What this test measures
Total serum calcium measures all calcium in serum — about 45% bound to albumin, 45% in the ionised (free, active) form, and 10% complexed with citrate and phosphate. The active form is ionised calcium; total calcium is a good surrogate as long as albumin is normal.
In patients with low albumin, total calcium will read falsely low — clinicians correct it with the formula: Corrected Calcium = Total Calcium + 0.8 × (4 − Albumin g/dL). When precision matters (sick patients, dialysis), an ionised calcium is preferred.
Calcium homeostasis is regulated by parathyroid hormone (PTH), vitamin D (calcitriol) and calcitonin, acting on bone, kidney and intestine.
Why it matters
In India, calcium abnormalities are common and almost always reflect either vitamin D deficiency (extremely common — affects 70–90% of Indians), parathyroid disease, chronic kidney disease, or malignancy. Persistently raised calcium in an outpatient is most often primary hyperparathyroidism (often with a parathyroid adenoma); in an inpatient it is most often malignancy (lung, breast, multiple myeloma).
Low calcium causes tingling around the mouth and fingertips, muscle cramps, tetany and (rarely) seizures. High calcium causes "stones (kidney), bones (pain), groans (constipation, ulcer), psychic moans (depression, confusion)". The test is a standard part of any electrolyte panel, bone evaluation, or thyroid / parathyroid workup.
How to prepare
No fasting strictly required, though fasting samples reduce day-to-day variability. Avoid prolonged tourniquet time (raises calcium falsely). Tell your doctor about calcium / vitamin D supplements, thiazide diuretics, lithium, and any recent vitamin D injections (large doses can elevate calcium for weeks).
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Total Calcium (mg/dL)[1][2] | 8.5 – 10.5 (correct for albumin if abnormal) | Hypocalcaemia — vitamin D deficiency (most common in India), hypoparathyroidism (post-thyroid surgery is a classic cause), chronic kidney disease, severe magnesium deficiency, acute pancreatitis, malabsorption, hypoalbuminaemia (apparent low calcium — correct for albumin first). Symptoms — tingling, cramps, tetany, Chvostek's sign, prolonged QT on ECG. | Hypercalcaemia — primary hyperparathyroidism (outpatient), malignancy (inpatient — multiple myeloma, lung, breast, lytic bone metastases), vitamin D toxicity, sarcoidosis / granulomatous diseases, prolonged immobilisation, milk-alkali syndrome, thiazide diuretics. Calcium >12 — symptomatic; >14 — emergency. PTH is the most useful next test. |
Working up an abnormal calcium
| Pattern | PTH | Phosphate | Likely cause |
|---|---|---|---|
| High Ca, high PTH | High | Low | Primary hyperparathyroidism (parathyroid adenoma) |
| High Ca, low PTH | Low / suppressed | Variable | Malignancy (PTHrP), vitamin D toxicity, sarcoid, milk-alkali |
| Low Ca, high PTH | High | Low / normal | Vitamin D deficiency (commonest in India), CKD |
| Low Ca, low PTH | Low | High | Hypoparathyroidism (post-surgical, autoimmune) |
| Low total Ca, low albumin | Normal | Normal | Apparent (pseudo-) hypocalcaemia — correct with albumin formula |
Frequently asked questions
Do I need to fast for a calcium test?
Not strictly. Fasting samples are slightly more reproducible. Avoid prolonged tourniquet time during sample collection — it can falsely raise calcium.
My calcium is 8.0 mg/dL but albumin is 2.8 g/dL. Is this low calcium real?
Low albumin lowers total calcium without changing the active ionised fraction. Correct using: Corrected Ca = Total Ca + 0.8 × (4 − Albumin) = 8.0 + 0.8 × 1.2 = 9.0 mg/dL — normal. An ionised calcium can be checked when precision matters.
What is the difference between total and ionised calcium?
Total calcium includes calcium bound to albumin (inactive) and the free ionised fraction (active). Ionised calcium is what the body senses and acts on. Most lab panels report total — ionised is requested in critically ill patients, dialysis, and pancreatitis.
Why is calcium often low in Indians?
Vitamin D deficiency is extremely common in India (70–90% prevalence in many studies), and without vitamin D the gut cannot absorb dietary calcium efficiently. Low vitamin D, low calcium, raised PTH and raised alkaline phosphatase together make a typical "vitamin D deficiency rickets / osteomalacia" picture.
My calcium is 11.0 mg/dL — what now?
Confirm with a repeat sample (correct for albumin). If genuinely raised, your doctor will check PTH, vitamin D, phosphate, kidney function and a urine calcium. Primary hyperparathyroidism is the leading outpatient cause and is curable with surgery.
Can a high calcium cause kidney stones?
Yes — chronic hypercalcaemia increases urine calcium and the risk of calcium oxalate / phosphate stones. Recurrent stone formers should have serum calcium, PTH, vitamin D and a 24-hour urine calcium evaluated.
Does diet alone correct low calcium?
For mild deficiency yes — dairy, ragi, green leafy vegetables, sesame seeds and fortified foods help. Moderate-to-severe deficiency usually also needs vitamin D supplementation (the deficiency is rarely just calcium), and oral / IV calcium under medical guidance.
Related Kidney / Electrolytes tests
Tests commonly ordered alongside CALCIUM, or that help interpret an unexpected result.
Sources & references
- NIH MedlinePlus — Calcium Blood Test · accessed 2026-05-30T00:00:00.000Z
- NCBI StatPearls — Hypercalcemia · accessed 2026-05-30T00:00:00.000Z
- Endocrine Society — Primary Hyperparathyroidism Guidelines · accessed 2026-05-30T00:00:00.000Z
Book with Zelnoo
Get your CALCIUM test done at home — transparent prices, NABL-accredited labs.
Zelnoo lets you compare diagnostic test prices across NABL-accredited labs in Mumbai & Thane, book a free home phlebotomist visit, and receive digital reports in 24–48 hours into a consent-first report vault. No subscriptions, no membership fees — pay only for the test you book.
Book CALCIUM now