Question 1

Why does serum calcium need to be corrected for albumin?

Accepted Answer

Approximately 40–45% of serum calcium is bound to albumin. When albumin is low (hypoalbuminaemia), total serum calcium is falsely low, even though the physiologically active ionised (free) calcium may be normal. The corrected calcium formula adjusts for this: Corrected Ca = Measured Ca + 0.8 × (4.0 − Albumin in g/dL). Without correction, hypoalbuminaemic patients may be incorrectly diagnosed with hypocalcaemia and treated unnecessarily.

Question 2

What is the formula for corrected calcium?

Accepted Answer

Standard formula (using g/dL for albumin): Corrected Ca (mg/dL) = Measured Ca (mg/dL) + 0.8 × (4.0 − Albumin g/dL). In SI units: Corrected Ca (mmol/L) = Measured Ca (mmol/L) + 0.02 × (40 − Albumin g/L). Normal serum calcium: 8.5–10.5 mg/dL (2.12–2.62 mmol/L).

Question 3

What is the normal range for serum calcium?

Accepted Answer

Normal total serum calcium: 8.5–10.5 mg/dL (2.12–2.62 mmol/L). Ionised (free) calcium normal range: 1.15–1.35 mmol/L (4.6–5.4 mg/dL). Values below the lower limit of normal define hypocalcaemia; values above the upper limit define hypercalcaemia.

Question 4

What are the causes of hypocalcaemia?

Accepted Answer

Common causes of true hypocalcaemia include: hypoparathyroidism (post-surgical, autoimmune, genetic), vitamin D deficiency, chronic kidney disease (reduced 1α-hydroxylation), hypomagnesaemia (impairs PTH secretion), pancreatitis (calcium saponification), hungry bone syndrome post-parathyroidectomy, and certain drugs (bisphosphonates, cinacalcet, denosumab, foscarnet).

Question 5

What are the causes of hypercalcaemia?

Accepted Answer

The mnemonic CHIMPANZEES covers most causes: Calcium supplementation excess, Hyperparathyroidism (primary — most common outpatient cause), Iatrogenic (thiazides, vitamin D, lithium), Metastatic malignancy or Multiple myeloma, Paget's disease, Addison's disease, Neoplasia (PTHrP-secreting tumours — most common inpatient cause), Zollinger-Ellison/MEN syndromes, Excess vitamin D (granulomatous disease — sarcoidosis, TB), Endocrine (hyperthyroidism), Sarcoidosis/immobilisation.

Question 6

Is the corrected calcium formula accurate?

Accepted Answer

The Payne formula (0.8 correction factor) is the most widely used but has significant limitations. Multiple studies show poor correlation between corrected calcium and measured ionised calcium, particularly in critically ill patients, those with nephrotic syndrome, malignancy, and acid-base disturbances. When accurate calcium assessment is critical, direct measurement of ionised calcium (blood gas analyser) is preferred over the corrected formula.

Question 7

What are the symptoms of hypocalcaemia?

Accepted Answer

Hypocalcaemia symptoms include neuromuscular irritability: Chvostek sign (facial muscle twitch on tapping the facial nerve), Trousseau sign (carpopedal spasm on blood pressure cuff inflation), perioral and fingertip paraesthesias, muscle cramps, tetany, and in severe cases seizures and laryngospasm. ECG findings include prolonged QTc interval. Chronic hypocalcaemia can cause cataracts, basal ganglia calcification, and cognitive impairment.

Question 8

What are the symptoms of hypercalcaemia?

Accepted Answer

The mnemonic 'Bones, Stones, Groans, Psychic Moans': Bones — bone pain, osteitis fibrosa cystica; Stones — renal calculi, nephrocalcinosis, polyuria/polydipsia (nephrogenic DI); Groans — nausea, vomiting, constipation, anorexia, peptic ulcers; Psychic Moans — depression, confusion, lethargy, psychosis, coma. ECG shows shortened QTc interval.

Condition	Total Ca (mg/dL)	Total Ca (mmol/L)	Ionised Ca (mmol/L)
Severe hypocalcaemia	< 7.0	< 1.75	< 0.8
Mild hypocalcaemia	7.0 – 8.5	1.75 – 2.12	0.8 – 1.15
Normal	8.5 – 10.5	2.12 – 2.62	1.15 – 1.35
Mild hypercalcaemia	10.5 – 12.0	2.62 – 3.0	1.35 – 1.55
Moderate hypercalcaemia	12.0 – 14.0	3.0 – 3.5	1.55 – 1.8
Severe hypercalcaemia	> 14.0	> 3.5	> 1.8