Feline Hypercalcemia: Total Ca Over 11 mg/dL

A total calcium above 11 mg/dL is abnormal and should never be treated from a single result. Repeat the measurement with albumin and add ionized calcium before you touch a drug, because roughly a third of high total calcium values in cats are artefact or protein-bound and need no treatment at all. True hypercalcemia has a short differential list, and lymphoma sits at the top of it.

Total, corrected and ionized calcium

Reference total calcium in cats is about 8.5–10.5 mg/dL, and the biologically active fraction is ionized calcium at 1.1–1.3 mmol/L. Total calcium includes protein-bound calcium, so a cat with high albumin or a paraprotein can look hypercalcemic while its ionized calcium is normal. The classic correction is total minus 0.4 times (albumin minus 3.5), but the formula performs poorly in cats and should never replace a direct ionized measurement.

Sample handling matters. Ionized calcium needs an anaerobic heparinised sample analysed within about 1 hour, because exposure to air raises pH and lowers the reading. A plain EDTA tube gives a falsely low total and ionized calcium, which is one reason a cat can have two contradictory results in one week.

First, exclude the artefacts

Young growing kittens normally run total calcium up to about 12 mg/dL, and dehydration concentrates albumin enough to lift total calcium by 0.5–1 mg/dL. Lipemia, hemolysis and delayed separation of serum all shift the number. Repeat after a 12-hour fast, in the same laboratory, with albumin and ionized calcium on the same draw. If ionized calcium is normal and the cat is well, the diagnosis is a spurious result, not a disease, and repeat in 3 months.

The differential list in cats

Cause	Clue	Confirm with
Lymphoma	Middle-aged to older, enlarged nodes, mediastinal mass, weight loss	FNA cytology, PTHrP
Idiopathic hypercalcemia	Indoor middle-aged cat, no mass, mild signs	Exclusion plus PTH low-normal
Primary hyperparathyroidism	High ionized calcium, low or normal phosphate	PTH high, ultrasound of neck
Vitamin D toxicosis	Rodenticide, vitamin D cream, some plants	History, 25-hydroxyvitamin D
Chronic kidney disease	Usually low total calcium, high phosphate	Creatinine, SDMA, phosphate
Granulomatous disease	Fever, uveitis, weight loss, raw diet	Toxoplasma titre, imaging
Osteolytic or myeloma	Bone pain, lytic lesions, high globulin	Radiographs, protein electrophoresis

Idiopathic hypercalcemia is the most common diagnosis in an otherwise well middle-aged cat; lymphoma is the most common cause in a cat that is losing weight. A cat with uveitis and fever deserves a toxoplasma titre, and the respiratory and ocular form of that infection is described in toxoplasmosis in cats. If the cat also has chronic ocular or nasal signs, look at feline herpes long-term care before adding another diagnosis to the chart.

Order of treatment: fluids, then drugs

Fluid first, always. 0.9% sodium chloride at 2–3 times maintenance, roughly 60–90 mL/kg per day, corrects dehydration and drives calcium out through the kidneys. Avoid lactated Ringer's in the initial phase because it contains calcium, and avoid any fluid containing phosphate. Rehydrate over 12–24 hours, then reassess the ionized calcium before adding a second drug.

Second, furosemide 1–2 mg/kg every 8–12 hours once the cat is volume-replete; it increases urinary calcium loss. Third, a glucocorticoid at 0.5–1 mg/kg per day of prednisolone, but only after a lymph node aspirate, because steroids lyse lymphoma cells and can erase the diagnosis. If calcium stays above 14 mg/dL, add a bisphosphonate: zoledronate 0.1 mg/kg intravenously over 15 minutes, or pamidronate 1–2 mg/kg over 2 hours in saline, repeated at 3–4 weeks with renal values checked first.

What not to do

Do not give calcium supplements, vitamin D, cod liver oil or a raw diet heavy in whole fish and liver. Do not use thiazide diuretics, which reduce calcium excretion. Do not start a low-calcium prescription diet as first-line therapy in a cat that is still being diagnosed, and do not treat a number without a clinical sign and a repeat test.

Monitoring and the kitten exception

Recheck ionized calcium at 24–48 hours during an acute episode, then at 1–2 weeks, then monthly once stable. For idiopathic cases, the target is ionized calcium below 1.5 mmol/L with a cat that eats and gains weight. Increasing dietary fibre and keeping the cat hydrated are the usual maintenance moves.

Growing kittens are a separate case: total calcium up to 12 mg/dL can be normal, and the useful signal is the trend rather than one value. Weigh kittens weekly on the kitten weight tracker and interpret calcium alongside growth, not against adult reference ranges.

Emergencies and red flags

• Total calcium above 16 mg/dL, or ionized calcium above 1.8 mmol/L
• Vomiting, refusal to eat, or constipation lasting more than 48 hours
• Muscle twitching, weakness, or a stiff gait
• Heart rate under 120 per minute, or an arrhythmia on auscultation
• Excessive drinking and urinating with rapid weight loss

These need hospitalisation with intravenous fluids and an ECG, not an outpatient recheck. Severe hypercalcemia causes acute kidney injury, and the damage can become permanent within 48 hours.

Five steps after an abnormal result

1. Repeat total calcium with albumin and ionized calcium on one fasting sample.
2. Palpate every lymph node and image the chest if any node is enlarged.
3. Run a renal panel with phosphate, plus PTH and PTHrP if ionized calcium is high.
4. Start 0.9% saline at 60–90 mL/kg per day and recheck ionized calcium in 24 hours.
5. Add furosemide only after rehydration, and biopsy before any steroid.

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