as far as i know, only diference is the window period, for agab about 28 days and for ab 12 weeks, but both conclusive in 12 weeks, that means that even if you have agab done before 12 weeks you must retest it with ab test at or after 12 weeks for conclusive answer.
Go to the CDC website, you will find everything you need about tests and testing :) all the best
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