Reference chart for relative weight change to detect hypernatraemic dehydration

Objective: The validity of the rule of thumb that infants may have a weight loss of 10% in the first days after birth is unknown. We assessed the validity of this and other rules to detect breast-fed infants with hypernatraemic dehydration. Design: A reference chart for relative weight change was constructed by the LMS method. The reference group was obtained by a retrospective cohort study. Participants: 1544 healthy, exclusively breast-fed infants with 3075 weight measurements born in the Netherlands and 83 cases of breast-fed infants with hypernatraemic dehydration obtained from literature. Results: The rule of thumb had a sensitivity of 90.4%, a specificity of 98.3% and a positive predictive value of 3.7%. Referring infants if their weight change is below 22.5 SDS (0.6th centile) in the reference chart in the first week of life and using the rule of thumb in the second week had a sensitivity of 85.5%, a specificity of 99.4% and a positive predictive value of 9.2%. Conclusions: The rule of thumb is likely to produce too many false positive results, assuming that for screening purposes the specificity needs to be high. A chart for relative weight change can be helpful to detect infants with hypernatraemic dehydration.