Differentials
Hypertonic hyponatraemia
SIGNS / SYMPTOMS
Due to hyperglycaemia or administration of an active osmolyte such as mannitol.
Signs and symptoms are variable depending on the underlying condition.
In hyperglycaemia, patients may be asymptomatic, have polyuria due to uncontrolled diabetes mellitus, or present with hyperosmolar hyperglycaemia and/or diabetic ketoacidosis.
INVESTIGATIONS
Serum blood glucose level: elevated.
Serum osmolality: high.
If the patient is hyperglycaemic, a sodium correction formula should be used. If the glucose level is >5.5 mmo/L (100 mg/dL), the most accurate correction formula is: corrected sodium (mmol/L) = measured sodium (mmol/L) + 0.024 {(glucose [mmol/L] x 18)-100}. This formula should be used to determine if true hyponatraemia is present.[42]
Pseudohyponatraemia
SIGNS / SYMPTOMS
Also known as isotonic hyponatraemia.
An artifact of incorrect measurement of serum sodium concentrations due to high lipid and/or protein in the plasma.
Most common cause of high protein levels is multiple myeloma.
Signs and symptoms are variable depending on the underlying condition.
INVESTIGATIONS
Serum lipids and proteins: elevated.
Serum osmolality: normal.
Use of ion-specific electrodes to measure sodium directly may help reduce the incidence of pseudohyponatraemia.
Assessment of hyponatraemia
SIGNS / SYMPTOMS
See our Assessment of hyponatraemia topic for a full review of the differential diagnosis.
INVESTIGATIONS
Laboratory investigations such as serum osmolality, urine osmolality, and urine sodium concentration help to distinguish the various causes.
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