Prader-Willi Syndrome (PWS) is diagnosed with a blood test that looks for the genetic abnormalities that are specific to PWS – called a “methylation analysis.”
A FISH (fluorescence in-situ hybridization) test identifies PWS by deletion, but it does not diagnose other forms of PWS. The methylation test will detect all types of PWS and is the preferred test for diagnosis. If a methylation test is completed first, additional testing may be needed to determine whether PWS is caused by a paternal deletion, UPD, or an imprinting mutation. In cases where an imprinting mutation is suspected, blood may also be drawn from the parents.
Interesting fact: PWS was the Syndrome that first demonstrated the Imprinting of a portion of genetic material on a chromosome. Knowing that an area of data can be turned on or off has led to new areas of research in epigenetics.