National proficiency test for molecular detection of white spot syndrome virus in China, 2021

National Aquatic Animal Disease Surveillance Program (NAADSP) has started since 2007 to conduct nationwide surveillance for the prevalence of major crustacean pathogens in China, including white spot syndrome virus (WSSV). Standard diagnostic procedures and reliable detection capability are critical for ensuring accurate and comparable surveillance results, hence competent authority of China launched annual proficiency test (PT) synchronously to screen competent aquatic clinical diagnostic laboratories to undertake NAADSP. Here we present the results of PT for molecular detection of WSSV in 2021. Six tissue samples containing different WSSV concentrations were provided as blind samples to a total of 170 laboratories nationwide. All the participating laboratories were asked to perform qualitative or quantitative analyses by using standard molecular assays or commercial detection kits. In the qualitative assessments, 86.8% (125/144) of the participants achieved satisfied results by using conventional nested-PCR. Comparing to the strong/medium positive samples, the identification of negative samples and weak-positive samples have been shown to be more challenging. A relatively higher error rates were reported in results of sample S3 and S4. In the quantitative assessments, cycle threshold (Ct) values of real-time PCR submitted by 47 participants had been analyzed by using robust statistical method. The overall satisfied performance was achieved by 74.5% (35/47) of the participants. Acceptable between-laboratories z-score (ZB) and within-laboratory z-score (ZW) was achieved by 93.3% and 91.1%, respectively, in the analysis of the pair of identical samples S6 and S8. WSSV-PT 2021 supplied valuable reference information for selecting competent laboratories to participate in the NAADSP. Furthermore, the national PT scheme in the study provided an effective tool to access the performance of laboratories in molecular tests, and also gathered practical experience for organizing PTs on other emerging crustacean pathogens in the future.