To confirm hypocalcaemia, the initial laboratory test should be a measurement of serum total calcium, ideally corrected for albumin concentration, or alternatively ionised calcium, which directly reflects physiologically active calcium levels. This confirms the presence of hypocalcaemia NICE CKS Feingold et al. 2000.
Once hypocalcaemia is confirmed, further tests are essential to identify the underlying cause. These include serum phosphate, magnesium, and parathyroid hormone (PTH) levels to evaluate parathyroid function and electrolyte disturbances NICE CKS Feingold et al. 2000. Low PTH suggests hypoparathyroidism, while elevated PTH indicates secondary causes such as vitamin D deficiency or chronic kidney disease.
Vitamin D status should be assessed by measuring serum 25-hydroxyvitamin D to detect deficiency, a common reversible cause of hypocalcaemia NICE CKS Feingold et al. 2000. Additionally, renal function tests (serum creatinine and estimated glomerular filtration rate) are important to exclude chronic kidney disease as a cause NICE CKS Feingold et al. 2000.
Magnesium levels are critical because hypomagnesaemia can cause functional hypoparathyroidism and refractory hypocalcaemia NICE CKS Feingold et al. 2000. In some cases, measurement of urinary calcium excretion may help differentiate causes related to calcium loss.