Hypocalcaemia is defined as a plasma or serum concentration of total calcium <2.20 mmol/l (<8.8 mg/dl) with normal plasma or serum protein concentration or a plasma or serum concentration of ionised calcium <1.17 mmol/l (4.7 mg/dl). In healthy people, only 48% of calcium, in the ionised form, is metabolically active. The acid-base balance is also important because an acidosis is associated with a higher level of ionised calcium, while an alkalosis with a lower level of ionised calcium shows a higher probability of hypocalcaemia symptoms.