Clinical pathology of hypocalcaemia in adult grey parrots (Psittacus e erithacus).

CALCIUM exists in three forms in avian serum: as an ionised salt, bound to proteins and in a complex bound to a variety of anions (citrate, bicarbonate and phosphate). The majority of veterinary pathology laboratories report a total calcium concentration, measured by spectrophotometer, which reflects the total combined levels of ionised calcium, protein-bound calcium and complexed calcium (Bush 1991). Measurement of total calcium in a bird with abnormal protein levels would not truly reflect its calcium status, as any changes in serum albumin values will affect the total calcium concentration, leading to an imprecise result. For example, in laying female birds, serum albumin levels may rise by up to 100 per cent to provide albumin for yolk and albumen production (Williams and others 2001). A blood sample analysed at this time would show an inflated total calcium concentration due to an increased proteinbound calcium fraction, while the ionised calcium level would not be affected. The binding reaction between the calcium ion and albumin is strongly dependent on pH, so acid-base imbalances would be expected to affect concentrations of ionised calcium. Therefore, an individual with metabolic acidosis would be expected to show ionised hypercalcaemia due to decreased protein binding. With an alkalotic individual, ionised hypocalcaemia would occur as the protein-binding reaction increases. Thus, the measurement of ionised calcium provides a more clinically relevant indication of an individual's calcium status, especially in diseased animals (Portale 1999).