Pièges et contraintes des dosages du calcium, des phosphates et du magnésium

Abstract

Calcium, phosphate and magnesium are three current parameters of the “bone” laboratory tests. We describe the conditions of sampling, assaying, and discuss how to interpret these three parameters in the blood and urine. Total serum calcium remains the first-line setting. Correction in total serum calcium for albumin is justified only in cases of hypoalbuminaemia. Outside this context, a correction of serum calcium may mask diagnosis of hypercalcemia. The measurement of ionized calcium, non-automated, requires a rigorous preanalytical and can be reserved for some difficult situations. It eliminates the determination of serum albumin whose variability is important in case of inflammation. It allows a more sensitive diagnosis of moderate hypercalcemia. It ensures better monitoring of calcium status of patients with renal insufficiency. The presence of a monoclonal gammopathy is the main source of interference on the determination of phosphate, causing false hyperphosphatemia or rarely false hypophosphatemia. Serum magnesium, imperfect marker, remains the most accessible parameter for determining the status of magnesium in the body. Urinary assays are an integral part of the bone laboratory tests. The quality of the result depends on a prior acidification of the total urine volume considered.