Читать книгу Vitamin D in Clinical Medicine - Группа авторов - Страница 12

Calcium in blood is partly (45%) bound to protein, especially albumin, in a pH-dependent manner, and partly (10%) complexed to anions such as phosphate (PO₄) and citrate [1]. Approximately 45% circulates as a free or ionized fraction and is the biologically active portion of total blood Ca. Although only the ionized Ca is available to enter cells and activate cellular processes, most clinical laboratories report total serum Ca concentrations. Concentrations of total Ca in normal serum generally range between approximately 2.12 and 2.62 mM (8.5 and 10.5 mg/dL) with levels above this considered hypercalcemic and levels below this considered hypocalcemic. Ionized concentrations can be measured, and the reference range is 1.16–1.31 mM (4.65–5.25 mg/dL). When protein concentrations, and especially albumin concentrations, fluctuate, total Ca levels may vary, whereas the ionized Ca may remain relatively stable. During venipuncture, dehydration, or hemoconcentration may increase serum albumin and falsely elevate total serum Ca measurements. However, changes in total Ca, when albumin levels are increased or decreased, can be “corrected” mathematically. Alterations in blood pH can change the equilibrium constant of the albumin-Ca complex, even in the presence of a normal serum albumin, with acidosis reducing the binding and alkalosis augmenting it. A major shift in serum protein or pH, therefore, requires direct measurement of the ionized Ca level to determine the physiologic serum calcium level.