Diagnostic Use
Abnormailites in sodium concentration are more commonly due to a primary problem with water homeostasis, than abnormal sodium homeostasis. Serum or plasma sodium concentration is dependent on the state of hydration, and the shift of water between the vascular compartment and other body fluid compartments. Mineralocorticorids directly influence sodium excretion or retention.
Increased sodium concertrations are therefore seen where there is water loss in excess of sodium loss (e.g. excess sweating, vomiting or diarrhoea), or with excessive water loss alone (e.g. diabetes insipidus, dehydration). Excess sodium can be due to hyperaldosteronism or Cushing’s syndrome, or excessive IV saline therapy.
Decreased sodium concentrations are seen with water retention (oedema, ascities, diabetes mellitus, hepatic failure, nephrotic syndrome, hypothyroidism, SIADH), or where there is both water and sodium loss with inadequate sodium replacement, as well as with diuretic abuse and osmotic diuresis. Adrenocortical insufficiency can also cause excess sodium loss.
Pseudohyponatraemia: Gross hyperproteinaemia or hypertriglyceridaemia can cause falsely low sodium concentration when the plasma sodium is measured by main laboratory analysers due to the “electrolyte volume exclusion effect”. This effect does not apply to sodium measured using a blood gas analyser, hence sodium analysis by blood gas analyser in these cases can be helpful.
High levels of ethanol may falsely lower sodium measurement on the Atellica analyser.