Magnesium

5 mL blood in lithium heparin or plain tube.

Spectrophotometry, atomic absorption spectrophotometry.

See Table 6.

Detection of hypomagnesaemia as the cause of unexplained cardiac arrhythmias, neuromuscular disorders, refractory hypocalcaemia.

Patients with clinical features suggesting hypocalcaemia and a normal plasma calcium, or with refractory hypocalcaemia/hypokalaemia, should have plasma magnesium measured.

Hypermagnesaemia may occur in renal failure, but assessment is rarely required.

Monitoring magnesium sulphate during anticonvulsant therapy (especially in pre-eclampsia) or therapy causing renal magnesium loss (eg, cisplatinum).

Decreased levels may be due to increased renal or gastrointestinal loss, or to decreased intake.

Levels fall rapidly in response to reduced intake and may not reflect tissue levels.

Increased levels are usually due to renal failure.