Use of C-terminal parathyroid assays in a large metropolitan hospital

The use of C-terminal parathyroid (C-PTH) assays, performed at a commercial laboratory, was evaluated for one year at Charity Hospital, New Orleans. Of 72 patients, the most frequent diagnosis was primary hyperparathyroidism (1 degree HPT) (n = 17, 24 percent), followed by malignancy (n = 15, 21 percent), chronic renal disease (n = 10, 14 percent), and thiazide diuretics (n = 5, 7 percent). In the 1 degree HPT group, all were hypercalcemic (mean serum calcium, 11.6 ml per dL) and had elevated C-PTH. Patients with malignancy had higher mean serum calcium levels than patients with 1 degree HPT. Three patients with malignancy had elevated C-PTH; two had suspected coexisting 1 degree HPT and neoplasm; and one had suspected pseudohyperparathyroidism. Patients with chronic renal disease undergoing dialysis treatment had the highest C-PTH recorded (all had elevated C-PTH) but only 50 percent were hypocalcemic. Three of the five patients on thiazide diuretics had increased C-PTH, indicating that the withdrawal of this drug may unmask underlying hyperparathyroidism. Appropriate reasons for ordering C-PTH include evaluation of hypercalcemia (n = 32, 44 percent) or hypocalcemia (n = 12, 17 percent); indicated clinical reasons not included in the hyper- or hypocalcemic groups, including chronic renal disease in patients undergoing dialysis or transplant (n = 5, 7 percent); in suspected 1 degree HPT, (n = 5, 7 percent); and in patients with renal stones (n = 2, 3 percent) or on thiazide diuretics (n = 2, 3 percent). Based on laboratory and clinical criteria, C-PTH assay was appropriately used in 81 percent of the patients surveyed (58/72).