Indirect test weighing: a new method for measuring overnight breast milk intakes in the field

Indirect test weighing (ITW), a new method for measuring nighttime breast milk intake, is based on separate weighings of the mother and infant at the start and end of the nighttime sleep period. All other incidental weight changes overnight are measured, so that the corrected infant's overnight weight gain and the mother's overnight weight loss are then accounted for by breast milk transfer and evaporative water loss (EWL) alone. The combined EWL is partitioned between mother and infant on the basis of metabolic body size (body weight raised to the power of 0.73). The infant's EWL is added onto the corrected overnight weight gain to give nighttime milk intake. A field validation study on 24 northern Thai infants, conducted in the subject's homes, in which ITW was compared with nighttime direct test weighing (DTW) on the same night, gives 95% confidence intervals for the method of +/- 39 g for a range of milk intakes of 77-344 g. Confidence intervals for estimates using feeding patterns alone, or multiplying 12 h intakes by a constant, range from +/- 104 to 180 g, indicating that these methods are insufficiently accurate for individual estimates.