Why and when is it necessary to use carrier DNA/RNA in the extraction procedure?
FAQ ID -1489

The addition of carrier nucleic acids increases the yield of extracted DNA/RNA from analytical samples. Especially when starting from samples low in pathogen or DNA/RNA concentration (e.g. cerebrospinal fluid), we highly recommend to use poly(A)-homopolymers (AmershamBiosciences, Cat. no.: 27-4110-02) as a carrier nucleic acid.

Carrier RNA is not required when sample material containing an excess amount of DNA is used, such as stool, blood cells and others.

Please note that some extraction kits, e.g. the QIAamp Viral RNA Mini Kit, already contain carrier RNA (supplement to the lysis buffer). In these cases, additional poly(A) carrier RNA is not required