Corrected calcium versus ionized calcium measurements for identifying hypercalcemia in patients with multiple myeloma.

BACKGROUND: Dysregulated bone turnover is an important clinical manifestation of multiple myeloma (MM), and 30% of patients present with hypercalcemia. Serum calcium levels are routinely monitored using total calcium measurements corrected for albumin. However, myeloma-related paraproteins may bind calcium, confounding these measurements. PATIENTS AND METHODS: We retrospectively analyzed correlation between corrected calcium and ionized calcium in a sample of patients with MM and a control sample of patients with breast or non-small cell lung cancers (n = 200). Multiple linear regression was used to identify variables affecting corrected calcium measurements. RESULTS: Correlation between corrected calcium and ionized calcium was stronger in the control group compared to the MM group (Spearman correlation coefficient 0.85 versus 0.76, respectively). Sensitivity of corrected calcium in identifying hypercalcemia defined by elevated ionized calcium was 36% in patients with MM and 76% in the control group. Multiple linear regression did not reveal variables significantly influencing corrected calcium in the MM group, although serum paraprotein trended toward significance (p = 0.09). CONCLUSION: Ionized calcium may be better than corrected calcium for detecting hypercalcemia in patients with MM. Additional analyses are needed to better quantify the clinical impact of paraprotein calcium-binding.