Risultati qualitativi e quantitativi del programma di Valutazione Esterna di Qualità (VEQ) “VEQ SARS-CoV-2 RNA” condotto dai laboratori in Lombardia nel 2021 = Qualitative and quantitative performance of SARS-CoV-2 nucleic acid detection tests: results from the 2021 External Quality Assessment in Lombardy

Introduction: to evaluate the qualitative test performance and inter-assay variations of SARS-CoV-2 nucleic acid detection tests by analysing the results of the External Quality Assessment (EQA) programme carried out in Lombardy in 2021. Methods: the 2021 EQA for SARS-CoV-2 molecular test programme consisted on testing of 12 exercise (12 samples of cells culture supernatants): 5 were SARS-CoV-2-negative and 7 SARS-CoV-2-positive samples [3 had viral load (VL) >5×103 copies/mL, 4 VL=1-5×103 copies/mL]. Participating laboratories provided qualitative (positive/negative) and quantitative (Cycle threshold, Ct value) results for each molecular test in use. Qualitative test performance was evaluated by positive/negative percent agreement (PPA/NPA); inter-assay variation of quantitative results was evaluated by coefficient of variation (CV). Results: 79 to 90 laboratories participated by using 199 to 231 systems, returning 2 623 qualitative/quantitative results. PPA ranged between 99.5% and 100%, NPA between 69.9% and 100%. NPA range was 99.1-100% by ruling out the “invalid” results (n=89). CV ranged between 11.6% and 13.5% (samples with VL=1-5×103 copies/mL) and between 10.8% and 14.2% (samples with high VL). The two most used systems gave the following results. System A (275 results) and B (245 results): CV by viral target ranged between 2.4-3.8% and 6.4-9% (samples with VL=1-5×103copies/mL) and between 2.8-4.2% and 7.3-11.9% (samples with high VL) respectively. Discussion: within 2021 EQA for SARS-CoV-2 molecular test, PPA and NPA were above 99.1%. However, the inter-assay variability among systems was notable and related both to viral load and to viral target detected by the test. This variability, thought limited, needs to be carefully evaluated and continuously monitored within quality assurance programs.