Which laboratory tests should be performed to confirm hypocalcaemia and identify its underlying cause?

To confirm hypocalcaemia and identify its underlying cause, the following laboratory tests should be performed:

• Measure serum total calcium and adjust for albumin to confirm hypocalcaemia, as low albumin can affect total calcium levels.
• Measure serum ionised calcium if available, as it reflects the physiologically active calcium fraction, but total calcium adjusted for albumin is usually sufficient.
• Measure serum phosphate, as hypocalcaemia is often associated with hyperphosphataemia in hypoparathyroidism.
• Measure parathyroid hormone (PTH) levels to differentiate causes related to parathyroid gland dysfunction; low or inappropriately normal PTH suggests hypoparathyroidism, whereas elevated PTH suggests secondary causes.
• Assess serum 25-hydroxyvitamin D levels to identify vitamin D deficiency, a common cause of hypocalcaemia.
• Consider magnesium levels, as hypomagnesaemia can cause hypocalcaemia by impairing PTH secretion and action.
• Additional tests may include renal function tests to assess for chronic kidney disease, which can cause hypocalcaemia.

Summary: Initial tests to confirm hypocalcaemia and investigate its cause include serum calcium (adjusted for albumin), phosphate, PTH, 25-hydroxyvitamin D, magnesium, and renal function tests ³,²,¹.