If a person is infected with an RNA virus what percentage of the RNA in the blood of the person contains RNA of the virus?
I think your question is:


  Can you diagnose someone with an RNA virus by looking for that RNA in their blood?


The use of nucleic acid tests, or NATs in diagnosis has been around for a long time. The specific for profit company you mention is not the first to use them. Almost all NATs work by first amplifying the genetic material with PCR or other methods, so there does not need to be a large proportion of pathogen RNA vs total RNA in a sample in order to use a NAT.

NATs, rather than culture or antigen/antibody based tests, provide the definitive diagnosis for many viruses, RNA viruses included. As the CDC describes, NATs (among other tests) are also used to screen donated blood for RNA viruses (HCV, HIV, and West Nile).

Incidentally, the for profit company you mentioned does NOT detect all bacteria and DNA-viruses in the sample as you said. It screens for a large number of specific pathogens in their


  proprietary micro-organism reference database