the last/final word

さきほどと同じ記事から拾います。

Eye Lifts — Fine Line Between Cosmetic and Therapeutic

“We’re the final word on it,” said Iwamoto. ” She said she’ll tell them, “‘It’s not medically necessary.’ In their hearts they know that.”

There are no national Medicare criteria for what determines a functional blepharoplasty. Different Medicare contractors that cover different regions of the country have their own criteria, although Migliori said that the criteria were fairly uniform across the country.
http://abcnews.go.com/blogs/health/2013/05/31/eye-lifts-fine-line-between-cosmetic-and-therapeutic/

the last wordは過去にも取り上げられています。http://d.hatena.ne.jp/A30/20120415/1334480847

the last/final wordはLDOCEに"a) the power to decide whether or how to do something"と定義されていました。今回はこの意味で使われています。
他にも"b) the last statement or speech in a discussion or argument""c) in sports, the last hit or kick in a game, especially when it is successful"とあります。(ibid)(Hatahata)