Accuracy of postthyroidectomy parathyroid hormone and corrected calcium levels as early predictors of clinical hypocalcemia

Abstract

Objective: To evaluate the accuracy of measurement of different parathyroid hormone (PTH) and corrected calcium (cCa) levels at different times as early predictors of postthyroidectomy hypocalcemia.

Design: A retrospective cohort study.

Setting: King Fahad Medical City, Riyadh, Saudi Arabia, between January 2006 and March 2009.

Methods: Patients who underwent total or completion thyroidectomy were followed until hospital discharge. Patients were observed clinically for hypocalcemia; at the same time, the postoperative PTH and cCa levels after 6, 12, and 20 hours and then twice daily were recorded.

Main outcome measures: Postthyroidectomy hypocalcemia.

Results: Seventy-nine of 116 patients were enrolled in our study; 26.60% of them had hypocalcemia. PTH measurement at 6 hours postoperatively was an excellent predictor of hypocalcemia (area under the curve = 0.95, 95% CI 0.88-0.99). The mean PTH at 6 hours for hypocalcemic patients was 0.93 (+/- 0.60). A 1.7 pmol/L as a cutoff level of PTH at 6 hours has 95.2% sensitivity, 89.7% specificity, 76.9% positive predictive value (PPV), and 98.1% negative predictive value (NPV). On the other hand, a 2.1 mmol/L as a cutoff level of cCa has 81.0% sensitivity, 81.6% specificity, 65.3% PPV, and 90.9% NPV in predicting hypocalcemic patients.

Conclusions: PTH measurement 6 hours after surgery with a cutoff level of 1.7 pmol/L is more accurate than serial calcium level measurement for early prediction of patients at risk of hypocalcemia. Thus, a single PTH measurement postoperatively will help in discharging patients safely within the first 24 hours, improving bed use and cost-effective care.