Predictive value of serial calcium measurements for occurrence of hypocalcaemia after thyroid surgery
The occurrence of hypocalcemia is an important and frequent complication of thyroidectomy that occurs in up to 75% of all surgeries. The value of serial measurement of serum calcium in postoperative period has a questionable validity. We aimed to evaluate the value of serial calcium measurements and other clinical and biochemical factors as predictors for occurrence of hypocalcaemia.
We prospectively evaluated 50 patients subjected to thyroid surgery due to various indications during the year 2011. Predictive values of serial calcium measurements were evaluated by using Receiver Operating Characteristics (ROC) analysis.
We recruited 50 patients with average age (SD) of 49.32 (12.15) years, spanning from 18 to 72 years. There were 38 (76%) female patients with women to men ratio of 3.17 to 1. There were 13 (26%) patients with the occurrence of hypocalcemia, out of which the most had temporary hypocalcemia (11; 22%). when comparing malignancy (8/17; 47.1%) vs. other reasons for surgery (5/33;15.15%), there were proportionally more patients with hypocalcaemia, among patients referred for surgery due to thyroid malignancy (X2=4.39; df=1; p=0.036). We compared predictive value of each particular calcium measurement for prediction of occurrence of hypocalcaemia and there was no statistical difference between AUROCs. A detailed ROC analysis for calcium measurement on day 2 was performed with the best performing threshold value of calcium of 1.95 with sensitivity of 70%, specificity of 97% and positive and negative predictive value of 90%.
Serial measurement of calcium in postoperative period after thyroid surgery is not necessary and that measurement on second day after surgery is sufficient for predicting the occurrence of hypocalcaemia. Values of calcium measured 6 hours after the surgery may be used as an early and less precise predictor for occurrence of hypocalcaemia.