Falsely elevated parathyroid hormone-related protein (PTH-RP) in a patient with hypercalcemia and renal failure

Abstract

Humoral hypercalcemia of malignancy (HHM) is the cause of hypercalcemia in the majority of patients with hypercalcemia and cancer. Parathyroid hormone-related protein (PTH-RP) has been identified as the circulating factor that mediates HHM. An N-terminal and a C-terminal PTH-RP are clinically useful assays for screening patients for HHM, and both assays are elevated in such patients. C-terminal PTH-RP depends on glomerular filtration and accumulates in patients with renal failure without malignancy, resulting in falsely-elevated levels, whereas N-terminal PTH-RP is low or undetectable in such patients. We present a case of a patient with renal failure and hypercalcemia who did not have an obvious malignancy and who presented with an elevated C-terminal PTH-RP level and a normal N-terminal PTH-RP. In patients with renal failure and hypercalcemia without cancer, C-terminal PTH-RP may be falsely elevated, especially if the eGFR is <20 mL/minute, and in such patients, N-terminal PTH-RP, because it is less affected by renal function, is the preferred test.