Calcium, Magnesium, and Phosphate

### Calcium #### Overview of Analyte Calcium plays significant roles in cell structural components and biochemical functioning through signaling within and external to the cell. Calcium is essential for normal neuromuscular function and for correct functioning of the coagulation factors. It is the most prevalent cation in the body and is found in the skeleton, soft tissue, and extracellular fluid. In serum, the total calcium concentration is approximately 8.4 to 10.2 mg/dL (2.1–2.6 mmol/L). This calcium exists in 2 different forms, namely, bound and free. The protein-bound form of calcium accounts for approximately 40% of total serum calcium, of which 80% is bound to albumin and the remaining 20% is bound to globulins.1 A total of 10% of the total calcium is bound to small anions, including bicarbonate, phosphate, citrate, and lactate.1 The free, physiologically active calcium (often called ionized calcium) concentration in the plasma is approximately 4.4 to 5.2 mg/dL (1.1 to 1.3 mmol/L). The intracellular cytoplasmic calcium concentration is typically very low in unstimulated cells—approximately 0.1 μM. This is 4 orders of magnitude lower than the serum free-calcium concentration. The cellular machinery is exquisitely responsive to very small elevations in cytoplasmic calcium, making it a highly sensitive second messenger. The amount of serum calcium that is bound to plasma protein can be impacted by a number of factors, one of which is the pH of blood. In a patient with alkalosis, the higher pH allows for greater binding of free calcium to proteins, effectively decreasing the concentration of free calcium. Ionized calcium will decrease 0.16 mg/dL (0.04 mmol/L) with every 0.1-unit increase in pH in patients with acute respiratory alkalosis.2 Scientists have developed various empirical formulae to adjust the total serum calcium measurement according to the serum protein concentration. This method is used to exclude protein effects when interpreting total calcium … [↵][1]* To whom correspondence should be addressed. E-mail: lbazydlo{at}ufl.edu [1]: #xref-corresp-1-1