Table 3

 Investigations to differentiate gastrointestinal magnesium losing states and excessive enteral administration

InvestigationReason for investigationFinal result
*Urinary fractional excretion of magnesium  =  [Ur Mg2+]/[Pl Mg]×[Pl Mg]/[Ur Mg2+].
Dietetic historyPotential innocent explanation for high Mg2+ inputHigh intake of Neocate factitious, prescriptions never collected
Pharmacological historyInadvertent ingestion of magnesium or calcium containing drugsNo magnesium or calcium containing drugs taken
Faecal magnesium concentrationConfirm chronic lossFigure 3
PTH concentrationAssess body stores17 ng/ml (low normal)
Fractional urinary magnesium excretion*Assess body stores3.36% (normal <4%)
Renal functionAssess body storesNormal
Chest x rayEvidence of secondary hyperparathyroidismNormal
Intravenous challenge with magnesiumTo detect increased intestinal magnesium loss if inborn error of intestinal magnesium handlingFigure 3
Washout periodTo enable concentrations to fall to baselineFigure 3
Oral magnesium challengeTo replicate baseline findingsFigure 3