Does a person with a positive helicobacter pylori antibody test always remain seropositive even after successful treatment?

Once +, always +. The blood test for h pylori looks for your immune response to the infection. Once you turn positive, you never revert back to negative, even after appropriate antibiotic treatment. That's how you're protected in the future. On the other hand, if you want a test of cure to prove that the infection is gone, you need a breath test to confirm eradication.