0092079 Magnesium, RBC 0092079 0020039 Magnesium, Plasma or Serum 0020039 About This Topic Magnesium, RBC (0092079), measures the amount of magnesium contained in red blood cells. It may be useful in the assessment of tissue stores. Recommendation For routine assessment of magnesium deficiency, Magnesium, Plasma or Serum (0020039) is preferred. ARUP Consult