Ninety-nine percent of total body Ca is contained in bone, making the skeleton a Ca reservoir; the remaining 1 percent is distributed in the intracellular and ECF compartments. Because 1 percent of skeletal Ca is freely exchangeable with the ECF, it can help buffer acute changes in the serum Ca, whereas the kidney regulates calciuresis.y Total serum Ca reflects dietary intake, GI absorption, calciuresis, and transfer from the bone to the ECF. Parathyroid hormone (PTH), 1,25-dihydroxycholecalciferol (DHCC), and calcitonin regulate both Ca absorption from the gut and calciuresis. PTH is released whenever the serum ionized Ca level falls; conversely, hypercalcemia suppresses the release of PTH. Because PTH promotes the synthesis of 1,25-DHCC, it indirectly influences GI absorption.
Although ionized Ca is the physiologically active form, total serum Ca is the laboratory parameter usually measured. This value reflects the ionized (unbound, 50 percent), the bound (mostly to albumin, 40 percent), and the chelated (10 percent) fractions. In the presence of normal serum protein levels, hypocalcemia exists when the total serum Ca value is below the normal range of 8.8 to 10.4 mg/dl. However, in the setting of hypoalbuminemia, although the measured total serum Ca is low, the patient is asymptomatic because the physiologically active, ionized fraction is normal. y Although the total serum Ca value may be corrected for hypoalbuminemia (e.g., by adding 0.8 mg/dl to the measured Ca value for every 1.0-g/dl decrement in the serum albumin value), this correction formula may be inaccurate in critically ill patients. y For that reason, it is best to measure the ionized Ca value in this patient population.
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