Welcome to HCM
It is a test done to know genetic predisposition to certain rheumatological diseases called SSA(Seronegative spondyloarthropathy
) one of these is Ankylosing Spondylitis(AS).
In general about 5% may test positive for HLA B27 it association with Ankylosing Spondylitis is > 85% and if you are already having backache
for last 4 years it is quite possible to have some association.
MRI will be good to know extent of damage and any compression on neural roots if any.
Please see your Rheumatologist
and take physiotherapy
Good luck to you