"A week ago" means "a week before today". If today is the 17th, and the biopsy was taken on the 10th, you should say that the biopsy was taken a week ago. The phrase "a week before" usually means "a week before that other event". If you told me that the biopsy was taken "a week before", then I would probably wonder: it was taken a week before what? More @Wikipedia
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