserves the full panel. Blood pressure above 160 mmHg on 2 visits is another reason to move quickly.
A workup you can book in one call
- Ask for ionized calcium on a heparin tube, run within 2 hours, plus total calcium and albumin.
- Add phosphorus, creatinine, and urine specific gravity to the same draw.
- Request 2-view chest radiographs and an abdominal ultrasound in the same week.
- Hold steroids until a lymph node or liver aspirate has been taken, unless the cat is collapsing.
- Repeat ionized calcium 2-4 weeks after any single borderline result before starting treatment.
If the calcium is confirmed high, the hypercalcemia overview lists the remaining differentials, and a node the size of a grape is worth aspirating early rather than watching, as covered in lymphoma in cats. A full workup with PTH, PTHrP, and imaging lands in the same bracket as 2 years of food, which the cost calculator can put in context before you agree to it.
Frequently asked
Is 1.45 mmol/L ionized calcium dangerous in a cat?
Mildly high, and not an emergency by itself. Values between 1.40 and 1.50 mmol/L without symptoms are common in cats over 10 and often normalize on a repeat sample 2-4 weeks later, especially if the first sample sat around before testing. It is a reason to repeat ionized calcium properly, check creatinine and urine specific gravity, and look at the lymph nodes, not a reason to start drugs the same day.
How do PTH and PTHrP tell lymphoma from hyperparathyroidism?
PTH should be low when calcium is high, so a PTH that is normal or high alongside high calcium means the parathyroid is driving the problem. PTHrP comes from tumour cells, so a raised PTHrP with a suppressed PTH means the calcium is paraneoplastic, most often lymphoma in cats. Drawing both at once on a borderline result usually produces 2 ambiguous numbers instead of one answer.
What calcium level needs emergency treatment?
Ionized calcium above 1.8 mmol/L, or total calcium above 14 mg/dL, needs same-day intravenous fluids and monitoring. At that level cats show vomiting, constipation, weakness, and heart rhythm changes, and the kidneys take damage within days. Recheck ionized calcium every 24 hours while the cat is on fluids, and treat the cause as soon as the tests identify it.