Is a DNA match with a very high X-DNA cM value always valid (or could it be a false-positive)?

Question

Recently I've been taking special notice of X-DNA matches, and I've come across several DNA matches where the X-DNA cM value is very high (but the atDNA cM value is very low). I have first cousins with smaller X-DNA cM values, but their atDNA cM value always are within the expected amount for a first cousin.

An example from GEDmatch is:

I'm wondering if a match with such a large X-DNA cM value is always valid (or can this be a false-positive result)?

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If this is a valid match, it posses a problem because the two donors/matches don't share a "common ancestor" (which should be at most a few generations away).

Answer 1

No, you need to carefully compare the X Chromosome overlap with the shared amount of other autosomal DNA. The catch is that X Chr is relatively small and men have only one X Chr and women - two X Chr. So it makes very difficult to interpret results correctly.

Also as X Chr is investigated alone the provided results may prove very distant (and old!!!) relationship.

I believe that any match with X-DNA below 100cM (almost half of X Chr length) is false positive (i.e. does not have value)

I would like to add that you have a small amount of shared autosomal DNA with that particular person. It means that you share the same ethnicity (i.e. your and that person's ancestors lived in the same region), but there is no common ancestor.