Fig 1: Dilution linearity comparison of the nucleocapsid concentration of the three quantitative commercial ELISA kits (Abcam, ab274341; ProteinTech, KE30007; GeneTex, GTX535824) using serial dilutions (2x, 4x, 8x,16x and 32x) of EQA SARS-CoV-2 sample (ID 410,004, Table 1). Mean optical density (OD) values were obtained according to the kit’s specifications and the measurements were obtained using a BioTek Synergy H4 plate reader (Biotek GmbH). Linear regression analysis was performed using Gen5 analysis software. The LOD and LOQ of the different kits were 79 pg/ml and 130 pg/ml, respectively, for the Abcam assay, and 90 pg/ml and 172 pg/ml for the Proteintech assay. For the GeneTex assay, LOD was > 200 pg/mL, so the LOQ was not calculated in more detail. The Roche-Elecsys and EU SARS-CoV-2 antigen tests had a good correlation (R2 = 1) (data not shown)