When should carrier be used with the QIAamp DNA Mini or the DNeasy Blood & Tissue Kit?
FAQ ID -100

For DNA isolation using the QIAamp DNA Mini, or the DNeasy Blood & Tissue Kit, we recommend using carrier DNA when expected yields are below 10 ng. If possible, carrier DNAs such as poly-dA, poly-dT or poly-dA:dT should be used. Other carrier DNAs such as herring sperm DNA may interfere with subsequent PCR by binding primers nonspecifically.

Please note that poly-dA may interfere with oligo-dT primers, and, in this case, a different carrier DNA should be used. The concentration of carrier DNA should be at least 10 µg/ml. Optimal amounts need to be determined empirically for each application. The size distribution of carrier DNAs is typically in the range of 100 bp to 10 kb.