Sodium potassium

Plasma levels may be elevated but poorly reflect intracellular (approximately 3-5mmol/l for Na + , 140-150mmol/l for K+) or total body levels. Plasma potassium levels are affected by plasma H + levels; a metabolic acidosis reduces urinary potassium excretion while an alkalosis will increase excretion.

Older measuring devices such as flame photometry or indirect-reading ion-specific electrodes gave spuriously low plasma Na+ levels with concurrent hyperproteinaemia or hypertriglyceridaemia.

Urinary excretion depends on intake, total body balance, acid-base balance, hormones (including anti-diuretic hormone, aldosterone, corticosteroids, atrial natriuretic peptide), drugs (particularly diuretics, non-steroidal anti-inflammatories and ACE inhibitors), and renal function.

In oliguria, a urinary Na+ level <10mmol/l suggests a pre-renal cause whereas >20mmol/l is seen with direct renal damage. This does not apply if diuretics have been given previously.

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