Incidence of hypophosphataemia in patients on parenteral nutrition

Refeeding syndrome is a major cause of hypophosphataemia in hospital inpatients. It is defined as “severe electrolyte and fluid shifts associated with metabolic abnormalities in malnourished patients undergoing refeeding, whether orally, enterally or parenterally”. After a period of fasting (the length of which is not well defined, but might be as little as 48h), gluconeogenesis and protein catabolism are promoted and a negative protein balance occurs.When feeding with glucose is re-established (for example, when parenteral nutrition [PN] is given) the cellular uptake of phosphate, magnesium and potassium is promoted, causing the concentration of these minerals in the plasma to decrease. Plasma concentrations of thiamine are decreased and fluid and sodium balance are also affected. Hypophosphataemia generally occurs after one to two days of PN in patients who were malnourished before being given PN, and after three to five days in those who were “normally nourished”. Hypomagnesaemia would be expected to accompany phosphate depletion in malnourished patients, but it is not known whether magnesium levels reduce in line with phosphate levels in those not significantly malnourished at the start of PN. Although hypophosphataemia has long been recognised as perhaps the most important aspect of refeeding syndrome, it is not always appreciated by prescribers of PN. It would also appear that it cannot always be prevented using best current practice and serious cases continue to be reported. It should also be noted that, when a plasma sample is reported below the normal range, before refeeding hypophosphataemia is diagnosed, other causes of hypophosphataemia need to be taken into account, as do the duration of the hypophosphataemia and the temporal association with starting PN. We were unsure of the frequency of refeeding problems among the patients in our hospital receiving PN. Moreover, guidance as to when to treat any fall in phosphate levels is lacking. There is also debate about how much phosphate to give. Since 1994, the British National Formulary has recommended, giving 9mmol phosphate every 12h, based on a small study. However, reports of instances where a larger amount OBJECTIVE — To determine the effect of parenteral nutrition on plasma phosphate levels and to evaluate the incidence of refeeding hypophosphataemia.