hbsag quantity reflects hbv infection and cccdna (the template integrated in human dna which produces hbv virions and antigens) because hbsag is produced by transcriptionally active cccdna inside liver cells.
there can be many cccdna in one liver cell and when cccdna reaches less than 5 copies per cell also hbsag in serum gets low like 500iu/ml, at this point hbv is not able to continue infection and can t replicate even off drugs, slowly by years hbsag is usually cleared when it reaches such low levels and when 10-20iu/ml clearance is almost certain
hbvdna pcr negative with alt less than 30 means no relevant liver damage and low liver cancer risk but mean nothing regarding hbv infection, the replication cycle of hbv has nothing to do with hbvdna pcr in blood the virus keeps infecting other liver cells and spreading by cccdna and intrehepatic hbvdna while hbvdna in serum is und
hbsag quantitative reflects the number of infected cells and immune control of hbv when reaching very low levels like 500iu/ml or lower and also reflects hbv immune clearance, hbvdna is totally useless for this because hbv clearance can happen even with detactable hbvdna in serum