When a patient has a serum calcium level of 13 mg/dL, which test should be ordered next to assess the cause?

When a patient presents with a serum calcium level of 13 mg/dL, which is considered significantly elevated, assessing the cause is critical for appropriate management. Measuring the parathyroid hormone (PTH) level is particularly relevant because it helps distinguish between primary hyperparathyroidism and other conditions that can cause hypercalcemia, such as malignancy or vitamin D toxicity.

In cases of primary hyperparathyroidism, one would expect elevated calcium levels accompanied by elevated or inappropriately normal PTH levels. If the PTH is low or suppressed, this might indicate a non-parathyroid cause of hypercalcemia, such as malignancy or granulomatous disease.

While testing BUN and serum creatinine can provide insights into renal function, it does not directly address the underlying cause of hypercalcemia. A 24-hour urine calcium test would assess calcium excretion, which may provide additional information but is typically performed after the initial blood tests to evaluate hypercalcemia. A thyroid panel, although important for other clinical scenarios, is not directly relevant in evaluating hypercalcemia specifically.

Therefore, ordering the parathyroid hormone level is the most appropriate next step to ascertain the cause of the elevated serum calcium in this patient.