Serum tolerance of the dRT-PCR assay. (A) The dRT-PCR mixtures became turbid following the completion of thermal cycling. (B) Ct values of dRT-PCR assay where different concentration of HP-PRRSV serum and purified RNA were used. (C) Amplicons of dRT-PCR assays were electrophoresed on 3.5% (w/v) agarose gel. Lane 1: 1 μL of purified HP-PRRSV RNA derived from 1 μL of HP-PRRSV-positive serum; Lane 2–6: 1 μL of HP-PRRSV-positive serum diluted with negative serum to acquire different serum concentrations of 5, 10, 15, 20, and 25% (v/v) in a 25 μL reaction, respectively. The arrow indicates the 73-bp target bands for HP-PRRSV. (D) Ct values of dRT-PCR assays detecting the same amount of purified RNA supplemented with no serum or with negative serum in various concentrations (5-25%, v/v).