"Grandparentage" is not presently listed as a word in major English dictionaries. However, it is used by a number of DNA testing services to refer to a form of paternity test.
Certain companies that provide DNA testing services (like Genex and the DNA Diagnostic Center) advertise "grandparentage" testing intended to determine if a couple are the biological grandparents of a child. This differs from typical paternity testing in that the participation of the child's biological parents is not strictly necessary. However, the DNA Diagnostic Center claims that the participation of the child's biological mother speeds up the process. These tests indicate probability and likelihood rates rather than a definitive conclusion.