This episode was referenced in the video game The Simpsons: Bart vs. the World in the trivia round. Question: "How did Bart save Mr. Burns' life?" A: He pushed him out of the way of a falling plutonium rod. B: He donated blood to him (correct answer) C: He talked Homer out of murdering him.
If Smithers is B-Positive, his kidneys would work on a person with AB Positive blood, since they can accept any blood type, or another B-Positive person, like Homer. A B-Positive person donor is incompatible with a Double O negative, meaning that Bart wouldn't be able to use his blood to save Mr. Burns.