Hypercalcemia and elevated concentration of vitamin D: A case report too easy to be true.

Background: Endogenous (heterophile, human anti-animal …) antibodies are a known cause of interference in immunoassays.

Case Report: A patient with hypercalcemia and low PTH levels was investigated. The serum 25OH vitamin D (25OHD) concentration was above the analytical range of the automated analyser (>150ng/mL) but serum dilutions were not linear. A myeloma-related monoclonal peak of immunoglobulin G (30g/L) was found.

Results: Alternative 25OHD assays (RIA, automated analysers, mass spectrometry) all found concentrations <25ng/mL. NabTM columns (Thermo Scientific) eliminated the endogenous immunoglobulin from the serum thus allowing the initial analyser to provide correct results.

Discussion And Conclusion: The potentially misleading point was that the apparent very high 25OHD levels were concomitant with hypercalcemia and low PTH levels thus mimicking vitamin D intoxication. Identifying assay interferences requires clinical awareness but, when suspected, one should be aware that technical tools or alternate assays are available to correct some interferences, including monoclonal immunoglobulins.