you just met ignorants, not doctors, see also this, a liver specialist prepared this book for his collegues and presented at a conference years ago since there are too many ignorants that understand nothing about hbv and make more damage than good:
"The general book of ingnorance about hbv"
http://www.medhelp.org/health_pages/Hepatitis/the-general-book-of-ignorance-about-hepatitis-B/show/1152?cid=153
hbsag is available since hbsag itself is known, from 2000-2005 abbott made a good commercial assay.
since hbvdna pcr is very expensive and hbsag is extremely cheap (7euro in italy), there is a tendency to hide hbsag quant (US, canada dont use it yet whie even third world countries have it), hbsag can be used instead of hbvdna pcr or in combo
hbsag quant shows also that antivirals like tenofovir and entecavir, quite expensive, increase its quantity making hbvdna undetactabel.....and many might think they are useless on hbv, which is not quite wrong
hbsag reflects cccdna which are the infected cells in the liver, i have yet to understand what exactly is the use of hbvdna really apart the use for monitoring nucs response and resistance development.
to me hbvdna is totally useless to monitor infection, infected cells increase when hbvnda pcr is und by antivirals, infact if you stop antivirals you get an acute hbv that might be even dangerous for life