Predicting Refeeding Hypophosphatemia in Anorexia Nervosa

Electronic medical records of eligible participants were retrospectively reviewed. As outcome indicators, the following two measures were used: the extent of the decrease in serum phosphorus levels from admission to the nadir phosphorus level during the first 2 weeks after admission and the development of refeeding hypophosphatemia (< 2.5 mg/dl serum phosphorus). Explanatory variables included data at admission, i.e., age, sex, body mass index, anorexia nervosa subtype (restrictive or binge-purge), data obtained from laboratory tests (BUN/Cr ratio, serum phosphorus and potassium levels, hemoglobin, and albumin), and indicators involving treatment, i.e., the rate of weight gain during the first 7 days, caloric intake, and amount of intravenous phosphate administered. Classification of anorexia nervosa subtype, restrictive or binge-purge (bulimic-type), was carried out because binge-purge behavior often influences serum electrolytes levels through repeated vomiting, in particular, serum potassium level [7 (link)]. Body mass index was calculated as the weight of the individual (in kilograms) divided by the square of the height of the individual (in meters). To calculate the rate of weight gain during the first 7 days, we divided the kilograms gained during the first 7 days by the initial weight. Total caloric intake (kilocalories) refers to the average total caloric intake from day 1 through day 7 [4 (link)], including both oral intake and intravenous infusion therapy. If the patient ate only half the provided 1200-kcal meal, the actual amount of total caloric intake was reduced to 600 kcal. To accurately investigate the effect of energy intake on an individual patient depending on his or her weight, an indicator of total caloric intake per body weight was used for this analysis (total caloric intake divided by body weight), which is widely used for diet therapy for diabetes mellitus [8 (link)]. Intravenous phosphorus administration, which was not measured in previous studies of predictors for refeeding hypophosphatemia, was included in this study and was defined as the average total intravenous phosphorus administration (millimoles) from day 1 through day 7. In our unit, phosphorus administration is done intravenously, not orally, during the first 7 days. To better calculate the effect of intravenous phosphorus supplementation on refeeding syndrome, the amount of intravenous phosphorus administered was divided by total caloric intake, and this value was used in the statistical analysis. This is because intracellular movement of serum phosphorus is dependent on reintroduction of nutrients, which is mediated by surges in insulin [9 (link)].
A laboratory panel, including serum phosphorus, was carried out on admission. From the second examination onwards, each blood test was conducted at 7:30 in the morning before breakfast. To precisely identify the nadir serum phosphorus level, the patients underwent serial laboratory tests; 47 admissions (74.6%) had the test again on the second hospital day, 45 (71.4%) on the third hospital day, 45 (71.4%) on the fourth hospital day, 29 (46.0%) on the fifth hospital day, and 34 (53.6%) on the sixth day, and they continued to have blood tests until the serum phosphorus level went up again. We note that 59 of 63 admissions (93.7%) had the second laboratory test within 48 h of the first.