The invention relates to quality control of analytical assays, particularly NAT assays of blood samples containing nucleic acids. A control panel containing quantified amounts of substances known to interfere with an analytical assay is used and compared with a reference sample in the analytical assay. A comparison of the assay results interference panel validates the assay and can serve as a periodic quality control check for the analytical assay as well as related methods and protocols. The use of the control panel of the invention can also determine whether interfering substances are present and establish under what conditions the analytical assay reliable.