What causes spurious hypocalcemia?
Total serum calcium is routinely measured with colorimetric assays. The commonly used arsenazo–III reagent complexes with certain gadolinium compounds (like gadodiamide and gadoversetamide), blocking the detection of calcium. The artifact can be as large as 6 mg/dL and persists until the gadolinium is excreted from the body. The ionized calcium is unchanged in this situation and can confirm the spurious hypocalcemia.
