This test is used to evaluate Magnesium levels in red blood cells. Magnesium is a mineral that is important for strong bones and muscles heart health energy production and nerve function. A Magnesium RBC test can provide an earlier indicator of Magnesium deficiency that a standard Magnesium serum test. When levels are low the body will pull Magnesium from the cells to keep blood levels normal. In this case a Magnesium blood test may show normal levels while a Magnesium RBC test will give a more accurate results.