Indirect test weighing: a non-intrusive technique for estimating night-time breast milk intake.

Indirect test weighing (ITW), for estimating night-time breast milk intake, is based on separate weighings of mother and infant before and after night-time sleep. Weight changes due to the exchange of milk overnight are reciprocal, so that if all other sources of weight change (eg, urine loss, supplementary fluid intake) are either controlled or measured, the difference between the mother's overnight weight loss and her infant's overnight weight gain represents their combined net evaporative water loss (EWL). The proportion of the combined EWL due to the infant is partitioned out as a function of the relative metabolic body sizes (kWt0.73) of mother and infant, and added to the infant's net overnight weight gain to provide an estimate of night-time breast milk intake. Validation studies, conducted in Northern Thailand, in which ITW was compared with direct test weighing (DTW) at each night feed, are reported for 13 infants over 3 nights at 5 d of age, and for 19 infants over 2 nights at 6 weeks of age or more. The regression equation for estimated milk intake against measured milk intake across all ages was y = 0.988x + 2.75, r = 0.927, n = 69 with 95 per cent prediction intervals of +/- 36 g for a range of intakes of 0-250 g. Rigorous control of data collection and taking account of the infant's age suggest that the prediction intervals for individual estimates can be improved to +/- 18 g at 5 d, and +/- 27 g at 6 weeks and over. Theoretical aspects of the relationship of EWL to body weight are discussed with a view to optimizing the accuracy of the technique.