Background and Aims

Anti-cyclic citrullinated peptide (CCP) antibodies are the most specific markers for rheumatoid arthritis (RA). Different generations of assays (CCP1-CCP3) have been developed showing variability regarding their performance. The comparability of different assays is an important issue to address especially in the early stages of disease.

Methods

This study aimed to investigate the diagnostic performance of IgG and IgA anti-CCP2 detected by EliA^TM (Thermo Fisher Scientific) compared to the combined IgG/IgA Quanta Lite^R anti-CCP3.1 assay (Inova Diagnostics) in sera of 184 early RA patients, 98 healthy subjects and 360 disease controls.

Results

Anti-CCP2 IgG and IgA assays showed high specificity versus healthy (98.9%; 98%) and disease controls (98.8%; 99.4%). Sensitivity was 52.2% (IgG) and 30.4% (IgA), respectively, resulting in high positive likelihood ratios of 47.5 (IgG) and 50.7 (IgA). IgA antibodies had no added diagnostic value since all patients were also IgG positive. Anti-CCP3.1 was slightly more sensitive than the anti-CCP2 IgG (55.4%) but specificity was markedly lower and amounted to 95.9% versus healthy and 90.8% versus disease controls resulting in a LR+ of only 6.0. Out of 360 disease controls 33 (9.2%) were found to be positive for CCP3.1 but among these only four (1.1%) were positive for anti-CCP2 IgG. When applying 60 AU/ml (high positive) as cut-off value for CCP3.1, sensitivity (52.7%) became comparable to the anti-CCP2 assay and both specificity (97.5%) and LR+ (21.08) increased substantially.

Conclusions

Thus, specificity of anti-CCP assays should be taken into account when interpreting results in order to reduce the risk of a false positive diagnosis.

Background and Aims

Anti-RA33 antibodies have been observed in seronegative patients, and thus may provide added diagnostic value in early rheumatoid arthritis (RA).

Methods

134 sera from an investigation cohort (Vienna early RA cohort) and a validation cohort (Leeds early RA cohort) of 131 patients (both satisfying 2010 ACR/EULAR classification criteria) were tested for the presence of IgA, IgG and IgM isotypes of anti-RA33 antibodies by prototype assays using the EliA™ platform (Thermo Fisher Scientific). The cut-off values were chosen to achieve specificities of ≥95% against disease controls and 98% against healthy subjects*. In addition, RF-IgM as well as ACPA-IgG was detected by EliA™ (Thermo Fisher Scientific).

Results

In the investigation cohort anti-RA33 antibodies were detected in 8 out of 51 seronegative patients, reducing the ‘serological gap’ left by RF and ACPA routine testing by 15.7%. The anti-RA33 IgM isotype showed the highest sensitivity (10%) followed by IgG (6%) and IgA (3.7%) isotypes. Interestingly, the prevalence of anti-RA33 antibodies in the validation cohort was considerably higher without differences in titer. The highest sensitivity was found for the anti-RA33-IgG isotype (20.6%) followed by IgA (16%) and IgM (14.5%). Disease duration might have influenced the different distribution of anti-RA33 isotypes as as it ranged from 0.2 years (investigation cohort) 0.6 years (validation cohort). The added diagnostic value in the validation cohort was 24.5%.

Conclusions

Results suggests anti-RA33 antibody testing may aid in the diagnosis of (otherwise labelled seronegative) early RA. Furthermore disease duration might impact the distribution of anti-RA33 antibody isotypes.

* Sieghart et al. Front Immunol. 2018