Effect of phosphate or magnesium cathartics on serum calcium: observations in normocalcemic patients.

The administration of commercial phosphate cathartic in conventional doses to normal subjects prior to barium enema resulted in a striking increase in serum phosphorus levels followed by a decline in serum calcium levels in all subjects. Changes were highly significant (P less than .01) when compared with control subjects who were prepared for the same procedure with magnesium citrate. Levels of serum potassium also decreased significantly (P less than .01) but not serum sodium, chloride, bicarbonate, or magnesium.