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    Calcium: total and ionised calcium, albumin correction, causes of hyper- and hypocalcaemia and when to seek help.

    Calcium (Ca) is the most abundant mineral in the body; 99 % of it is stored in bones and teeth. The small amount in the blood is vital for nerve conduction, muscle contraction, heart rhythm, blood clotting and hormone secretion. About half of blood calcium is bound to proteins, mainly albumin, and the other half is free (ionised), the biologically active form. Blood calcium is tightly regulated by parathyroid hormone (PTH) and vitamin D, which act on bone, kidneys and gut.

    ElectrolytesParathyroid hormoneVitamin D
    Total calcium2.15–2.55 mmol/L (8.6–10.2 mg/dL)
    Ionised calcium1.15–1.30 mmol/L
    Hypercalcaemia> 2.6 mmol/L (corrected)
    Hypocalcaemia< 2.1 mmol/L (corrected)

    Correcting for albumin

    When albumin is low (in malnutrition, liver or kidney disease, or acute illness), total calcium falls even though free calcium is normal. Laboratories therefore often report albumin-adjusted ("corrected") calcium: corrected Ca (mmol/L) = measured Ca + 0.02 × (40 − albumin in g/L). The formula is an approximation. When precision matters (critical illness, kidney failure, acid-base disorders), ionised calcium should be measured directly.

    Corrected serum calcium scale: severe hypocalcaemia below 1.9, hypocalcaemia 1.9 to 2.1, normal 2.1 to 2.6, mild hypercalcaemia 2.6 to 3.0, moderate 3.0 to 3.5, severe above 3.5 millimoles per litre Corrected serum calcium (mmol/L) Severe low Tetany, seizures, long QT 1.9 Low Tingling, cramps 2.1 Normal ≈ 2.1–2.6 2.6 Mild high Often none, thirst 3.0 Moderate Polyuria, constipation 3.5 Severe Emergency: confusion
    Figure 1. Albumin-corrected calcium zones. Laboratory reference ranges vary slightly (about 2.1–2.6 mmol/L).

    High calcium — hypercalcaemia

    More than 90 % of cases are caused by primary hyperparathyroidism (usually a benign parathyroid adenoma, common in post-menopausal women) or cancer (bone metastases, myeloma, PTH-related peptide). Measuring PTH separates the two. Other causes include excess vitamin D, thiazide diuretics, lithium, sarcoidosis, immobilisation and hyperthyroidism. Symptoms include thirst and frequent urination, constipation, nausea, tiredness, bone pain, kidney stones, depression and confusion ("stones, bones, groans and moans"), and a short QT on the ECG.

    Low calcium — hypocalcaemia

    Causes include vitamin D deficiency, hypoparathyroidism (most often after thyroid or parathyroid surgery), chronic kidney disease, magnesium deficiency, acute pancreatitis, some medicines (bisphosphonates, denosumab, cinacalcet, loop diuretics) and massive blood transfusion. Symptoms are tingling around the mouth and in the fingers, muscle cramps and spasms (tetany), and in severe cases seizures, a prolonged QT interval and arrhythmia.

    Seek emergency care

    if corrected calcium is above 3.5 mmol/L or below 1.9 mmol/L, or if an abnormal level comes with confusion, drowsiness, severe vomiting, muscle spasms, seizures or palpitations.

    Planned evaluation

    A persistently abnormal calcium needs repeat testing with albumin, plus PTH, vitamin D, magnesium, phosphate and kidney function. Do not take high-dose calcium or vitamin D supplements without medical advice if your calcium is high.

    References

    1. Minisola S, Pepe J, Piemonte S, Cipriani C. The diagnosis and management of hypercalcaemia. BMJ. 2015;350:h2723. doi:10.1136/bmj.h2723
    2. Fong J, Khan A. Hypocalcemia: updates in diagnosis and management for primary care. Can Fam Physician. 2012;58(2):158–162. PMID: 22439169

    This article is for general information and does not replace a consultation with a doctor. Laboratory results must be interpreted by a physician together with symptoms and other tests.

    Published on 6 July 2019