Urea and electrolytes are carried out routinely on serum or plasma samples. The test is useful in numerous situations and the following highlights some of the most common interpretative challenges.
Hyponatraemia (Na < 120 mmol/L)
This may be due to either loss of sodium or overload of water or a combination of both. The following list contains the relatively common and important causes of hyponatraemia. For more complex cases, it is necessary to have simultaneous samples of both blood and urine for estimation of osmolality and electrolytes.
|Hyponatraemia and oedema
|Hyponatraemia and dehydration
Renal salt wasting
|Hyponatraemia and normal volume
||diarrhoea and vomiting followed by drinking excess water
diuretic therapy may occur within 1/2 weeks of initiation of therapy but may develop after many years of therapy
|Inappropriate ADH secretion
|Hypokalaemia (K < 3.5 mmol/L)
diarrhoea and vomiting
chronic purgative abuse
elderly patients with poor nutrition (catabolic state)
|Hyperkalaemia (K > 5.5 mmol/L)
||haemolysis and delayed separation over-night storage in a 'fridge
potassium retaining diuretics
chronic liver disease
transiently after eating high protein meal
following a significant gastrointestinal bleed
Monitoring diuretic therapy
Prior to the initiation of therapy and after 2/52, 3/12, 6/12 and then 6-12/12 or more frequently in the elderly or in patients with renal disease, disorders affecting electrolyte status or those patients taking other drugs eg corticosteroids, digoxin.