Cases of pseudo-hypocalcemia caused by phospholipids

Abstract

We reported cases of false low values in the enzymatic calcium assay using phospholipase D. There was no abnormality in the reaction time-course, but dilution reproducibility was not obtained. Therefore, we conducted the following study to investigate this issue. The patients had biliary stasis and high levels of phospholipids and free cholesterol. Lipoprotein X (Lp-X) presence was suggested by a confirmation test using the precipitation method. Based on the reagent principle, we suspected a non-specific reaction caused by phospholipids, which are abundant in Lp-X. To test this, lecithin was added to pooled serum or purified water as a source of phospholipids and the reaction were observed. Phospholipase D acted not only on the chromogenic substrates but also on the phospholipids. In the phospholipid coexistence effect test, calcium measurements were decreased in a phospholipid concentration dependent. These results suggest that high concentrations of phospholipids cause false low calcium values in the phospholipase D enzymatic assay due to competition with chromogenic substrates.