Objective To determine the relationship between serum sodium concentration and weight loss in newborns with hypernatraemic dehydration caused by insufficient breastfeeding, and to determine the sensitivity of the following rule of thumb “if weight loss compared with birth weight is less than 10%, the baby does not have hypernatraemic dehydration caused by insufficient breastfeeding”.

Methods Medline was searched using the terms “dehydration AND breastfeeding” for case reports on patients with “hypernatraemic dehydration caused by insufficient breastfeeding”. Reference lists from the articles retrieved were also searched. All cases that the author diagnosed as “hypernatraemic dehydration caused by insufficient breastfeeding” were included.

Results A total of 47 articles was found, containing 128 relevant cases. Of these, nine had less than 10% weight loss. Therefore, the sensitivity of the 10% rule was 93%. We found a linear relationship between the degree of weight loss and serum sodium concentration (Pearson’s correlation coefficient 0.71; p<0.001). For every 10% increase in weight loss, the serum sodium concentration increased by 16 mmol/l (95% CI 13 to 19).

Conclusions A relatively strong linear relationship was found between weight loss and serum sodium concentration. If the weight loss was more than 10%, the serum sodium concentration was beyond the range of normal values. The rule of thumb had a high sensitivity; however, the specificity should be determined before the rule of thumb is implemented.