Have you been supplementing magnesium? If yes, at what dose and how often? Your level is only a tiny bit above the top of the reference range and might be explained by something as simple as dehydration. Other reasons for high levels are discussed on the link above.
Serum magnesium assays are of little use, unless you are very high or very low. You would need 'free magnesium' if they were trying to determine serum levels and in any event this would have very limited use. Serum levels do not correlate with tissue free levels. You might even have low tissue levels in spite of this slightly high serum total magnesium. Ignore this result, it has no use.
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