Lol, you can put to much faith in the word of suigetsu, who isn't a medic or someone lie naruto's reaction. If it was Karins DNA, it would t be a 100 match, like the results show. Suigetsu is just assuming that it's Karin umbilical cord, how would she have it? She was an orphan. The cord is Sarada's. How Karin got it idk.This is actually the only scenario I can think of that can justify this while maintaining the dignity of their marriage.
Otherwise Sasuke was unfaithful. (or Karin stole his DNA and made her own baby but this is extremely far-fetched, almost impossible.)
The DNA test confirms she is Karin's daughter, there's no way around that, you are looking too much into specifics if you try to say otherwise, this was the buildup of the whole chapter with some pretty dramatic panels, something like this is no troll.
I don't think Sasuke was unfaithful or Sakura is incapable of having kids, madara didn't hit her uterus .