Accuracy and precision of test weighing to assess milk intake in newborn infants

Background: Test weighing is commonly used to estimate milk intake in newborn infants. Objective: To assess the accuracy and precision of test weighing in clinical practice. Methods: Infants fed by bottle, cup, or nasogastric tube were weighed before and immediately after feeding by a blinded investigator. Actual milk intake was determined by reading the millilitre scale of the milk container before and after feeding. The accuracy and precision of test weighing was assessed by examining the frequency distribution of the difference between weight change and actual milk intake. Results: Ninety four infants completed the study. The mean difference between weight change and actual milk intake was 1.3 ml, indicating good accuracy. The precision of test weighing, however, was poor: 95% of differences between weight change and actual milk intake ranged from −12.4 to 15 ml. The maximum difference was 30 ml. Imprecision was not influenced by the presence of monitor or oxygen saturation wires, intravenous lines, or vomiting of the infant. Conclusions: Test weighing is an imprecise method for assessing milk intake in young infants. This is probably because infant weighing scales are not sensitive enough to pick up small changes in an infant’s weight after feeding. Because of its unreliability, test weighing should not be used in clinical practice.