Pre-operative Serum Alkaline Phosphatase as a Predictor for Hypocalcemia Post-Parathyroid Adenectomy

BACKGROUND Post-operative hypocalcemia (POH) may complicate parathyroidectomy for primary hyperparathyroidism. This study investigates the relationship between POH and pre-operative risk factors to identify a simple method to predict POH risk. METHODS Retrospective data on risk factors for 29 patients was collected for age, pre-operative serum calcium, alkaline phosphatase (ALP), parathyroid hormone (PTH), adenoma size, gender, and bisphosphonate pre-treatment. These were screened to exclude those with small effect sizes, and analyzed using Univariate General Linear Modeling (GLM) with trough serum calcium (TSC) as the dependent variable. The regression function of the significant variables against TSC was plotted with 95% CI fit lines. The cut-off regression value was read from the lower fit line for the threshold TSC of 2.0 mmol/L. RESULTS After screening, log-transformed age (r=0.600), ALP (r=-0.415), and PTH (r=-0.433) were entered into GLM analysis, which showed that only ALP was significant (p=0.016 Eta-squared=0.220). The GLM model had a partial Eta-squared of 0.559 with 98% observed power. The plot of TSC against log-ALP gave an ALP cut-off of 340 U/L. CONCLUSIONS The study shows that there is a strong relationship between ALP and TSC, and that patients with a pre-operative ALP less than 340 U/L are unlikely to have symptomatic POH (100% sensitivity, 95% specificity). While vitamin D was not analyzed in this study, the ALP cut-off is conservative and should still screen out cases with severe vitamin D deficiency. We therefore recommend that pre-operative ALP be utilized to complement clinical protocols for POH management in parathyroid adenomectomy patients.