Hi Teo,
At page 35 in this article you will find the WHO criteria for MDS
http://www.hematologics.com/pdf/ref36.pdf
As you can see it is very important to exclude all other hematopoietic or non-hematopoietic disorders as primary reason for cytopenia/dysplasia.
If your mother is sedated when they perform the BMB they will hopefully get an aspirate and a biopsy that contains good material and make it possible to get a diagnosis.
I don't know if there are other ways to diagnose chromosome aberrations than cytogenetic testing.
Kind regards
Birgitta-A