Answer: You should report the diagnosis that the ordering physician indicates is the reason for the test. To select the proper code, you need to know if the screening is for a couple's genetic counseling prior to conception or for other reasons, such as a patient whose father has Huntington-s.
For procreative management genetic testing, look to V26.3x (Genetic counseling and testing). For the diseases you mentioned, you should report V26.31 (Testing of female for genetic disease carrier status) or V26.34 (Testing of male for genetic disease carrier status), depending on the patient's gender.
Red flag: Most couples will turn out to be non-carriers, so you should make sure that you don't use disease codes for the screening encounter. Even if one of the parents is a carrier, the individual does not have the genetic disease, so you shouldn't apply the disease codes.
If the genetic screening is for some reason other than procreative management, you should use V82.7x (Genetic screening). Because your case involves screening specifically for genetic carrier status, you should use V82.71 (Screening for genetic disease carrier status).
Alternative: When the physician orders a genetic screening to determine if a patient carries a gene for a specific disease when a family member is a known carrier, use V18.9 (Family history of genetic disease carrier).