[Weighing before and after feeding: an unreliable method for estimating milk intake in infants]

Weighing before and after feeding (test weighing) is commonly used to estimate the amount ofmilk intake in neonates. This study was designed to assess the reliability of test weighing in clinical practice, expressed as precision (scatter of measurements around the real value) and accuracy (the ability of a method to measure the real value).

Method study.

Bottle-fed infants were weighed before and immediately after feeding by an investigator who was unaware of the amount of milk drunk by the infant. Actual milk intake was determined by reading the ml scale of the milk container before and after feeding. The accuracy and precision of test weighing were assessed by applying the method of Bland and Altman, by examining the frequency distribution of the difference between weight change and actual milk intake. The precision of the weighing scale used was assessed by calculating the standard deviation of repeated measurements of standard weights of 1.5 and 4 kg.

Of 100 eligible infants, 6 were excluded because the measurements were taken with unknown scales. The accuracy of the test weighing was good (n = 94): mean difference between weight change and actual milk intake was 1.3 ml. The precision, however, was poor, with 95% of differences between weight change and actual milk intake ranging 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 or regurgitation ofthe infant. Based on the standard deviation of repeated measurements of the infant weighing scales, these instruments were not appropriate for assessing small changes in infants' weights after a single feed.

Test weighing was an unreliable method for assessing milk intake in infants, because infant weighing scales are not sensitive enough to detect small changes in an infant's weight after feeding. Because of its unreliability, the use of test weighing should be abandoned.