Application of a ddRT-PCR to quantify seasonal influenza virus for viral isolation

Viral isolation in cell cultures has been regarded for decades as the "gold standard" for the laboratory diagnosis of influenza viral infections. Not all viral strains could be isolated from clinical samples. This study aimed to quantify the viral load in the samples before isolation to save working time and improve working efficiency. Four hundred samples from patients with influenza-like cases were confirmed pdmH1N1 positive (200 cases) and B Victoria (BV) positive (200 cases) by whole-genome sequencing and analyzed by ddPCR for viral load in samples before isolation, and isolation results were verified by hemagglutination (HA) assay and hemagglutination-inhibition (HI) tests. Probit regression analysis was used to calculate the isolation viral load limit with a 95% probability level by SPSS 19.0 software. The results showed that the isolation limit of viral load was 4.9 × 10 4 (95% CI: 2.5 × 10 4-9.0 × 10 4) copies/mL for pdmH1N1 and 1.9 × 10 4 (95% CI: 7.8 × 10 3-3.6 × 10 4) copies/mL for BV. The isolation rate of clinical samples is positively correlated with the viral load in clinical samples, which can be used for viral culture, providing important guidance for daily work.