T topic of Calculation Skills this month is calcium supplementation for hypocalcaemia. In humans, over 99% of total body calcium content is present in bone mineral and calcified cartilage. In this state, it is largely unavailable, although bone undergoes a continuous process of resorption and deposition through the action of osteoclasts and osteoblasts, respectively. Calcium is also essential in the physiology of nerve and muscle cells and is a cofactor for a range of enzymes, including those of the blood-clotting cascade. The calcium level in the blood is tightly controlled by extracellular calcium-sensitive receptors, which enable the parathyroid gland and other tissues, including bone and the kidneys, to maintain calcium homeostasis (Willacy, 2011). The normal total serum calcium range is 2.25–2.5 mmol/L. Calcium levels above (hypercalcaemia) or below (hypocalcaemia) the normal range interfere with the normal function of most body cells, but particularly nerve and muscle cells. About 40% of plasma calcium is bound to albumin and only the unbound, unionized fraction is important physiologically. The serum calcium level is therefore reported as uncorrected or corrected, where an adjustment is made for changes in albumin level. An approximate correction of 0.1 mmol/L is added to calcium concentration for every 4 g/L that albumin is below 40 g/L, or subtracted in the same proportion for albumin levels above 40 g/L. So, for example, if the uncorrected serum calcium level is 2.1 mmol/L, but the serum albumin level is 32 g/L, the corrected calcium level is calculated as follows: ■ Serum albumin of 32 g/L = (40 – 32) 8 g below 40 g/L = 2 x 4 g ■ Corrected serum calcium = (2.1 + (0.1 x 2) mmol/L = 2.3 mmol/L. Calcium is usually present in the diet and supplementation is required only if dietary calcium intake is insufficient, although this varies with age and in certain conditions (Willacy, 2011). People at increased risk of calcium deficiency include those on a lactosefree diet, those with coeliac disease, and those with or at risk of osteoporosis (post-menopausal women, for example). Alison Eggleton