Q: What is the 'rule of thumb' for conversion between plasma and free magnesium?
Answer: Multiply by 0.7
Only 70 percent of the Magnesium in plasma is in free form. Said that this applies only in the normal situations. This value probably changes in ICU where gastrointestinal and/or renal losses are common.
It may be helpful to distinguish between GI and renal loss for persistent hypomagnesemia in ICU patients. 24-hour urinary magnesium excretion should not be more than 10 to 30 mg with normal renal function.
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