The differential diagnosis of hypernatraemia in children, with ...

The Diagnosis of Salt Poisoning Leading to Hypematraemia in Children – September 2009
6.2.2 Fractional urinary sodium excretion
The fractional urinary sodium excretion (FE Na) is calculated from simultaneous
measurement of the blood and urinary sodium and creatinine concentrations 27 , using the
following formula:
FE Na (%) = U Na x P Cr x 100
U Cr x P Na
Where U Na is urinary sodium, P Cr is plasma creatinine, U Cr is urinary creatinine and P Na is
plasma sodium. The units of measurement must be the same for the sodium concentrations
(although the conversion factor for sodium between mmol/L and mEq/L is 1) and for the
creatinine measurements (to convert from mg/dL to μmol/L, multiply by 88.4). Beware
also of differences between laboratory reporting using micromol/L in plasma and mmol/L
in urine, which are 1,000 times different: these have to be converted to the same units! The
FE Na is a reasonable proxy for the sodium excretion rate.
6.2.2.1 Excessive sodium intake
There are only reports of 3 cases of excessive sodium intake where fractional
sodium excretion was either calculated or could be calculated from the information
given in the publication. Details of the three cases are given below:
FE Na = 15% 167 (case 1) , aged 20 months, serum sodium 195 mmol/L, salt added to
yoghurt. The child received peritoneal dialysis, but details of this were not given.
FE Na = 21% 167 (case 2) , aged 7 months, serum sodium 178 mmol/L, rehydration
fluid with excessive salt content. The child received a rehydration solution with 40
mmol/L of sodium.
FE Na = 9.5% 258 , adult aged 26, deliberate self-harm, serum sodium 188 mmol/L,
ingested salt solution. The patient received fluids containing 5% dextrose and 4
grams per litre of saline.
None of these patients received intravenous 0.9% saline.
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