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That interpretation was by the laboratory, described verbatim from the reports of the test results.

Abrahamson also reported that the XY chromosomes were depicted photographically. So we know it must have been a karyotype test because that's what those tests show.

Here's an example from Wikipedia: https://commons.m.wikimedia.org/wiki/File:NHGRI_human_male_k...

We can see in this how the sex chromosomes make an asymmetrically sized pair, indicating XY. It's clear from the description of the lab reports that the same was observed for Khelif and Lin.