Having protected sex protected you. You'd know if the condom broke as that is a noticeable event. Condoms split wide open and people don't miss that. Pin holes are a myth. But regardless, yes, your test can be counted on. It means you do not currently have hep B. People can get a negative HBsAg result even if once positive for Hep B if they spontaneously cleared the virus. So, whether or not you've 'ever' had it, I don't know. but the result indicates you don't now and you won't now develop it from that episode 6 months ago.