Correlation of serum-adjusted calcium with ionized calcium over a 24-h period in patients with adult growth hormone deficiency before and after growth hormone replacement

Background: Difficulties associated with measuring ionized calcium in clinical practice have led to the use of total calcium, with or without adjustment for albumin concentration, as an estimate of calcium metabolism. We examined the correlation between ionized and total/adjusted calcium over a 24-h period in patients with adult growth hormone deficiency (AGHD), a group of patients with previously reported alterations in calcium metabolism. Methods: Four patients with AGHD were consented to the study. They were hospitalized for 24 h where half-hourly blood samples were collected for ionized calcium, total calcium, albumin and creatinine, before and one month after the commencement of growth hormone replacement. Total calcium concentration was adjusted for serum albumin. Results: Strong correlations were found between ionized calcium and adjusted calcium (r(2) = 0.840 and 0.766 for visits 1 and 2, respectively, P < 0.001), and between ionized calcium and total calcium (r(2) = 0.828 and 0.731 for visits 1 and 2, respectively, P < 0.001). Correlations remained significant during the day (ionized versus adjusted calcium: r(2) = 0.847 and 0.780 for visits 1 and 2, respectively; ionized versus total calcium: r(2) = 0.860 and 0.792 for visits 1 and 2, respectively, all P < 0.001) and at night (ionized versus adjusted calcium: r(2) = 0.831 and 0.802 for visits 1 and 2, respectively; ionized versus total calcium: r(2) = 0.767 and 0.722 for visits 1 and 2, respectively, all P < 0.001). Conclusion: The results of our study suggest that total calcium and serum-adjusted calcium can be used in place of ionized calcium as a reliable indicator of calcium metabolism over a 24-h period in patients with AGHD.