It means undetectable, so the hiv rna test has a lower limit. Say 12 copies per ml. Undetectable means it is below the level. You'd normally want to be UD for

It means undetectable, so the hiv rna test has a lower limit. Say 12 copies per ml. Undetectable means it is below the level. You'd normally want to be UD for 3 months before engaging in unprotected sex with your informed partner.