A total calcium above 11 mg/dL is abnormal in a cat, but it is a prompt rather than a diagnosis. The next number that matters is ionized calcium: at or above 1.4 mmol/L on a properly handled sample, real hypercalcemia is confirmed and 3 cause groups cover nearly every case. The whole first step fits inside 24 hours.
Total calcium above 11 is a prompt
Most laboratories print a total calcium interval of 8.5 to 10.5 mg/dL. About 30 percent of that total rides on albumin, so dehydration or a high protein sample can lift the number by 0.5 to 1.0 mg/dL with no disease at all. The classic albumin correction was validated in dogs and performs poorly in cats, so it should never replace a direct ionized measurement.
Sample handling decides more cases than the analyzer does. Ionized calcium needs a heparin tube, minimal air exposure and a run within 1 hour, because carbon dioxide escaping raises pH and lowers the reading. An EDTA tube gives a falsely low result, and a 6-hour-old sample drifts again. Two contradictory results in one week usually mean two tubes, not two diseases.
The 24-hour repeat that settles half the cases
Repeat within 24 hours, after a 6 to 8 hour fast, at the same laboratory, with albumin and ionized calcium on the same draw. If the second total is under 11 mg/dL, ionized calcium is under 1.4 mmol/L and the cat is eating well, the diagnosis is a spurious result and the next check is in 3 months. Kittens under 1 year normally reach about 12 mg/dL total, so age alone explains a value that would worry you in a 9-year-old.
If the repeat holds, the case is real and the clock starts. Do not start a bisphosphonate, a calcimimetic or a low-calcium diet before the ionized value and PTH are back; each one changes the numbers the next test depends on.
What 1.4 mmol/L actually means
Ionized calcium usually sits at 1.1 to 1.3 mmol/L and is the fraction the body uses. Between 1.40 and 1.50 with no signs, many cats over 10 repeat as normal in 2 to 4 weeks, so one borderline value is not a diagnosis. Between 1.50 and 1.80 the workup should not wait. Above 1.80 mmol/L, or a total above 16 mg/dL, soft-tissue mineralization and arrhythmia make it an inpatient problem.
Signs are vague until they are not: more drinking, more urination, less appetite, vomiting, constipation, then weakness and a stiff gait. A cat at 1.9 mmol/L with a heart rate under 140 beats per minute needs an ECG, not a diet change.
The 3 cause groups
Almost every case lands in 1 of 3 groups: cancer, primary hyperparathyroidism, or a third group of kidney, vitamin D and granulomatous causes. Lymphoma is the malignancy to look for first, especially in a cat over 8 with enlarged nodes or a mediastinal mass. Primary hyperparathyroidism is the group cured by surgery, which is why PTH has to be drawn before any drug.
| Cause group | Ionized Ca | Confirm with | Typical age |
|---|---|---|---|
| Lymphoma | 1.5 to 2.0 mmol/L | Node cytology, PTHrP, chest imaging | 8 to 14 years |
| Other cancer (carcinoma, myeloma) | 1.4 to 1.9 mmol/L | PTHrP, bone marrow, imaging | 9 to 15 years |
| Primary hyperparathyroidism | 1.5 to 2.2 mmol/L | PTH high or inappropriately normal | 8 to 13 years |
| Chronic kidney disease | 1.0 to 1.4 mmol/L | Creatinine, phosphorus, SDMA | 10 years and up |
| Vitamin D poisoning | 1.6 to 2.5 mmol/L | Exposure history, phosphorus | any age |
| Idiopathic, young cat | 1.4 to 1.6 mmol/L | Exclusion, then 3-month recheck | under 2 years |
Read the ionized column as a rough guide. A lymphoma cat can sit at 1.45 mmol/L for weeks, and a parathyroid cat can reach 2.2 mmol/L with 1 enlarged gland.
What gets tested in the first 24 hours
The first pass is cheap and ordered together: total and ionized calcium, albumin, phosphorus, creatinine, urea, potassium, a CBC, and urine for specific gravity and a calcium-to-phosphorus ratio. Then PTH and PTHrP, drawn before treatment. Imaging follows the exam: chest radiographs for a mediastinal mass, abdominal ultrasound for nodes, kidneys and parathyroid region, and cytology of any enlarged node, because a 5-minute aspirate can end the workup.
Two results redirect everything. High calcium with high phosphorus points at kidney disease or vitamin D; a low or normal PTH with high ionized calcium points at cancer. A PTH that is normal while calcium is high is not normal at all, it is inappropriately unsuppressed, and that is the parathyroid diagnosis.
Why the 24-hour window changes the plan
Above 1.8 mmol/L the kidneys lose water, dehydration concentrates calcium further, and the curve accelerates. Fluids are the first treatment in almost every group and the one step that does not depend on the diagnosis. Steroids are the trap: they lower calcium and shrink lymphoma nodes, so given before cytology they can make a treatable cancer unconfirmable for weeks.
Vomiting, a heart rate under 140, or 1 seizure turns the 24-hour plan into a today plan.
Recheck intervals once treatment starts
After fluids alone: ionized calcium at 24 and 72 hours. After parathyroid surgery: 24 hours, 48 hours, then 7 days, because the drop can be steep. On a calcimimetic or bisphosphonate: 7 days, 2 to 4 weeks, then every 3 months. Idiopathic cases in young cats: 4 weeks, 3 months, then twice a year.
Every recheck needs ionized calcium, total calcium, phosphorus and creatinine on the same draw at the same laboratory. Comparing 1.6 from one lab with 1.5 from another is how a real trend gets lost.
When it is an emergency tonight
- Ionized calcium above 1.8 mmol/L, or total calcium above 16 mg/dL.
- Heart rate under 140 beats per minute, or an irregular rhythm.
- Vomiting plus no urine for 12 hours, or 24 hours without drinking.
- Sudden hind-leg weakness, a stiff gait, or 1 seizure in 24 hours.
- Any known cholecalciferol (vitamin D) bait exposure, whatever the calcium reads today.
Bring previous panels and the exact diet, including any supplement with vitamin D or calcium. If a bait is part of the history, check the ingredient at /tools/toxic/ before the second draw: cholecalciferol rodenticide raises calcium within 24 to 72 hours and the treatment differs completely. A suspected urinary infection changes with the collection method, covered at /health/health-cat-urine-culture/. For an older cat with overdue vaccines, the titer decisions sit at /health/health-cat-vaccine-titer/.
Three steps: book ionized calcium and albumin within 24 hours on a fasting sample, hold steroids until cytology is done, and photograph the report so the next veterinarian compares intervals rather than memories.
Terms mentioned in this article
Frequently asked
Is a total calcium of 11.4 mg/dL an emergency?
Usually not by itself. A single value just above 11 needs a repeat within 24 hours with albumin and ionized calcium on the same draw, because dehydration and protein binding lift the total. It becomes urgent when ionized calcium is at or above 1.8 mmol/L, when total calcium passes 16 mg/dL, or when the cat is vomiting or has a slow heart rate.
Why does the vet want to wait for cytology before steroids?
Steroids lower calcium and shrink lymphoma nodes, so giving them before a fine-needle aspirate can hide the cancer that the aspirate was meant to find. A 5-minute node aspirate or a PTHrP result usually settles the cause group. If the cat is stable enough for a 24-hour workup, that sequence protects the diagnosis.
How often should calcium be rechecked after treatment starts?
After fluids alone, ionized calcium at 24 and 72 hours. After parathyroid surgery, at 24 hours, 48 hours and 7 days. On a calcimimetic or bisphosphonate, at 7 days, 2 to 4 weeks and then every 3 months. Young cats with idiopathic hypercalcemia are checked at 4 weeks, 3 months and twice a year once stable.