What initial investigations should I perform for a patient presenting with hypercalcaemia?

Initial investigations for a patient presenting with hypercalcaemia should include measurement of albumin-adjusted serum calcium to confirm hypercalcaemia and assess severity. This should be repeated at least once if the initial level is ≥2.6 mmol/L or ≥2.5 mmol/L with features suggestive of primary hyperparathyroidism. Concurrent measurement of parathyroid hormone (PTH) is recommended if albumin-adjusted serum calcium is ≥2.6 mmol/L on two occasions or ≥2.5 mmol/L with suspicion of primary hyperparathyroidism. A random sample for PTH with concurrent calcium measurement is advised. Vitamin D levels should be measured if primary hyperparathyroidism is probable. To exclude familial hypocalciuric hypercalcaemia, urinary calcium excretion should be assessed via 24-hour urine collection or random urine calcium:creatinine ratio. Additional investigations may include renal function tests (eGFR or serum creatinine), and imaging such as renal ultrasound to assess for stones or nephrocalcinosis, and bone mineral density assessment by DXA scan to evaluate for osteoporosis. Other blood tests to exclude malignancy or granulomatous disease may be considered based on clinical suspicion. If the patient is on medications like thiazide diuretics or lithium, these should be reviewed and possibly stopped with repeat calcium testing after 3 weeks. Referral to a specialist is recommended unless a reversible cause is identified and corrected with normalization of calcium levels.