What is a blood calcium test?
The serum calcium test measures total calcium in your blood. About 99% of body calcium is in bones. The remaining 1% in blood is critical for nerve transmission, muscle contraction (including the heart), hormone release and blood clotting. Total calcium is the standard test; ionised calcium (the active free form) is used in critical care.
Calcium normal range
| Test | Normal Range (mg/dL) |
|---|---|
| Total calcium (adults) | 8.5 – 10.5 |
| Total calcium (children) | 8.8 – 11.0 |
| Ionised calcium | 4.5 – 5.6 mg/dL (1.12–1.40 mmol/L) |
HIGH Calcium — Hypercalcaemia
The two most common causes are primary hyperparathyroidism (a parathyroid adenoma overproducing PTH) and malignancy (lung, breast, multiple myeloma, lymphoma releasing calcium from bones). Other causes: vitamin D toxicity, sarcoidosis, thiazide diuretics. Symptoms at high levels: nausea, constipation, excessive thirst, confusion, muscle weakness, kidney stones and cardiac arrhythmias (remembered as 'bones, groans, stones and moans').
LOW Calcium — Hypocalcaemia
Causes: hypoparathyroidism (after neck surgery), severe vitamin D deficiency, kidney disease, magnesium deficiency (low Mg blocks PTH release). Symptoms: muscle cramps, numbness and tingling around the mouth and fingers, muscle spasms (tetany), and in severe cases seizures and cardiac arrhythmias.
Tests ordered alongside calcium
| Test | Why ordered |
|---|---|
| PTH | Distinguishes hyperparathyroidism from malignancy |
| Phosphate | Low phosphate + high Ca = hyperparathyroidism |
| Vitamin D (25-OH) | Low vitamin D is a correctable cause of low Ca |
| Magnesium | Low Mg causes refractory hypocalcaemia |
Questions to ask your doctor
- What is causing my abnormal calcium?
- Do I need a PTH and vitamin D test?
- Should I see an endocrinologist?
- Do I need a bone density (DEXA) scan?