Investigations

FBE
- Osmolality
Decreases in hyponatraemia due to 1) sodium loss; 2) increased fluid in the bloodstream
- Sodium< 135 mmol/L

Urine test
- Osmolality
12 -14 hr fluid restriction: >850 mOsm/kg;
Affected by increased or decreased urine output
- Sodium 
May mirror serum sodium level or be the opposite depends on cause

Test result may be affected by food intake, medication e.g. diuretics, steroids, dehydration or overhydration.

Abnormal test results need further investigations to indentify cause of hyponatraemia:
If suspecting sodium loss with normal extracellular volume,
- SIADH
- Normal plasma osmolality in hyperlipidaemia / hyperproteinaemia (pseudohyponatraemia)

If suspecting an increased extracellular volume,
- Decreased urinary sodium in heart failure
- Decreased urinary sodium in liver failure
- Reduced GFR in renal diseases
- Hypoalbuminaemia?