IgM rheumatoid factor (RF) was measured using RapiTex RF (Dade Behring Inc, Newark DE, USA) (positive ≥40 IU/ml). Anti-CCP2 antibodies were measured using QuantaLite™, Inova Diagnostics (San Diego, CA, USA) using titer levels as suggested by the manufacturer (positive >20.0 U/ml) or, since 2009, using the EuroImmun assay (positive >5.0 U/ml). As both the EuroImmun and the Inova assays use the same enzyme linked immunosorbent assay (ELISA) plates coated with the same antigens (as do all other antibodies to cyclic citrullinated peptide, second generation (anti-CCP2) assays across the world), have similar sensitivity and specificity, and give linear results across a wide range of concentrations, their results are easily interconvertible using the following logarithmic transformation:
Inova anti-CCP2 = –26.44 + 28.86 * ln(EuroImmun anti-CCP2).
The anti-Sa/citrullinated vimentin in-house ELISA was described previously (positive threshold ≥0.20 optical density units) [16 (link)]. From 2012 on, we used a commercial anti-Sa assay (EuroImmun; positive >19 RU/ml), validated against our in-house ELISA [17 ].
Inova anti-CCP2 = –26.44 + 28.86 * ln(EuroImmun anti-CCP2).
The anti-Sa/citrullinated vimentin in-house ELISA was described previously (positive threshold ≥0.20 optical density units) [16 (link)]. From 2012 on, we used a commercial anti-Sa assay (EuroImmun; positive >19 RU/ml), validated against our in-house ELISA [17 ].