Comparison of derived and actual transferrin: a potential source of error in clinical nutritional assessment

A prospective study was undertaken to evaluate the utility of calculating transferrin from total iron-binding capacity in the nutritional assessment of burned patients. Regression analysis was used to compare total iron-binding capacity with radial immunodiffusion transferrin determinations. The method used for calculating transferrin (0.8 TIBC - 43) is a frequently published conversion formula for deriving transferrin. One hundred twenty-five data sets were obtained from 45 burned patients. Values for derived transferrin ranged from 39 to 235 mg/dl, averaging 121 mg/dl. Actual transferrin averaged 162 mg/dl, ranging from 41 to 320 mg/dl. Forty-eight actual serum transferrin samples were normal (greater than 172 mg/dl) whereas only 17 derived transferrin values were normal. While there is a correlation between total iron-binding capacity and serum transferrin (r = 0.85), to calculate transferrin according to the formula above would have resulted in significant error in the clinical assessment of the patients' nutritional status (p less than 0.001). From our studies, the formula for conversion of total iron-binding capacity to transferrin was found to be (0.68 TIBC + 21). These results suggest that the development of a universal conversion factor is not feasible. Modification of the formula may be necessary at each institution for clinically useful evaluations of serum transferrins are to be derived from iron-binding capacity for use in nutritional assessment.