Correlation between calcium and magnesium in plasma or serum?

Blood Ionized Magnesium Concentrations During Cardiopulmonary Bypass and Their Correlation with Other Circulating Cations

“…The literature suggests that, while there may be a weak correlation of total Mg with total Ca in healthy patients,27?28 there appears to be no such correlation during surgery. 28 The situation has been further confused by authors who refer to "Mg ++" when, in fact, they are discussing data which involves total Mg measurements. 29 Our data suggest that, when iCa is measured to assess circulating Ca status, iMg levels are not correlated with, and cannot be inferred from, the iCa levels.…”