Correct for albumin, then use the parathyroid hormone
Expansion
Correction: about 45 per cent of calcium is bound to albumin, so the total is misleading when albumin is abnormal.
Corrected calcium = measured calcium + 0.02 x (40 - albumin in g/l)
So a low albumin underestimates the true calcium, which matters in the malnourished, in liver disease and in the acutely unwell. Where the answer really matters, the ionised calcium on a blood gas is definitive and needs no correction.
Interpreting hypercalcaemia with the parathyroid hormone
| PTH | Cause |
|---|---|
| Raised or inappropriately normal | Primary hyperparathyroidism, tertiary hyperparathyroidism, lithium, familial hypocalciuric hypercalcaemia |
| Suppressed | Malignancy (bone metastases, PTH related peptide, myeloma), vitamin D excess, sarcoidosis and other granulomatous disease, thyrotoxicosis, immobilisation, thiazides, Addison’s, milk alkali |
Together, primary hyperparathyroidism and malignancy account for about 90 per cent of hypercalcaemia. Hyperparathyroidism is usually mild, chronic and found incidentally; malignant hypercalcaemia is usually higher, more acute and more symptomatic.
Symptoms: “bones, stones, abdominal moans and psychic groans”, plus polyuria, thirst, constipation, and a short QT interval.
Hypocalcaemia: with a raised PTH it is vitamin D deficiency, chronic kidney disease, pseudohypoparathyroidism or acute pancreatitis; with a low PTH it is hypoparathyroidism, most often post-surgical, or hypomagnesaemia, which must be corrected first because it blocks PTH release. Signs are Chvostek’s and Trousseau’s, perioral paraesthesia, tetany, seizures and a long QT.