A lyophilised reference serum from one patient with rheumatoid arthritis (RA) diluted with serum samples from healthy subjects was evaluated as a possible first international standard for anticitrullinated peptide antibodies (ACPAs).
The authors used 12 commercial ELISAs for ACPA detection in the reference serum and for testing the linearity of the assays by studying twofold serial dilutions. To test the effectiveness of the standardisation, sera from 20 RA patients with variable antibody concentrations were analysed, and the relative concentrations were calculated using both the kit's own curve and the six dilutions of the reference serum as a calibration curve. Fifty sera from normal healthy subjects were used to calculate cut-off values for the reference serum using each commercial kit.
The calibration curve obtained for each of the 12 methods using the reference sample dilutions as calibrator allowed harmonisation of the ACPA concentration of the 20 RA serum samples, significantly reducing the dispersion of the values. The mean coefficient of variation (CV) was reduced from 76.4% to 27.9% (p=0.018) and from 85.9% to 33.5% (p=0.028) for the medium/high and negative samples, respectively. Low positive sera CV was also reduced, but to a smaller degree, from 82.5% to 55.5% (p=0.043).
This first evaluation of the behaviour of the ACPA reference serum demonstrated that it tested positive in all the assays and that it may be used as a reference standard for establishing calibration curves, reducing the dispersion of antibody values and better comparing results obtained from different methods/laboratories.