Which criterion of intraoperative iPTH assay is the most accurate in prediction of true serum calcium levels after thyroid surgery?

Background and aims Intraoperative quick intact parathyroid hormone (iPTH) assay (IOPTH) has become a valuable adjunct in parathyroid surgery reliably predicting cure from hyperparathyroid state. Similarly to parathyroid surgery, the accuracy of the assay in predicting postoperative calcemia after thyroid surgery is related to blood sample timing and the criteria applied with no guidelines widely accepted, so far. This study compares different IOPTH criteria in predicting hypoparathyroidism-related hypocalcemia after thyroid surgery. Materials and methods The study included 200 consecutive patients undergoing total thyroidectomy. Three blood samples for IOPTH were taken in each patient: preoperatively-baseline (BL), at the end of surgery-skin closure (SC), and at 4 h postoperatively (4H). Serum calcium was routinely monitored at 4, 12, 24, 48, and 72 h postoperatively. The incidence and severity of hypocalcemia and related symptoms were matched to IOPTH results. The following criteria were tested: A, greater than 50% drop from BL at SC; B, greater than 70% drop from BL at SC; C, greater than 50% drop from BL at 4H; D, greater than 70% drop from BL at 4H; E, serum iPTH less than 15 pg/ml at SC; F, serum iPTH less than 10 pg/ml at SC; G, serum iPTH less than 15 pg/ml at 4H; H, serum iPTH less than 10 pg/ml at 4H. The accuracy of the tested criteria was calculated in predicting serum calcium level less than 2.0 mmol/l at any point after thyroidectomy. Results Tested criteria had the following value in predicting serum calcium level less than 2.0 mmol/l after thyroidectomy (sensitivity, specificity, positive predictive value, negative predictive value, and overall accuracy, respectively): A (60, 89, 38, 95, and 86%), B (80, 93, 57, 98, and 92%), C (70, 90, 44, 96, and 88%), D (85, 95, 65, 98, and 94%), E (80, 91, 50, 98, and 90%), F (90, 95, 69, 99, and 95%), G (90, 95, 70, 99, and 95%), H (95, 99, 90, 99, and 98%). Conclusions The criterion of iPTH serum level less than 10 pg/ml at 4 h postoperatively has the highest accuracy in predicting serum calcium level below 2.0 mmol/l after total thyroidectomy when compared with the other criteria.