Detection of endogenous RT-PCR inhibitors in a sample used for diagnosis. Prior to WNV RT-PCR, serial dilutions of competitor RNA were added to undiluted and diluted (1:20) RNA extracted from a spleen sample. The results of RT-PCR following agarose gel electrophoresis are shown. Lanes 1 to 7, 10, 100, 1,000, 10,000, 100,000, 1,000,000, and 0 molecules of competitor RNA, respectively; lane M, DNA molecular size markers.