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I'm trying to correct a letter for a friend and i wanted to know should a comma, semicolon, or nothing be used in this sentece especially after the me

He assured me yes it would because they had performed a diagnosis test.

thanks
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Then"yes" becomes an interjection, and so can go pretty much anywhere parenthetically - that is, without regard to the grammar of the rest of the sentence. As such, you should separate it off with commas on both sides of "yes".
Comments  
If you left out yes, and I think you should, then the sentence would read:
"He assured me it would, because they had performed a diagnosis test."
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what if i want to keep the yes in
 rommie's reply was promoted to an answer.