Hypernatraemic dehydration arises when there is a disproportionate deficit of body water relative to body sodium. Although the serum sodium concentration is elevated, whole body sodium content may be reduced, unchanged or increased. When the condition occurs in an otherwise healthy full term, breast-fed baby the cause is poor milk intake. In this situation there is loss of body sodium but a greater deficit in body water.

Hypernatraemic dehydration may have serious adverse consequences. At the most severe end of the spectrum these include cerebral oedema, convulsions, venous sinus thrombosis, intracranial haemorrhage, disseminated intravascular coagulation, renal failure, permanent brain injury and death. Infants admitted to hospital usually undergo extensive investigation and rehydration with intravenous fluids and formula. This has led to concern about how best to detect the condition, particularly in newborns in the community as biochemical testing would require referral to hospital. In this issue, a group of Dutch researchers describe the construction of a reference chart for relative postnatal weight change and propose the use of this chart by midwives assessing babies at home.¹ They conclude that beyond the first postnatal week, the guideline or “rule-of-thumb” that weight loss in excess of 10% of birth weight is abnormal is a reasonable trigger for referral to hospital. However, they argue that relative weight loss in the first week exceeding

−2.5 SDS is preferable because the 10% rule would result in a greater number of referrals. They recommend the use of a chart for relative weight change as a screening tool. Is this a reasonable approach to dealing with the problem of hypernatraemic dehydration?

Let us consider this from two perspectives: the utility of the approach as a guideline and the utility as …