13. Total and ionized calcium, total and ionized magnesium - common data for the parameters, indications for investigation, principles of the analytical methods, reference ranges, interpretation of results.

Total and ionized calcium, total and ionized magnesium - common data for the parameters, indications for investigation, principles of the analytical methods, reference ranges, result interpretation.

Calcium is the fifth most common element and is the most prevalent cation in the human body. Serum calcium exists in three distinct forms:

1. (1) free or ionized calcium (35-40%), active form
2. (2) complexed calcium (10%), bound to bicarbonate, lactate, phosphate
3. (3) plasma protein-bound calcium (40-50%).

Total serum calcium levels may remain unchanged, the relative distribution of the three forms is altered as a result of pH changes in extra cellular fluid.

Function: 99% of the body's Ca²⁺ is present in bones, it is required for vision, muscle contraction, nerve conduction, blood clotting, exocytosis, cell division and the activity of many enzymes and hormones.

Reference Range: Serum, or heparinized plasma may be used

• Total calcium 2.12-2.62 mmol/L
• Ionized (free) calcium 1.16-1.32 mmol/L

Indications for investigation for Calcium: liver transplantation, open heart surgery, neonatal hypocalcaemia, renal disease, hyperparathyroidism, malignancy, hypertension and pancreatitis.

Principles of the analytical methods:

1. spectrophotometric determination of the coloured complex when various metallochromic indicators or dyes bind calcium.
2. Atomic absorption spectrophotometry is the reference method for determining calcium in serum.
3. Instruments with calcium-selective electrodes provide accurate, precise, and automatic determinations of free (ionized) calcium. Calcium ion-selective electrodes consist of a calcium-selective membrane enclosing an inner reference solution of CaCl₂, AgCl and other ions, and a reference electrode.

Result interpretation for calcium:

Hypercalcemia due to: - overproduction of parathyroid hormone, or hyperparathyroidism. Hypocalciuric hypercalcemia is an inherited condition in which individuals do not excrete normal amounts of calcium, thus calcium levels are usually slightly elevated. Cancers are commonly associated with elevated calcium levels and are referred as "hypercalcemia of malignancy - excessive levels of vitamin D from: vitamins; excessive dietary calcium, diseases that may result in excess vitamin D production;

Hypocalcaemia due to: Hypoparathyroidism (reduced or absence of PTH secretion), usually: Vitamin D deficiency leads to hypocalcaemia when associated with decreased dietary calcium intake. The low calcium level stimulates PTH secretion (secondary hyperparathyroidism), Rickets, Hungry bone syndrome. Hungry bone syndrome is hypocalcaemia after surgery for hyperparathyroidism.

Magnesium: is the second most prevalent intracellular cation. One-third of skeletal magnesium is exchangeable and probably serves as a reservoir for maintaining a normal extracellular magnesium concentration.

Function:

• Magnesium is essential for the function cellular enzymes, including those related to the transfer of phosphate groups
• required for cellular energy metabolism and has an important role in
• membrane stabilization, nerve conduction, ion transport, and calcium channel activity.